Objective Reviews & Commentary - An Engineer's Perspective

July 28, 2011

O2 Design Process

o2 see thruRAISING THE BAR: Last week I introduced my own headphone amp design--the Objective2 (“O2” for short). The goal was to see how much objective performance and audio accuracy I could achieve with about $30 worth of parts. And to raise the bar further, the O2 is a “no excuses” headphone amp suitable for most any headphone and adaptable to most any source—at home or on the go. The first O2 article covered the premise, detailed performance measurements, and comparisons with a few other headphone amps including the Benchmark DAC1 Pre’s headphone output and it’s closest competitor, the AMB Mini3. If you haven’t seen the first O2 article, you might want to at least skim through it before reading this one. The final article covers all the other O2 Details, options, and a detailed circuit description.

I HAD SOME HELP: Some really smart and well respected guys, like Douglas Self, Bob Cordell, Bruno Putzeys, Jan Didden, Walt Jung, Cyril Bateman, Samuel Groner, Siegfried Linkwitz, and others, have done extensive audio hardware research and published their findings. These guys have solid numbers, math, measurements and science on their side. Their published results often have an “Ap” for Audio Precision watermark in the corner indicating they use professional instrumentation. Many have published multiple books, papers, technical articles, etc. Their work has been extensively peer reviewed and has stood the test of time. They’ve found what works best from input circuits to capacitors to grounding schemes. They helped perfect the “wheels” of high quality audio. So, rather than go off and try to re-invent the wheel as many DIY and audiophile designers seem bent on doing, I liberally took advantage of their well proven research. Very few can match their expertise in their respected fields and I’m certainly not going to pretend I can do better. So to all of the guys above: Thank you!

TRUSTING THE EXPERTS: An amazing number of audio designers apparently think they know more about audio performance than the component manufactures (and sometimes the guys in the paragraph above as well). For example, they try to design discrete op amps instead of using IC op amps. Samuel Groner tested two of Audio-GD’s discrete op amps and the results were extremely poor. In his comments he said he couldn’t understand why anyone would want to use them. You can find his impressive PDF op amp distortion paper via Google and read the results for yourself. The “roll your own” approach is something like a guy trying to build his own car from scratch in a shed thinking he can do a better job than the companies who specialize in designing and producing cars. Some audio designers take off the shelf parts, that were carefully optimized by skilled engineers using hundreds of thousands of dollars worth of equipment, and think can make them work better by using them in ways the designers never intended—like forcing certain op amps into Class A operation. It’s like buying a new Porsche, bolting on monster truck tires for more “traction”, and claiming the result is somehow better than what all those clueless Porsche engineers thought was best. The sad thing is, audio performance isn’t as obvious as car handling. And most of these designers lack the equipment to properly measure the results of their often crude efforts. If they did, they would probably realize the overall performance is very likely worse as Samuel Groner has demonstrated.


No Worry Audio

worry dbaldinger OSFA: A primary mantra for the O2 was “One Size Fits All” (OSFA). Basically I wanted an amp suitable for most any application. The first article has the details. (drawing:DBaldinger)

ACCURACY: A primary goal was maximum accuracy. The amp should get out of the way and deliver music as the recording engineering intended. That might sound like hype, but it’s mostly an objective engineering exercise to make that happen—not audiophile voodoo magic. Again, the first article goes into this further.

HIGH-END MEETS OBJECTIVITY: A lot of audiophile beliefs have some objective truth behind them. For example, resistors really can sound different. Some are relatively noisy and others don’t follow Ohm’s law very well—their resistance varies with the voltage applied creating distortion. There’s some truth to “burning in” electronics as some capacitors in certain applications perform better over time. Electrolytic capacitors in the signal path sometimes create measurable and audible problems. And if someone claims a coupling capacitor changes the sound, why not measure and analyze the differential signal across the capacitor while the amp is operating playing real music? If there’s nothing much to measure, by Ohm’s law, it can’t change the sound. If there is something there, you’ve found an area to possibly improve. I’ve done that and much more with the O2. Objective measurements help an audio designer focus on the things that matter most.


Overall Design Principal

design drawing dbaldingerDIFFERENT APPROACHES: There are many different approaches to designing a headphone amp. It’s useful to know where the O2 fits into the bigger picture of design philosophies (drawing: DBaldinger)

  • Cost (almost) No Object – Some go overboard with ultra-high end parts, exotic topologies (i.e. fully balanced), etc. These designs can end up being very costly as some “boutique” audiophile parts are ridiculously expensive and some of the topologies require 2+ times as many parts. Do they work any better? It’s unfair to generalize but I know lots of the parts and principals that go into overkill designs often have no measurable benefits and fail to survive blind listening comparisons. I’m sure some designs turn out great. And even those that don’t can be impressive works of art to be admired. You can put a Mercedes AMG V8 engine into a riding lawnmower but the result probably belongs behind a rope in a museum rather than trying to mow lawns. In other words, the engineers at Lawn Boy can probably build a better lawn mower than some guy in a shed using Mercedes parts. I go into this more later and it’s one of the key reasons why many Cost no Object designs are flawed—the implementation is at least as important as the parts and cost
  • Latest Ideas – Many in this hobby are used to getting a new phone/PC/iPod/etc. every year or two. An Intel Core i7 is easily faster and better than a Core 2 CPU for example. But that doesn’t translate to analog audio. Some of the best audio op amps are 10+ years old. The only thing analog I can think of that’s really changed in the last decade are “Class-D” power amplifiers. Some DIYers and boutique manufactures “invent” new topologies like 3 channel designs but those too rarely provide any real world improvements and are often a step backwards as I’ve shown. The challenges of implementing an accurate headphone amp were solved a long time ago. The “newer is better” axiom might apply to smartphones, but it doesn’t usually apply here.
  • Design By Ear – A lot of DIY designers, and apparently even some small commercial ones, lack the right test equipment. Some use RMAA but it’s loaded with limitations and problems. So most of them, not wanting to spend five figures on instrumentation, are at least partly designing in the dark. But they creatively argue that’s ”OK” because they depend mostly on their ears. But that’s been proven deeply flawed in multiple ways. Most audiophiles dislike blind listening (as it could undermine a lot of their beliefs and hence is often criticized) so they use what’s known as sighted listening. And along with that comes a strong unavoidable psychological bias that’s hardwired into all our brains. Multiple studies have shown we humans easily hear things that don’t exist. So the “design by ear” crowd are genuinely fooling themselves. NuForce admits to designing this way and their products have had some embarrassing problems as a result. There’s a lot more information and many references on this topic in my Subjective vs Objective article.
  • As Cheap As Possible (but make it look nice) – This is the mantra for a lot of commercial designs being sold. And “cheap” often extends to the R&D time that went into the design or lack thereof. It took Benchmark a few years to develop the first DAC1. But some companies, like Audio-GD, FiiO, etc. crank out new designs every few months. Obviously there’s not a lot of R&D going into each Audio-GD product unless they have an army of design engineers but one guy claims to design all of it. A lot of companies are trying to profit from the “headphone craze” however they can and it shows. Cheaper designs, faster to market, means more profit. This assumes, of course, enough people buy them. And if you’re an advertiser on Head-Fi the odds seem good you’ll have a loyal fan club no matter how bad the product is. Samuel Groner tested a few Audio-GD products, a Head-Fi sponsor, and the measured performance was awful. But they’re still immensely popular among many Head-Fi members who drink the subjective Kool Aid. So while this approach might work, I honestly don’t know how some of the purveyors sleep at night.
  • Purely Objective – This is how most of the mainstream big guys do it—like say Behringer or Sandisk. They survey the competition, come up with detailed specs and requirements to be a bit better than Brand X, and figure out how to make a product that meets all the goals yet can sell for less. It’s not sexy but it’s a formula that works. Unlike in the audiophile world, their products usually do meet the claimed specs and consumers at least get what they pay for.
  • Harmless Excess – This is my philosophy. When it can’t hurt, and the cost is low, I’ll throw in an audiophile tweak here, a better part there, etc. Douglas Self and others have shown in many applications polypropylene capacitors don’t measure or sound different than similarly constructed polyester caps. But if the the two types are close in price, I’ll go with with the audiophile preferred polypropylene. And I do the same with performance goals. A lot of studies claim you can get away with 0.05% distortion at least everywhere but the midrange. But if I can keep everything to 0.009% or better across the board, without other compromises, it provides extra peace of mind. Those are just two examples.

ISN’T LISTENING MORE IMPORTANT THAN MEASUREMENTS? I think both are important. And that’s why, for the first O2 article, I conducted blind listening tests comparing the O2 to the well reviewed Benchmark DAC1’s headphone amp. It’s best to make all the right measurements and do lots of listening. If you want the listening to be unbiased, you have to do it blind. This has been demonstrated dozens of times.

CAN DIY BE BETTER? I think it can but I suspect it rarely is. Someone who really knows what they’re doing can certainly beat the misguided “Design By Ear” and profit hungry “Cheap as Possible” guys. The “Purely Objective” camp is a bit more challenging and depends on the product, price, etc. Can I beat Behringer’s current headphone amps? Yes. But that’s because they’re designed for musicians monitoring a mix while playing live which isn’t a terribly critical application. If Behringer made an audiophile-grade headphone amp that would be a bigger challenge. And it’s even harder to beat a company like Benchmark, Violectric, or Grace Designs as they’re much less concerned with shaving every penny out of the design and they’re very good at audio design. It’s all they do. In general, those engineers with their knowledge and equipment can turn out a better product with inexpensive mainstream parts than nearly any DIY designer can manage in his basement with RMAA no matter what parts the DIY guy uses. I know that probably sounds harsh to lots of DIYers in their basements, but in my experience, it’s just reality. They often have no idea how poorly their designs perform in some areas.


Designer Components

op amp chipsDESIGNER COMPONENTS: Some audiophiles are “component snobs”. Someone once told me the Benchmark DAC1 isn’t worth considering because its Alps volume control only costs a few dollars. But, being objective, the DAC1 has great crosstalk performance (a weakness of some volume controls), good channel balance tracking, the volume control feels solid, turns smoothly, and doesn’t make any audible noise when turned. So what exactly is wrong with the volume control? The answer: Nothing significant. But some think you’re supposed to spend way more to get those hidden designer labels. They can go enjoy their latest issue of the Robb Report. They’re after something very different than simply getting the most accurate sound and the O2 isn’t their kind of amp.

DESIGNER PARTS GONE WRONG: I tested a commercial headphone DAC with a fashionable trendy DAC chip and op amp in it. The PC board layout looks fancy with everything arranged neatly in rows (always a bad sign—more on that later). But whoever designed it apparently couldn’t be bothered to read (or perhaps understand) the datasheet for the DAC. The oversampling digital filter in the DAC chip—a very critical aspect of a DAC—defaults to 24/192. But as a USB DAC it runs at 16/44. Because the filtering is all wrong high frequency content in music creates alias artifacts that are “mirrored” down into the audio band. It measures poorly on the dScope and I’m pretty sure you can hear all the extra high frequency garbage. A typical Head-Fi subjectivist might buy this DAC, hear the extra high frequency crud, decide the added HF content is newfound musical “detail”, and give the half baked DAC a glowing review on Head-Fi. Others run out and buy one and, courtesy of subjective bias, hear what the first reviewer described. Next thing you know the Half-Baked DAC Company becomes a Head-Fi sponsor, and well, you can see where this is going. But the real crime is whoever designed it either never properly measured it, or if they did, they didn’t care they got it wrong. But hey, it looks nice and uses all the right fashionable components. That’s what matters most, right?

OP AMP ROLLING: I’ve written two follow up articles regarding testing op amps for the O2. The results are rather surprising! See: Op Amp Myths and Op Amp Measurements.

DESIGNER COMPONENT CHALLENGE: Some claim specs alone don’t tell you how something like an op amp will sound. I believe if two op amps meet clear some basic measurement criteria, they will sound so similar it’s next to impossible to tell them apart. Anyone’s who’s skeptical might be interested in my Op Amp Blind Listening Challenge.

IMPLEMENTATION IS EVERYTHING: Like the DAC mentioned above, I’ve seen all sorts of products that use the right parts but got the details wrong and don’t work very well. Just routing a single ground signal wrong on the PC board can seriously harm performance. I’ve seen designs that measure great on RMAA but are simultaneously oscillating at RF frequencies. The O2 demonstrates proper implementation can yield genuinely excellent performance without using any designer or expensive parts. Some of the O2’s measurements are pushing the limits of even my dScope audio analyzer.



requirements ivan walshHEADPHONE AMP DESIGN 101: For those interested, the rest of this article discusses what goes into designing a headphone amp, some of the trade-offs, and how the steps were applied to the O2. I also bust, or at least dent, a few myths here and there. If you just want to build an O2, it’s not required reading. But if you’re interested in learning more about what makes a good headphone amp, and specifically why the O2 ended up the way it did, it’s worth checking out. And if you consider yourself a DIY or commercial audio designer it might be genuinely useful.

STEP 1 REQUIREMENTS: It’s considered good engineering practice to start with fairly detailed requirements. Without decent requirements you don’t know where you’re doing, it’s harder to keep your eye on the ball, you end up wasting a lot of time, and it’s harder to know when you’re done and if you even got it right.


  • As much of a “One-Size-Fits-All” (OSFA) design as reasonably possible
  • Performance as accurate as possible for the best sound quality (see the first O2 article)
  • Portable (rechargeable battery) and desktop (AC line) operation
  • Reasonably small portable size to fit an inexpensive off-the-shelf enclosure
  • 7 hours minimum battery life with a 20+ hour low power version
  • Brief short circuit protection
  • No risks to headphones with power up/down, low batteries, etc.
  • Switchable gain (2 gain modes)
  • Power LED
  • DIY friendly design with no surface mount components
  • Reproducible: No critical components, matched parts, critical construction techniques, etc.
  • As many components available from a single vendor as possible to save shipping costs
  • As low cost as possible while meeting all other requirements

1-2 WHAT HEADPHONES? Obviously a headphone amp is to drive headphones. But which headphones? In keeping with the OSFA requirement, I wanted to find close to the “worst case” headphones possible. After quite a bit of research it seems the 38 ohm planar HiFiMan HE-4/HE-5LE and 50 ohm HE-6 are especially tough cans that need lots of current. They’re the kind of headphones that make tube amps run and hide in the closet. And for needing lots of voltage, the Beyer DT880-600 needs the most of voltage of any dynamic/planar cans I could find. For wild impedance swings and ultra high sensitivity my Ultimate Ears Super Fi Pro 5s and Etymotic ER-4s help round out the assortment.

1-3 HOW LOUD? Music with a wide dynamic range is the most challenging to play loud because the peaks are much louder than the average level. Such music may not seem loud but the peaks can be seriously challenging for your audio hardware. For such music to approach live levels, you need to cleanly reproduce peaks of 110 dB SPL. If you want to know where that number comes from, see More Power?

1-4 THE CALCULATIONS: To start with a well known example, the Sennheiser HD650 is rated at 103 dB at 1 V RMS input into 300 ohms.The HD650 needs 2.2 Vrms to hit 110 dB SPL. The math can be found in the More Power article. 2.2 Vrms is about the limit of say the Mini3. And, not coincidentally, most find the Mini3 gets loud enough with the HD650s—but just loud enough So this provides some correlation that 110 dB, and this approach, seem to really work in the real world. Now to apply it to our two worst case cans:

  • HiFiMan HE-4/5 – They’re rated 87 dB SPL at 1 mW and 38 ohms. To drive them to realistic peaks of 110 dB SPL they need a whopping 200 mW (enough to fry some headphones). That’s about 2.8 V RMS and a peak current of 104 mA per channel in 38 ohms.
  • HiFiMan HE-6 – Rated 83.5 dB SPL at 1 mW at 50 ohms. They need 447 mW to reach 110 dB which is 4.7 V RMS and 133 mA of current. Despite their higher impedance they’re even more power and current hungry than the HE-4/5 above. There’s good reason here for tube amps, and plenty of other amps, to run and hide.
  • Beyer DT880-600 – These need 43 mW at 600 ohms to hit 110 dB which is 5 V RMS. This is way beyond what most portable amps can manage. Even the new FiiO E11 can’t come close nor can the Mini3.
  • Extra Headroom – In the spirit of OSFA, it’s best to have some extra headroom above and beyond the theoretical limits. That way the O2 won’t end up on the ragged edge. Generally 25% is considered enough headroom for voltage and current capability. So 133 mA * 1.25 = 166 mA and 5 V * 1.25 = 6.25 V RMS. I’m sure in the real world there are still a few cans that won’t quite get loud enough, but I’ve tried to find some of the most challenging that are currently in production (the K1000 “ear speakers” don’t count).
  • Noise – Testing shows noise that’s 85 dB below the maximum listening level will usually be inaudible under nearly all conditions. The Ultimate Ears and Shure IEMs hit 110 dB SPL with only about 100 mV of input. 85 dB below that is 5.6 uV or –105 dBv (103 dBu). This would be a noise level of 25 dB SPL with these headphones. See: Noise and Dynamic Range

1-5 AUDIO SPECIFICATIONS: Here are the complete audio specs. This isn’t some watered down list of specs with the bar set conveniently lower than it should be. It’s the real deal. I believe this to be the true point of diminishing returns and amps that can pass all of the following on a real audio analyzer get my seal of approval. Most of these criteria are supported by well respected research and/or are generally accepted guidelines as to the thresholds of audibility. Some thresholds are not black and white so it’s best to error on the side of being conservative (more accurate) and that’s what I’ve done here (many of these are explained a bit more in the first O2 article):

  • Output impedance less than 2 ohms
  • Input impedance >= 10K
  • Frequency response +/- 0.25 dB 20 hz – 20 Khz 400 mV 16-600 Ohms
  • Phase response less than +/- 2 degrees error 100 hz - 20 Khz 16-600 Ohms
  • Absolute phase: Preserved
  • Slew Rate greater than 3 V/uS using 10 Khz square wave near full output 600 Ohms
  • Distortion under 0.01% 20hz – 20 Khz into 16 – 600 ohms from 10 mV – 400 mV RMS
  • Channel separation better than -40 dB @ 16 ohms and –60 dB @ 150 ohms 400 mV RMS
  • Channel balance error less than 1 dB at any setting down to –45 dB below max volume
  • Noise under –105 dBv (103 dBu) unweighted (5.6 uV or -97 dBr referenced to 400 mV)
  • DC offset under 5 mV typical, and ideally, under 20 mV worst case
  • 100% stable with any realistic reactive load from 16 – 600 ohms
  • Transient ringing and overshoot tightly controlled with all realistic headphone loads and 0.01 uF
  • 166 mA per channel, both channels driven, peak current capability at < 1% THD
  • 6.25 volts RMS on AC power at < 1% THD into 150 ohms
  • 4.5 volts RMS on DC power (nominal battery voltage) at < 1% THD into 150 ohms


Circuit Design

scope trace powerSTEP 2 CIRCUIT DESIGN: With the above defined here’s how I arrived at the O2’s design:

2-1 DISCRETE, IC OR BOTH? Tubes and single ended designs were ruled out in the previous article as they’re notably less accurate and far more likely to let their presence be known and get in the way of the music. They’re also not battery-friendly. That leaves a push-pull solid state design with 3 main choices:

  • Fully Discrete – Some designers, such as Kevin Gilmore and John Lindsay Hood, prefer fully discrete designs. And Douglas Self has published some truly excellent fully discrete power amps for driving speakers (his “blameless amplifiers”). While such designs can work very well, they require a fair amount of work to get right. And some aspects can be especially tricky—most notably finding the optimal bias level in a Class-AB design and stabilizing the bias over a wide range of device temperatures. PSRR and CMRR typically suffer without hand matching components. High frequency stability can be rather challenging as well. And discrete designs are generally more power hungry than an IC design as the transistors will require more bias for low distortion—not a good thing for battery operation. Discrete designs can also be fairly fussy about components which conflicts with the “easily reproducible” requirement. And, finally, it’s really hard to beat the big IC semiconductor companies that spend millions on R&D. Want proof? Check out Samuel Groner’s Operation Amplifier Distortion paper (Google it) and compare the discrete op amps he tested to the ICs. No contest. The discrete designs fail by a wide margin.
  • Hybrid – There are many headphone amp designs floating around using an op amp to drive a pair of discrete output transistors in each channel such as the AMB M^3. There are many in databooks. But you don’t see many of them fully tested. Basically they suffer most of the same bias and stability issues as described in the paragraph above. Crossover distortion is especially difficult to fully correct with feedback. It helps a lot if they’re class A but that’s not suitable for battery operation. And some are more difficult to stabilize without using output inductors or 10+ ohm series resistors. It can be done, but it’s very difficult for such designs to match an IC with a Class-AB discrete output stage in most areas except current capability.
  • ICs (myth busted) – If you want to compare a 60+ watt chip amp for speakers to say a Doug Self blameless discrete design I’ll put my money on the discrete Self amp. But at headphone levels, it’s a different ball game. You don’t see Doug Self off trying to build a better op amp or buffer IC out of discrete parts. With only a few exceptions ICs are the best way to go. For those who claim not to like the alleged “sound” of op amps they should consider the entire signal chain. Some favorite recordings widely used by audiophiles as demo material were recorded and mixed using hundreds of op amps. The mixers, equalizers, compressors, processors, etc. were all full of op amps. And there are very likely op amps in whatever digital source gear they’re using. So the majority of music is already well steeped in op amp goodness. And there have been plenty of blind tests that support the transparency of op amps. To those who insist op amps sound bad, I say let’s arrange a blind test!

2-2 CLASS-AB OR CLASS-A? Class A amps can have some significant advantages—especially if you’re unable to fully optimize a Class-AB design including the bias operating point. Given the relatively low currents in a headphone amp the main advantage of Class A is getting rid of crossover distortion. But here’s where those IC designers at the semiconductor companies and their expensive R&D labs get to show off. They can nearly always remove crossover distortion from Class-AB designs much better than a discrete designer can. This is partly because it’s far easier to manage the thermal tracking issues when everything is on a single die so the bias point can be very precisely controlled in an IC. They also have tricks at their disposal a discrete designer can only dream about. So, in a nutshell, they can get class A performance from an IC on a class B power budget. It’s the best of both worlds. This is shown in the residual THD measurement from the last article. And for those forcing op amps into Class A that weren’t designed to be operated that way, unless you can fully measure the results of your hack, you may well be making things worse. If It’s not broken it’s usually best not to try and “fix” it—especially if you can’t fully test the result.

2-3 SINGLE OR MULTI STAGE TOPOLOGY? As explained in the Cmoy With Gain and Mini3 articles there are some significant compromises trying to use one stage for both the gain and output stage. It’s like a high performance front wheel drive sports car—few exist because the front wheels have to handle all the steering and power duties which ends up compromising both. So a single stage design was ruled out and a two stage topology is the obvious choice for the following reasons:

  • Lower Distortion – Adding voltage gain requires using less feedback, and less feedback means higher distortion. Most of the distortion in a headphone amp is in the output stage so you want the most feedback possible for that stage which means operating at unity, or nearly unity, gain.
  • Lower Noise – The gain stage is where most of the noise comes from. If the volume control is after the gain stage reducing the volume also reduces the noise with it. In a single stage design, with the volume control at the input, you get all the noise all the time at any volume setting. With the volume control at the input you also amplify the Johnson Noise of the volume control itself which, in many headphone amps, dominates the overall noise. So there are huge noise improvements to having a second stage with the volume control between them. See the O2 noise measurements in the previous article for more details and the proof.
  • Lower DC Offset – The gain stage is where significant DC offset is usually generated. Using two stages allows isolating this DC from the output stage and hence the headphones.
  • Higher Stability – A 2 stage design can be inherently more stable see 2-5 Feedback below.
  • Component Optimization – With two stages the components (ICs) for each stage can be optimized for their task. High current devices that can drive headphones don’t make the best gain stages and vice versa. With two stages no such compromises are required.
  • Controlled Impedances – The first stage provides a known low impedance source for the second stage. This allows the components, like the volume control, coupling caps, bias resistors, etc. to be optimized for maximum performance without worrying about what’s used to drive the amp.
  • Lower Power Consumption – Compared to 3 or more stages, a 2 stage design will generally use less power for longer battery life.

2-4 AC OR DC COUPLED (myth busted)? I know many high-end subjective audiophiles don’t like caps in the signal path but, in reality, their benefits can far outweigh their negatives when you use the right cap properly. Douglas Self, Cyril Bateman, and others, have conducted tests of capacitors and how they affect audio signals. And, lending some truth to the audiophile beliefs, there are circumstances where capacitors cause problems—including electrolytics in the signal path and EQ circuits where caps have a significant AC voltage across them. But a properly sized high quality film coupling capacitor isn’t one of those circumstances. For a cap to do more than simply attenuate the sound by a tiny fraction of a dB, requires a non-linear voltage across it in operation. The isolated inputs of the dScope allow analyzing the voltage across any cap while the amp is operating. And, besides a miniscule bit of linear attenuation (which is expected due to the ESR), the non-linear components are lost in the noise which is a few microvolts. It’s not even remotely close to any threshold of audibility. Proper coupling caps have also been shown transparent in blind and audio differencing tests. So the only negatives are usually cost and space but the benefits are substantial:

  • DC Input Protection – A direct coupled amp like the Mini3 will amplify any DC offset at its input and send that amplified DC straight into the headphones. And, because audiophiles often dislike coupling caps, audiophile and DIY gear (including DACs) are more likely than consumer gear to have significant DC offsets. At 14X gain, just 70 mV of DC at the input would mean 1 full volt of inaudible headphone destroying DC output. Is that risk really worth the zero proven benefits from DC coupling?
  • DC Offset Reduction – As mentioned in 2-3 above, the gain stage generates most of the DC offset. AC coupling keeps it away from the output stage so it never reaches the headphones.
  • Silent Volume Control – DC bias currents through a pot wiper are hard to avoid in a fully DC coupled design. And with bipolar input opamps, it’s usually audible like it is with the Mini3 and FiiO E9. When you change the volume with no signal you hear an obvious “rustling” noise in the headphones. That’s just tacky. It makes it sound like the pot is worn out when it’s just a marginal design that’s to blame.

2-5 LOCAL OR GLOBAL FEEDBACK? With a multi-stage amp you have to decide between global feedback, local feedback or a combination of both. To most easily meet the goals of stability and transient response local feedback was chosen. It has the following advantages:

  • Gain Stage Isolation – Reactive loads create phase shift between the output voltage and output current. Local feedback isolates the phase shift enhancing stability.
  • AC Coupling – Each stage needs DC feedback to operate. So an amp using global feedback must be fully DC coupled. That prohibits using AC coupling between the stages which has other advantages (see 2-3 and 2-4 above).
  • Volume Control – Global feedback generally requires the volume control be outside the feedback loop. This has significant noise consequences as discussed in 2-3 above.

2-6 DC SERVO? Does the design need a DC servo? Using a servo lets the amp be fully DC coupled without worrying about the offset voltage. But servos require power and this is a portable amp where battery life is important. Servos are generally more applicable to larger discrete power amplifiers where they can address compromises with the design of the feedback loop (different AC and DC gains). The O2 doesn’t have these problems to begin with. And the DC offset and frequency response of the O2 are  genuinely excellent without a servo. A servo would just drain the batteries faster with zero benefit, take up board space, make the amp more expensive and add complexity.

2-7 POWER SUPPLY VOLTAGE? Before going too far with selecting components (like op amps) it’s good to know the power supply voltage. To achieve 6 - 7 volts RMS output you need about 20 V peak-to-peak. Accounting for the appropriate voltage drops, a +/- 12 volt (24 volt total) power supply should work depending on diode drops, and how close the stages can swing to the rails under load. This is another area where some designers get carried away. Higher supply rails limit your choice of components and create much greater power dissipation (waste heat) in the output stage. Power is a function of the square of voltage. So as the power supply voltage goes up, the thermal losses are exponentially greater and the amplifier is less efficient. You want the voltage just high enough to get the job done and no higher.

2-8 INPUT CIRCUIT? The higher the input impedance the more stray noise pickup you get if the amp is connected to an un-terminated cable or gear that is powered off. And much less than 10K can excessively load the outputs of some DACs, preamps, etc. So 10K was chosen as optimal. An RC filter provides RF protection with a cutoff around 3 Mhz. This is low enough to filter out most RF energy while not creating phase shift in the audio band with even higher impedance sources. Cell phones operate at 800+ Mhz and are the most common source of RF problems. A good input circuit should also have some series resistance to help limit current into the op amp if it’s overloaded and provide greater ESD protection. But the larger the series resistor, the worse the noise performance of the amp so it’s a trade-off.

2-9 GAIN STAGE: It’s an IC-based design so the gain stage will be an op amp. The first choice is what topology? Inverting (shunt feedback) or non-inverting? The inverting stage has some benefits for common mode rejection but it’s at a disadvantage for noise performance which is critical in a OSFA headphone amp. A non-inverting design is also absolute phase correct keeping the purists happier (assuming the output stage is also non-inverting which it is).

2-10 WHICH OP AMP? With a topology defined, which op amp out of the hundreds available is best? The semiconductor websites are the best place to start, then all the datasheets, and ultimately you have to start testing the best candidates in the desired application. A given op amp won’t be specified at the exact gain, impedances, etc. of your circuit so the datasheet can only tell you so much. And testing can reveal other surprises. Doug Self, for example, found the expensive Analog Devices OP275 performed worse than several less expensive options. Nearly two dozen op amps were tested in developing the O2 and I have published two articles on the process:

2-11 VOLUME CONTROL: This is a critical part of a headphone amp and what’s mostly responsible for meeting the channel balance requirement. So it’s worth keeping a few things in mind:

  • Location - See 2-3 above for why the volume control is best positioned between the two stages rather than before the input/gain stage. The main downside to the “volume control in the middle” approach is it’s easier to overload the gain stage which always runs wide open. The O2 has a gain switch to address this problem but it’s something those altering the default gain settings should be aware of. See: Gain Stage Overload. A compromise is a “split gain” design where you divide up the gain between two sections but this requires the output stage, in a 2 stage amp, be capable of gain (many buffers are not) and usually requires additional components taking up board space.
  • No DC - The same high quality film capacitor that provides DC input protection in the O2 also isolates the bias current of the next stage from the volume control rendering it completely silent with no “rustling” noise when the volume is changed. If you put even the tiny DC input bias current of most op amps through a pot wiper—especially if it’s before the gain stage—you’ll get noise in your headphones when you adjust the volume. The Mini3 and FiiO E9 suffer this problem.
  • Channel Balance – All pots have some channel balance error. Due to the logarithmic nature of perceived volume, and the way voltage dividers work, the channel balance error will be greatest at the lowest volume settings. And it’s different for every pot. The dScope has a real time channel balance measurement so you can literally turn the pot and watch the number in dB change in real time. It’s not uncommon for the louder channel to swap back and forth as the pot is trying to average a 0 dB error over its range. The dScope makes it relatively easy to find the worst case imbalance and it’s nearly always in the first 5% of the range. The key is to intelligently set the gain so you avoid using the few few percent of the volume control’s range.
  • Stepped Attenuators – These are great when they’re well implemented. But beware of the eBay versions. All those tiny SMT resistors on a DACT? What are they exactly? Thick film SMT resistors perform poorly for audio. And even thin film SMTs get much worse in smaller sizes. They get more noisy, and more alarming, their voltage coefficient rises dramatically. That means they don’t follow Ohm’s law as well—their resistance literally varies with the voltage applied. That creates distortion as the voltage is constantly changing in an audio amp. And the switch contacts, wipers, etc. in a cheap stepped attenuator are prone to wearing out as they get a lot of use. They’re rather expensive to make properly. So, without detailed measurements with an audio analyzer, I wouldn’t trust an eBay (or other unknown) stepped attenuator.
  • Electronic Pots – Most electronic pots are not well suited for high-end audio. Many run from 3V or 5V single power supplies and the audio signal has to stay in that range. So they have to be capacitor coupled and the audio signal has to be referenced to the midpoint of their power supply. That’s a big problem in a high quality amp. They’re also, at best, limited to about 1.7 V RMS before they severely clip the signal. There are some higher voltage chips and even some with bipolar power supplies but they’re expensive (around $10 each). In most cases you need a microcontroller to run them but a few have pins for up/down buttons. Sometimes they’re built into chip amps (like the one in the FiiO E5 and E7).
  • Choices – Lower values are generally better to reduce the Johnson Noise. But if it’s in the input stage, you have to consider loading on the source gear. I think so many use the Alps RK097 because it's hard to find anything better without spending a lot more. Noble, Panasonic and Bourns makes some similar pots but they’re not better that I know of. There are some interesting motorized options as that’s more typically what the high-end manufactures want. The Alps RK27 "Blue Velvet" pot is a really nice pot. But it's currently only stocked in a 100K value which is way too high for most headphone applications (except tube amps) and it wouldn't fit on the board. There are also cheap RK27 clones around made in China.
  • Taper - Alps makes two slightly different audio tapers for volume control use “3B” and “15A”. The 3B taper is at 50% (-6 dB) at half volume. The 15A taper has a more gentle taper up to 70% and then it becomes more steep. If you listen mostly at lower volumes the 15A taper is likely the better choice. If you listen often close to full volume, the 3B might be better.

2-12 OUTPUT BUFFER CHIP? I considered a lot of options for the output stage. A dedicated high current buffer seems like an obvious choice and here are the main options:

  • National LME49600 - This part performs very well. But it’s only available in a surface mount package, it’s relatively expensive, and worst of all, it’s too power hungry for a battery operated design. And, because it has no gain, it doesn’t work with the chosen 2-stage topology here without giving up several other things (especially noise performance and potentially stability). See the BUF634 below for more.
  • Linear Tech LT1010 – This is a fairly old part and only is rated for 150 mA of current which is below the 166 mA spec. It also has relatively high quiescent current and several specs are not as good as the 49600 above. It suffers the same topology issue as the 49600.
  • TI BUF634 – This is available in a TO220-5 through-hole package, and it has a lower power mode, but they’re over $12 each at Mouser and there’s still no voltage gain so you can’t use local feedback. Running it without feedback creates excess distortion. And using global feedback creates stability issues and requires a DC coupled design with the volume control at the input rather than between the stages. That creates DC offset issues—especially at higher gains—and far more noise for all the reasons already mentioned. So to properly use the BUF634 in this design, you need a third op amp stage to provide the feedback for the buffer. And if you use global feedback you may need a DC servo. And either one would help kill the battery life. Not to mention there’s not enough space on the small PCB or either option. Plus, just the required pair of BUF634s would cost more than all the other parts on the board combined.
  • TI TPA6120 – This has been a popular part I suspect partly because it’s much cheaper per channel than the above 3 choices. But as I documented in my FiiO E9 and DIY QRV09 reviews, TI specifies at least a 10 ohm output impedance and it has seriously high distortion at low frequencies. Plus it’s nearly impossible to properly solder (it’s surface mount with a hidden heat sink pad). It’s also power hungry with high quiescent current. It can, however, be configured for voltage gain and local feedback.

2-13 OUTPUT STAGE CHIP AMP? I looked at several “chip amps” designed to drive headphones and I didn’t find any that met the requirements. Most are designed for low voltage operation from around 3 – 5 volts. I also looked at “chip amps” made to drive speakers as they have higher voltage capability. But the only ones I found with suitably low quiescent current for battery operation have relatively poor performance specifications as they’re designed for lo-fi portable audio gear. So much for chip amps.

2-14 OUTPUT STAGE OP AMP? By process of elimination (discrete designs were ruled out earlier) the choice is down to an op amp. But which one? I scoured all the semiconductor websites looking for high current output, low distortion, through hole packaged, 24+ volt power, low distortion, op amps, and came up with only a few parts that even came close to meeting most of the criteria. Here’s the breakdown:

  • Analog Devices AD8397 – The 8397 is is used in some “fashionable” headphone amps like the new FiiO E11 and the AMB Mini3. The good news is it can manage around 300 mA of peak current which is impressive. It will also (barely) run from a +/- 12 volt supply to meet the voltage swing requirement. The bad news is it’s surface mount only which makes it impossible to directly socket and opens up lots of DIY issues. The AD8397 can be pre-mounted to a SOIC-to-DIP8 adapter and used that way but that makes an already expensive ($6.50) op amp more expensive and decreases its already marginal stability. Because this op amp is much faster than it needs to be for audio use it’s far harder to properly stabilize. AMB documented having problems with it and being unable to get it to work at supply voltages higher than the +/- 4.5 volts in the Mini3. There are quite a few stern warnings in the datasheet. It’s also not short circuit protected and known for blowing up if it’s even slightly abused. So it needs some form of protection—usually series output resistance which compromises the performance. It’s also rather dissipation challenged in this application. Both channels in a single SOIC8 package with peak currents of 166 mA and a 24 volt power supply will far exceed the dissipation limits. So it’s off the list.
  • TI Burr Brown OPA551/2 – These single op amps were my first choice as they look good on paper, are rated for 200 mA of current, have high open loop gain, and even some cool features like thermal shutdown. In testing, however, they proved to be something like the AD8397—not quite as bad but still high strung. Admittedly my prototype board wasn’t optimal but the OPA551 didn’t like reactive loads without using output inductors or substantial series resistance. You can get clever with the feedback to presumably compensate for capacitive loads but it kind of works against the OSFA mantra as it’s hard to optimize it for all possible loads without using an output inductor. The very high current limit of 380 mA is also less than optimal. Finally, they’re relatively expensive at $5 each and you need two of them.
  • TI TLE2062 – This is an interesting part that’s specified into loads as low as 100 ohms—headphone territory. The quiescent current is less than 0.5 mA per amplifier section which is great for battery operation. Amazingly, despite the lower power, the slew rate is more than fast enough. It’s also rated at 80 mA max which is far higher than most op amps and it’s short circuit protected. But the unity gain bandwidth is less than 2 Mhz which is less than optimal. Likewise the CMRR and open loop gain could be better. The open loop output impedance is also rather high which, in real world use, limits the voltage swing into low impedance loads. Still, this part has some promise as I’ll discuss later.
  • JRC NJM4556 – JRC is an interesting company with its roots in audio and analog. Unlike the big US companies like National, TI, etc. they tend to either offer lower cost versions of existing designs, or they design application-specific analog parts. I suspect the NJM4556 is one of the latter as it’s unique and almost made to order as a headphone op amp. It’s rated at 70 mA of current specified into 150 ohm loads. It was designed for audio use. In the eBay Cmoy it managed around 100 mA peak and overall rather impressive performance. In the same prototype setup as the OPA551 the NJM4556 performed significantly better in several tests and was much more stable. The NJM4556 is the optimal speed for audio use and no faster. This makes it very stable without any fussy special requirements, output inductors, or series resistance required. That’s worth a lot in this application. The one remaining problem is the output current. The 4556’s 70 – 100 mA obviously falls short of the 166 mA requirement. But, the 4556 is a dual op amp with two well matched amps on a single silicon substrate (die). What will it do with both amps in parallel to double the current capability? The answer is, with appropriate measures, it does good things! It produces over 200 mA which approaches the 250 mA of even the LME49066 and BUF634 above. It turns out using one 4556 with paralleled sections for each channel meets all the requirements. I’ve also tried to blow one up with brief short circuits playing music at clipping and so far so good. And you can buy 7 of them for the price of one OPA551. To my knowledge, this is the first time the 4556 has been paralleled for headphone duty and it’s a big reason the O2 can deliver great performance at such a low price.

2-15 OUTPUT STAGE DESIGN – With the 4556 specified, the rest of the output stage needs to be optimized. First, you usually can’t simply connect two op amp outputs in parallel. They may not share the current equally and any difference in offset voltage will significantly increase the quiescent current. Measurements with the dScope demonstrated just 1 ohm of series resistance works nicely with half a dozen different 4556 samples (some from different production lots). These are effectively in parallel so the output impedance is approximately 0.5 ohms which is well under the 2 ohm goal.

2-16 CURRENT LIMITING – It’s important to protect the output stage from at least brief short circuits and current limiting can help a lot. But intelligent current limiting can also help protect the headphones in a OSFA design like the O2. The FiiO E9, for example, can put out over 1 watt into 16 ohms. That’s enough to send many headphones up in smoke and much more than any 16 ohm headphone I’m aware of needs. FiiO’s solution was to toss extra resistors in series with the 3.5mm jack resulting in serious frequency response and damping problems with balanced armature IEMs. A much better solution is active current limiting. This is discussed more in the first O2 article. The end result is the O2 still has more more power than any headphone I know of in the 16 – 32 ohm range should ever need. But it’s only about 1/3 as much as the E9 puts out at 16 ohms which should help save expensive headphones if someone gets careless with the volume, plugs something in with it cranked up, etc. And it does it without 43 ohms (FiiO E9) or even 10 ohms (QRV09) of output resistance. Active current limiting is the solution that makes the most sense. Why don’t more amps use it?

2-17 STABILITY (one myth confirmed) – Audiophiles claim at least some output inductors sound bad and there just might be something to that. I tried at least half a dozen different inductors in the output of the QRV09 trying to get rid of the datasheet mandated 10 ohm resistor for the TPA6120 and it didn’t go well. All the ferrite inductors significantly increased distortion. And trying to use an air core inductor, on that very fast amp, caused instability. Output inductors are common on power amps driving speakers but seem a bit more problematic in this application. Some of them also are relatively large in terms of PC board real estate. For these reasons I wanted the O2 stable into any reasonable load without inductors. The two stage design with local feedback goes a long way towards achieving that by isolating reactive loads from the rest of the amp. Combine that with an output stage that’s not’s better suited as a video amp, the right PC board layout, proper power supply decoupling, what’s described in the next paragraph, and you have a nicely stable amp.

2-18 COMPENSATION - Op amps get complex, literally, when you near the end of their useful bandwidth. There’s complex math involved, complete with imaginary numbers, to calculate their behavior, including things like phase margin in a given circuit. To do it right, you have to take into account stray parasitic capacitance, stray inductance, and more. Those things are difficult to model in calculations or simulations. Despite the fact the O2 uses internally compensated op amps you still have to verify their stability and transient response. In a low gain application, an op amp might still show ringing on square waves due to the phase margin becoming small. The generally preferred solution is to apply additional compensation in the feedback loop. This, in addition to the dominant pole compensation, is used to optimize the transient response and stability. Amplifier circuits that exhibit significant ringing often have dangerously low phase margin. In my experience this can have a negative impact on their sound quality although some have perceived the “different” sound of a ringing amplifier as somehow better. See Op Amp Myths and Op Amp Measurements for more details. The compensation in the O2 was optimized using a very fast ( < 20 nS) rise time square wave and a fast (> 50 Mhz) scope. The resulting –3 dB point of the O2 is around 250 Khz and the phase shift at 10 Khz is less than 1 degree. Tthe slew rate is still in excess of 3 V/uS. Why does anyone need more bandwidth and “speed” for an audio amp? Would you rather it starts oscillating at 500 Khz when your friend plugs his $1500 HD800s into it? Output stages don’t like being RF transmitters and get really hot trying. The heat ultimately makes them fail and the resulting DC could easily destroy the HD800s. Your friend will hate you and your marginally stable amp. Half-baked stability is never good.

private star ground2-19 GROUND TOPOLOGY: Anyone who’s read my Mini3 review, Cmoy review and/or my Virtual Ground article knows I strongly prefer a proper bipolar power supply with a real ground. And it needs to be a star ground as shown at the right where each functional area of the amplifier has its own private path to a single central ground reference. A true zero volt referenced bipolar dual power supply almost always works best in a headphone amp.

2-20 BATTERY POWER SUPPLY: The O2 only needs less than 200 mA DC clipping a sine wave in both channels into 15 ohms. In real world use with music the current is under 60 mA DC total under even difficult conditions. Just considering battery operation for the moment there are several choices:

  • Bipolar DC-DC Converter – This allows using a relatively low voltage battery like a single 3.7 V li-Ion cell, pair of AAs, etc. A DC-DC converter generates dual bipolar supply rails at the desired voltage (+/- 12 volts for the O2). If the battery becomes too low the converter shuts down and the amp shuts off gracefully. But DC-DC converters are expensive and are typically only 50% - 80% efficient when not fully loaded so 20+% of the battery capacity is wasted. They also create substantial electrical and magnetic noise (EMI) that will find its way into the audio circuitry no matter how hard you try to keep it out. You can see an example of this noise on the blue square wave at the end of the FiiO E7 review.
  • Single DC-DC Converter Or Charge Pump – This is a lopsided version of the above solution. Only the negative rail is generated and the battery is used “raw” for the positive rail. For the O2 that would mean at least a 9 volt battery. The FiiO E5 and E7 use a single charge pump built into the output chip amp. The supply has asymmetrical impedances which can degrade performance. So you would be lucky to get 3 hours instead of 7 – 9 hours. Battery life would be rather poor with only 50% of the total battery capacity (watt/hours) plus the losses in the converter. And it may have unpredictable behavior when the battery gets low. The plus is the amp could be smaller.
  • Dual Batteries – A battery is quieter than any power supply. Which helps explain megabuck battery powered phono preamps like the Nova Phonomena. Two batteries are also 100% efficient—all the battery power goes to the amplifier rather having some wasted in a power converter or even more wasted in a virtual ground/third channel. The main downside is the cost and space of two batteries and, more important, possible headphone damage if one battery becomes disconnected or dies first. This issue is addressed in 2-23 below.

2-21 AC POWER SUPPLY: There are several options for the AC power as well:

  • DC Trickle Charge - A cheap solution is to always run the amp from the batteries and simply trickle charge them with a 24 volt DC wall adapter. The batteries would equally divide the DC charge current between them neatly solving the problem of needing two DC supplies from an AC source. The downside is the batteries are required even for AC operation making this a poor solution for a full time desktop amp. And, worse, the power drain exceeds the maximum continuous “float” charge rate for 9 volt Ni-MH batteries so the AC adapter would extend the battery life but the amp would still eventually die playing even while plugged in. Or the current would have to be set high enough to eventually “fry” the batteries if the AC was left connected with the amp turned off. Neither is acceptable.
  • Center Tapped or Dual Transformer – A conventional bipolar power supply is made using either a center tapped (3 wire), dual secondary (4 wire), or two independent AC transformers (like the QRV09). So far so good. The problem is wall transformers with a 3 or 4 wire AC output are extremely rare. So you have to build the power supply from scratch which involves working with hazardous AC line voltages. The supply would also need to be built into a separate enclosure (expensive) or the O2 itself greatly increasing the size for a portable amp. 
  • Dual DC Power Adapter – I’m not aware of any wall adapters with suitable dual DC outputs, but there are table top “brick” power supplies (like most laptops use) with dual outputs. The problem is the decent ones are relatively expensive (around $40+) and are overkill for this application. They’re also switching power supplies intended for digital devices and typically have some noise in their outputs which probably will find its way into the audio circuitry.
  • Single AC/AC Wall Adapter - An inexpensive AC/AC two wire wall transformer can provide a true split supply using half-wave rectification. While half-wave might seem less than ideal, the performance is in the implementation and the advantages are numerous. Wall transformers are already safety agency approved and available for local power/plugs in various countries. They come with standard barrel connectors allowing the use of a small inexpensive power jack on the amp. The amp can operate perfectly without the batteries installed. There’s no high frequency switching “hash” to worry about. All that makes an AC wall transformer the best choice for the O2. Anyone doubting the performance of a half-wave power supply should check out the performance results in the first O2 article including the incredible noise performance. All those measurements (marked “AC”) were using this power supply. It works great!

2-22 BATTERY CHARGING: Again, there are more options:

  • Charge Controller IC – These typically use multi-stage charging with a faster rate until the battery is close to full then switch to a trickle charge. They’re great but the only options for a dual 9 volt bipolar battery set up are expensive and not DIY friendly (surface mount).
  • Constant Current IC – This solution is used in the Mini3. But, it turns out, it’s actually a disadvantage for 9 volt Ni-MH batteries. See the next option.
  • Resistive Taper Charging – The battery specs say you can charge a NiMH battery at 1/20 the battery capacity indefinitely. Higher than that and you’ll cook the battery, and much lower will take forever to charge. That limit is about 10 mA for the O2’s batteries. A resistor can be sized setting the charge current well under that (about 6 - 7 mA) when they’re fully charged which helps prolong the life of the batteries when the amp is left plugged in. But, even better, the charge current in the O2 is proportional to the battery voltage. This speeds charging compared to a constant current source. The maximum current is around 50 mA if the battery is completely dead which is still a safe value. So no constant current limiting IC is required. It’s also dirt cheap and takes up very little board space.

2-23 POWER MANAGEMENT: As mentioned under 2-20, dual batteries present a significant risk. The problem of potential headphone damage has to be addressed. I developed a unique circuit that manages the DC power rails of the amplifier and offers some additional benefits besides DC protection. I’m not aware of anything similar in other headphone amp designs. It serves five purposes:

  • Headphone Low Battery Protection – The circuit shuts the amp down long before the amp can become unstable due to one battery dying before the other. So there’s never any DC at the output.
  • Battery Disconnect Protection – If one battery becomes dislodged the amp will immediately shut down preventing a large DC output. In my development adventures this is a more likely scenario than low batteries. There’s also the chance one battery is not fully connected when you turn on the amp. The circuit prevents the amp from turning on unless it’s safe to do so.
  • Power On Transient Suppression – Controlling the timing of the power rails greatly reduces turn on transients. The power circuit reduces the turn on “click” by approximately a factor of ten.
  • Cell Reversal Protection - 9 volt batteries, with 7 cells, are easily damaged by what’s known as cell reversal if they are discharged much below 1 volt per cell. Without protection the O2 (or a Cmoy) will operate down to battery voltages of under 3 volts. The listener may have no idea the batteries were being over-discharged and permanently harmed. The power management circuit shuts the amp down when the batteries drop much below 1 volt per cell (7 volts each). So users are free to just listen until the amp shuts off. No harm. No foul.
  • Hysteresis – When the amp shuts off due to low batteries the battery voltage will rise because most of the load is removed. A simple circuit would just turn the amp right back on again. And you end up with the O2 doing DJ rap special effects with your tunes. So the circuit is designed to keep the amp off when the batteries are low.

2-24 LOW POWER VERSION: A lot of portable music players these days will play 20 – 40 hours on a battery charge. So one of the O2’s requirements is 20+ hour battery life in a low power version so you can fly from LA to Sydney with ease on a single charge even with a multi-hour layover along the way. Those who want to use the amp on the go, or where there’s no AC power handy, will probably be happier with the low power version. See: Low Power Option

2-25 ENCLOSURE: With most of the amplifier design roughed out, how small can the enclosure be? There are not that many high quality low cost enclosures on the market—especially if you want a metal one. The best I could find was the “BEX Series” by Box Enclosures and the B2-080 was the smallest one that would conceivably hold two 9 volt batteries and the 80-ish components on the O2’s circuit board. The B2-080 is a high quality rigid extruded aluminum case that you could probably drive a car over and it would survive. Plus it’s under $11 with front and rear panels and hardware. It has slots to slip the PC board into so no mounting hardware or modifications are required. There’s also the slightly taller B3-080 that can accommodate the same PC board but has more height to allow extra panel mounted jacks such as a 1/4” headphone jack, RCA input jacks, etc.

2-26 FRONT AND REAR PANELS: While it’s temping to put external components at both the front and back of the amplifier that requires using two of the most expensive single component on the entire Bill of Materials—the custom machined panel. That makes a big difference in the total cost. The B2-080 and B3-080 come with both front and rear panels so if the rear panel is blank, it’s free. So all the controls and jacks are arranged along one side of the PC board. That way only one machined panel is needed (it’s under $17 from Front Panel Express) instead of two. It also allows using deeper custom enclosures without having to worry about anything matching up with the back of the enclosure.

2-27 PC BOARD: The enclosure mandates the PC board be no larger than 100mm x 80mm. This is where things get really, um, “fun”. Figuring out if everything will fit can take many hours or sometimes even days of trying with the PCB layout software. And if it doesn’t fit, you’re forced to either start compromising your design and toss out parts, or you give in and move to a bigger enclosure, a bigger board, and more or less start over. The more cramped a PC board is the harder and more time consuming it is to place and route. For those who have never routed a board, nothing can cross over anything else that’s not supposed to be connected. So it’s like a 2D Rubik’s Cube puzzle to get all the signals where they need to go, especially when you take 2-29 into account.

2-28 CHANGES & ADDITIONS: Just like in software development, hardware is often a moving target. You might get the first prototype built and finally working right and then you or someone else discover it needs to be changed. For the O2 the suggestion was made to add a gain switch. But there wasn’t space. The solution was to get creative and switch to 1/8 watt sized resistors. There are 20-ish resistors in the O2 so that made room for the gain switch but required re-routing the entire board. It also created a new problem I would discover later in 2-31.

2-29 PC BOARD ROUTING: The O2 has nearly 80 components. Where they’re placed and how they’re interconnected is a seriously critical aspect of performance. While you can find lots of info on how to route a tricky HDMI digital video signal, or make a strip-line antenna feed for a WiFi module, there’s surprisingly little info on routing analog audio boards. Even Doug Self hasn’t published much on the topic. It’s much less black and white than most other aspects of audio design and, to be honest, it’s something of an art. It’s very much an acquired skill you can’t master overnight. There’s even an underground blotchy photocopied “manuscript” paper dating back to the 80’s that’s passed on from senior engineer to junior engineer like some kind of secret manual and right of passage. It’s all about the “black magic” of PCB design. I’m seriously not making this up. Here are just some of things that can make a big difference in measured audio performance:

  • shared ground pathGround Routing – As mentioned earlier a star ground is essential. The diagram to the right is what often happens but it’s not how to do it. That oval bit at the bottom is a typical “ground fill” island on a PC board. And within it critical ground currents overlap and interact with each other. That creates distortion and other problems. With a 2 layer board, and a design of any complexity, you will quickly run out of routing options if you try to return every single grounded component to a single point. So you have to know which things can share a ground return and that’s design specific. And you have to plan for anything “off board” that can destroy your best efforts. For example if someone uses non-isolated metal connectors, and panel mounts them,  those grounds you so carefully routed end up connected together elsewhere (via the panel) and you end up with ground loops and/or other serious problems. The Mini3, as another example, has exposed ground “strips” on both sides of the PCB in an attempt to ground the PC board to the enclosure but this creates two giant ground loops through the upper and lower halves of the metal case. An enclosure should always be grounded at a single point and never have ground currents flowing through it.
  • EMI Loops – Think about where the higher currents flow. You want to keep the “loop area” of those currents as small as possible. For example, I’ve seen several boards that more or less route a power supply rail up each edge of the board and the ground up the center. It’s very logical, neat and tidy. But it’s very poor practice for analog audio. All the currents flowing from the rails, through the load, and back to ground are spread wide apart creating a single turn coil or inductor with your entire circuit nestled right in the middle of each “loop” where all the resulting EMI fields are the worst. And even a 3 channel or bridged (balanced) amp doesn’t solve the problem because the return current is to the opposite rail which is still on the other side of the board. It takes some serious thought to imagine all the current paths and the loops they form. Then it’s even more of challenge, especially on a cramped board, to minimize those loops while still being able to route the entire PCB in 2 layers.
  • Inductive Currents – Don’t run high gain input signals parallel to anything with much current flowing. There will be inductive coupling that can (depending on the signals) significantly increase distortion, degrade crosstalk, and worst of all, create instability if it’s out of phase with whatever it’s next to or generates positive feedback to an amp input. Doug Self has examples of 100 times greater distortion and I’ve seen similar results from various half-baked designs.
  • Watch Parasitics – If you dig deep into datasheets and/or application notes, especially for faster parts, you’ll often find some advice on keeping the part stable. One problem can be parasitic capacitance and inductance. Amplifiers hate positive feedback as it turns them into oscillators. Yet I see all sorts of questionable routing where outputs are coupled to positive inputs in ways they never should have been. You may not always want ground fills around the input pins of op amps, for example, as that creates parasitic capacitance that can degrade stability.
  • Use The Reference PCB Design – I know some engineers who work for semiconductor companies. It’s not uncommon for one or more engineers, more or less full time on the project, to spend months getting their reference designs correct. They might spin a half dozen iterations of the PC board and they’re making measurements with hundreds of thousands of dollars worth of instrumentation. Their reference design is essentially the company’s business card for that part. It’s the first thing many potential customers will lay their hands on and evaluate. It’s important it be as correct as reasonably possible so the part makes a good first impression and they get more design wins. Why then do so many DIYers and small commercial designers just ignore these carefully engineered reference designs and screw everything up thinking they somehow know better? This is also blatantly a problem with DIY DACs. Use the reference design as much as possible. If you really think you know better than the guys at National, Analog Devices, TI, Linear Tech, etc. you’re very likely wrong. I’d love to be in the same room when you try to make your case to the guys (or gals) who sweated all the tough details of their reference boards.
  • Forget Aesthetics – It’s apparently hard for some to resist but the optimal layout rarely has everything arranged in neat tidy rows and columns like a spreadsheet. Electrons have no concept of aesthetics but they don’t generally like going further than they need to because someone thought an IC or resistor looked better over there. See the next item.
  • Think Small – It’s tempting to think of copper PCB tracks as nearly “perfect” but they’re not. A four inch trace at a typical width of 15 mils from one side of the O2 to the other is around 0.13 ohms. That may not sound like much, but it can be a lot in some parts of the circuit. Traces also have inductance and capacitance and couple to both components and each other. A little math helps put this in perspective. For the distortion goal of 0.01% at a realistic listening level of 400 mV (my standard reference level) that distortion can be reached with just a single unwanted anything of only 40 microVolts (0.00004 volts or 40 uV). It doesn’t take a lot of ignorance to end up with an extra 40+ uV here and there from a sloppy layout. And before you know it, even using the best op amps and parts on the planet, your design won’t produce anything resembling really low distortion. Moving a single track can change the distortion from 0.005% (-86 dB) to 0.1% (a lousy –60 dB). Really! PCB design is way more important than many realize.
  • Ground Planes vs Floods/Fills – A ground plane, by definition, is uninterrupted. The idea is ground currents will follow the shortest path back to their source. And that generally keeps loop areas very “tight” which is what you want. But if the ground area is interrupted, it’s no longer a plane, and the ground currents likely will have to take long paths around obstacles that interrupt the “plane”. It’s like walking anywhere you want on a football field, versus breaking the same field up into a bunch of little islands surrounded by water with only bridges here and there to get across. It’s entirely different. And it rarely works very well. The random divided areas are more accurately called “floods” or “fills” and they’re what you get when you fill unused areas on the PC board with copper after you have routed all your traces. If you want a real ground plane use a 4 layer board (which are much more expensive—especially in small quantities) and don’t route any signals on the ground plane. Otherwise you have to carefully plan for every ground current and use star grounding practices—floods or not.
  • Never Use Auto-Routers – A low speed digital design will typically at least work if you simply route the power and ground tracks (or set up suitable design rules for them) and turn the software auto-router loose on everything else. But it’s suicide for high quality analog audio work. The auto-router has no concept of what makes an op amp oscillate as just one example.

2-30 COST ENGINEERING: This step is sort of sprinkled throughout the process. But in choosing everything from the topology to the components if you’re designing to a budget you have to keep an eye on the pricing. Nothing is really expensive in this design but with 80 parts it does add up. The most expensive items, especially in low quantities, are the PC board, enclosure, front panel, rechargeable batteries, and the AC wall adapter.

2-31 SPECIFYING COMPONENTS: This gets surprisingly complex if you’re trying to keep the costs down and not compromise the design. Some things to consider:

  • Try To One Stop Shop – Shipping costs can really add up. If you’re forced to buy parts from 5 distributors that’s probably $50+ in total shipping costs right there. The goal here was to get as much as possible from one source and I managed to do get everything that’s on the PC board (which is a complete working amplifier) and the AC wall adapter from Mouser. You can even get the batteries, solder, and tools from them if you want.
  • Try To Combine Values – I won’t name any names but I’ve seen DIY designs with a 1K resistor in series with the LED and a 1.1K resistor in the feedback loop of an op amp and no other 1K resistors in the entire design. Why not use 1.1K for the LED? It will be ever so slightly dimmer. Big deal. There are lots of examples like this where I see similar, but different, non-critical values being specified. It just makes the design more complicated, more likely something won’t be in stock, etc. In the commercial world it also usually costs more money.
  • Look Up Similar Values – As explained earlier, there was a need to switch to mostly 1/8 watt sized resistors due to a lack of space. A few resistors in the O2 are especially critical for noise. But when I tried to find low noise resistors in that size I discovered Mouser only had limited values. And the prices for the best mil-spec ones were $0.30 or $1.19 (same resistor, slightly different resistance value). Often what matters in audio is the two channels be well matched (hence the use of 1% resistors) but the absolute value isn’t that critical. Nobody cares if the input impedance is 10K or 10.1K for example. Mouser might have a big customer that uses huge quantities of the 10.1K value so they can offer a much better price on that value. So there are a few weird looking resistor values in the O2 and that’s why. Same part, way cheaper, no difference in performance. This also happens with many other passive parts like capacitors.
  • Stick To The Big Companies – Distributors like Mouser don’t sell junk like you’ll find on eBay and at smaller outfits like Jameco, etc. They sell what are known as franchised lines which means they’re a top tier distributor for those parts with full factory support from the component manufactures. Most of their customers are making hundreds or thousands of units at a time. And the last thing a distributor wants is for a customer to have to re-work 1000 boards because of some substandard part. Avoid no-name components, surplus, small dealers, and eBay parts if at all possible. Always remember eBay is often used to liquidate reject merchandise. 
  • Passive Components Can Matter – The issue here isn’t so much the brand, but the details. Thick film SMT resistors for example are awful for audio use. Always use thin film. Metal film through hole resistors work much better than carbon film. The ESR of power supply capacitors can vary by a factor of 10X for the same value capacitor but you have to download the datasheets and look it up. Other capacitors can have very significant differences. Doug Self and others have written extensively on this topic. You can’t just use any parts and expect similar performance.
  • Avoid eBay Components – A lot of the components being sold on eBay are rejects that can’t be sold elsewhere. Big companies do incoming inspection on parts, and if they fail, they’re sent back. In China many of the parts that weren’t good enough to even use in kids toys end up on eBay. And many of the audiophile components are fake knock offs. That Alps Blue Velvet pot from a Chinese or Hong Kong vendor? It’s probably not made by Alps.
  • Try To Avoid Single Source Items – Lead times on many parts are horrific right now. Some parts have 52 week lead times. So, more than ever, single source parts may become an impossible roadblock. Sometimes you can’t avoid it but at least try to look for two versions of a component that can work. Like the B2-080 or B3-080 from Box Enclosures both work for the O2. Plus each comes in several colors. So it’s unlikely they’ll all be out of stock even if they are made by only one manufacture.


Prototypes and Testing

nwavguy o2 pcb enclosuresSTEP 3 PROTOTYPES: With the above 30+ steps you hopefully have something resembling an initial design. The next step is to turn it into reality!

3-1: SIMULATION: I have multiple simulation tools including some with four figure price tags. All of them are ultimately based on SPICE which dates back to at least the early 80’s if not the 70’s. It’s basically from an era when a computer the size of a living room struggled to do in a day what an iPhone can do in a fraction of a second. Despite its very crude origins, computer simulation is still useful for some things. For example I used it in designing the power management circuit for the O2. But when you’re worried about things like –90 dB vs –70 dB of crosstalk, or 0.007% THD vs 0.07% THD, or 105 dB S/N or –95 dB S/N, simulation falls flat on its face. The reason is implementation. A lot of high-end performance is more about getting the power supply, grounding, PC board layout, decoupling, parasitics, etc. correct. And simulation glosses over most of those or it gets seriously complex trying to model all of them. It’s also really easy to leave things out or otherwise have flaws in the underlying models you’re not even aware of. Simulation can be a useful, but crude, tool for getting in the ballpark. But don’t pretend it represents real world performance.

3-2: DIGITAL VS ANALOG: If your design is say a PIC microprocessor that measures the room temp you just hack together a protoboard and test it out. Digital signals under 5 Mhz or so are fairly tolerant of sloppy implementation. But if it’s a piece of high-end audio gear things are different. You can’t build a headphone amp like the O2 on a piece of perfboard, protoboard, etc. and expect it to work anything like what it’s capable of. Unfortunately, the only way to properly test it is with a properly routed PC board. And that takes a lot of time to design. Getting the board made is usually at least $70+ with shipping and takes at least a week or more. If you want it faster, plan on more like $200.

3-3: ITERATIONS: All that work you put into the first PC board above is usually thrown out the window rather quickly after you build the first prototype. There are often enough changes after you test it you’ll have to significantly modify the design which usually makes all those painstakingly hand routed traces unusable. Sometimes, just for the prototype, you can get out the X-acto knife, cut traces, tack wires on, hang parts off the bottom of the board, etc. to patch things together temporarily. But, ultimately, you have to spin a whole new PCB.

3-4 MECHANICAL ENGINEERING: We electrical engineers sometimes have to put our mechanical engineering hat on. Everything that’s supposed to interface with the front panel has to be mounted on the PC board the correct distance from the edge of the board. If something sticks out too far, or not far enough, it won’t work in the enclosure. You also have to worry about parts interfering with each other, hitting protrusions inside the enclosure, or being too tall. And things like 3.5mm connectors, the power jack, Alps volume pot, power switch, gain switch, battery connections, right angle LED, etc.are unlikely to be in your PCB CAD software library. So their mechanical “footprints” must be defined from scratch. And if you get even one pin or hole wrong by a fraction of a millimeter, the component may not even fit in the board. All these things can also require multiple spins of the board to get correct.

STEP 4 TESTING: The test results for the O2 are shown in the first O2 article. Before those measurements were made, however, there were many similar measurements conducted unveiling problems that needed correcting. Lots of DIY designers, and even some commercial ones, get a design to where they think it sounds good and they “blindly” release it based mainly on their ears. But that’s often a serious mistake. Just ask Schiit Audio and the guy who designed the Half Baked DAC mentioned earlier. If designers run the sort of measurements you’ll find in the first O2 article, they will discover if their design has serious problems. NuForce has been caught with their pants around their ankles multiple times because they failed to make the right measurements. And the AMB MIni3 doesn’t come close to some of its performance claims. Many don’t seem to bother and/or lack the capability to make all the right measurements. Everyone should remember you can’t hear ultrasonic oscillation, headphone damaging DC, and other things that are serious problems and/or red flags. So just because you think something sounds fine doesn’t mean it is fine. And RMAA still leaves a lot of stones unturned.


Design Summary

BOTTOM LINE: I’ve tried to document what goes into designing and testing something like the O2. There are many more steps involved than have been covered here. But, hopefully, I’ve at least touched on the highlights. In the bigger picture a headphone amp like the O2 is relatively simple. A headphone DAC, for example, is considerably more involved and a remote controlled pre-amp/USB DAC like is much more involved still. Unless you’re in a position to know you can somehow do better than the experts, including the semiconductor companies and published audio designers (i.e. Self, Cordell, et al.) I would suggest following their proven solutions as closely as possible.


  1. Wow...I'm really at a loss for words...

    Once again, I'd like to thank you for spending your own time to bring sanity back to this hobby.

  2. Fantastic detail on the design process .... I knew it would be worth staying up late tonight! I feel as if Santa came down the chimney but instead of giving me my gifts, he just described in excruciating detail what I might be getting. Can't wait for the next instalment!

  3. Well done!
    Looking forward to next installment.


  4. Arg waiting for the rest , I'm dying :p

  5. You leave me speechless. Awesome.

  6. Impressive sir, lots of information as expected from you. I can see I wasn't wrong the first time I saw this blog and noticed I can learn a lot in here! keep working this hard.

  7. Incredible post! Your blog keeps getting better. Eagerly waiting for the next post.

    Surely your blog is among the 0.00001% top audio blogs ;)

  8. Looking forward to this, Guy. I'd love to build one, measure it with my AP, and write a review. Lemme know!

  9. Hey everyone. Thanks for all the kind words and encouragement. And to Tyll, as long as you give the O2 a fair shake (considering your employer and such), that would be great!

    And you might want to hit the refresh button periodically as I'm still making lots of changes to the article. I clicked the Publish button around midnight all bleary eyed and this is far from my best effort in terms of writing. Parts of it were/are pretty bad. Besides being tired last night I have a head cold and a bad case of "brain fog" so my journalistic skills are even worse than usual.

  10. This was an absolutely fascinating read, you just can't get information this concise anywhere else, and in such great detail! And as always it's very enjoyable and easy to read, even for a guy who's never used a soldering iron. This has been the highlight of my day, very well done indeed.

  11. With measurements like yours, I would like to see the O2 compared to something much farther up the food chain than the Mini3 and the FiiO. I know you're annoyed at AMB for banning you from his forum, but I don't think it's enough to say your amp beats out budget amps like the Mini or the FiiO. I know you think that the integrated amp bundled with the Benchmark DAC sounds good, but in the years the Benchmark has been out, there must be a reason why you don't read much, if anything, about people using that built in amp.

  12. I haven't tested the AMB M^3 or Beta22 but I'm fairly confident the O2 will beat every AMB design with the criteria I've established--i.e. what matters most for real world headphones. And that's despite the fact they cost more than the O2.

    The Benchmark's headphone output has received quite a few positive reviews including from publications like Stereophile, The Absolute Sound, etc. The big magazines, obviously, have more on the line than some semi-anonymous person on a forum somewhere. Stereophile lists the DAC1 as a Class A recommended component under their Headphone section. So I'm not sure I agree with your assessment.

    But, if someone doesn't like the DAC1, is there any consensus as to popular headphone amps that are "higher up the foodchain"? What are some other good (pardon the pun) Benchmarks? I'm happy to compare the O2 with just about anything in ways that are realistic for driving real world headphones.

    Ultimately, if someone knows what they're doing and isn't cost constrained, it's not that hard to beat the O2 objectively. I can do it myself. But the question is: Would such an amp produce better sound? And, from everything I know for 99% of the headphones out there, it wouldn't. The big thing with the O2 is achieving that level performance at a very reasonable price.

    The main idea here is an amp that's well beyond the point of diminishing returns on a tight budget. Imagine a $15,000 Hyundai posting the same lap time on a track as a $300,000 Ferrari. Some might still want the Ferrari for the looks, badge, status, snob appeal, etc. But everyone else would be delighted to have genuine Ferrari performance at a tiny fraction of the price. That's what the O2 is all about.

  13. Amazing! I'm in my third year working towards an EE degree and a confirmed headphone freak. Until I stumbled on your blog, I was frustrated with most of what I've found on the web.

    I was starting to wonder if the simple act of putting on headphones destroyed your brain and made you susceptible to headphone propaganda? How else do you explain all the idiots posting on headfi? I might not be some gray haired engineer but I know enough to call out most posts on headfi as total BS.

    I can't wait for the schematic and PCB to build my own O2! Keep fighting the good battle. And don't worry about the clueless drug addicts on headfi. They're obviously beyond all hope.

  14. Don't worry about your prose, Guy. There's no distortion, just a little noise here and there. PS Don't liken this to a Hyundai. I think it's more of a Lamborghini. They started out making industrial pumps and such before going up against the established 'big boys' including Ferrari with a product specifically designed to deliver great real-world performance.

  15. The O2 effectively removes the need to ever look at an amp again. It drives every headphone in existence to live levels, and it does so without imparting it's sound on- or degrading the signal. It doesn't really matter if another amp does it even better - it's impossible to tell the difference. That's the whole point, audiophiles believe that more expensive = better, this proves them wrong.

    I'd love to see Tyll review the O2 but I wonder how it'd read. "It plays loud, REAL loud... But for the life of me, I can't hear it!"

  16. Anonymous ... maybe there are certain low-level EM fields encoded in some music that affect the brain directly when using headphones. Like this .....http://en.wikipedia.org/wiki/God_helmet
    Presumably Head-Fi sponsors are paying the RIAA to insert these signals.

  17. Interesting and entertaining as always.

    I think I will take that guy's advice in the last article's comments and stop wasting my time on Head-Fi.

    I'll read all your articles instead, even if I can't possibly understand it all. My head hurts just from reading. >_<


  18. To continue a bit:

    "I'm fairly confident the O2 will beat every AMB design with the criteria I've established--i.e. what matters most for real world headphones.... What are some other good (pardon the pun) Benchmarks? "

    Well, if it will sound better than the M^3 and the Beta22 it's pretty darn good.

    Now your talking !!!!!!!!!!!!!!!!! (big Grin)

    Other "Benchmarks"... hummm, you might consider the Gilmore designs.... dynalo, dynahi. They have been standards for a long time, and they are known quantities like the M^3 and Beta.

  19. At the risk of sounding redundant... congrats on another great blog. I wish more "high end" audio designers used your design approach, but it is apparent there a lot that simply don't bother with a number of steps. (Thanks on the cliffhanger on the output stage btw... not nice at all ;) ).

  20. Thanks for all the added positive comments. As for other "benchmarks" I'd prefer something commercial, in part, to avoid what happened with the Mini3. Several of my critics, to this day, seem convinced the Mini3 I tested was built by an idiot out of parts recycled from a toaster oven. A commercial product helps remove the whole "yours was a poor example" argument.

    Surely there must be a few well respected commercial headphone amps that are less expensive than the DAC1?

  21. There's also the AKG K1000 which I forgot about. 120 ohm impedance and 74dB/mW sensitivity. If the calculator that came up on google is accurate, 7.3V will only get you 100dB.

    That's one halfway popular non 'stat the O2 won't quite drive.

    Of course that one was pretty much designed to be used with speaker amps from the beginning...

  22. Also, there were a few commercial versions of those Gilmore designs like the HeadAmp Gilmore Lite but that one is out of production but still regularly sold on the used market. HeadAmp also currently makes a few modified (and more expensive) versions of that basic design called the GS1 (I don't remember how this one different) and GSX (which is "balanced").

  23. Yeah, the K1000 is both long out of production and they're not really headphones in the regular sense but "ear speakers". I did get to hear a pair once and they're rather interesting. They're so different than anything else they're hard to describe.

  24. Everything about this is great!

    When do you think the PCB design will be available? I just completed my first PCB, so I'm especially interested in seeing your layout.

  25. To Continue:

    "As for other "benchmarks" I'd prefer something commercial, in part, to avoid what happened with the Mini3."

    That’s a tough one. If you’re going to rule out all AMB’s non commercially produced DIY designs, the only thing we’re left with is one of HeadAmp’s Gilmore designs.

    So, if I had to pick a commercial amp that is still in production to go up against the O2, it would have to be the Gilmore designed GS-1 otherwise known as the masses as the dynalo.

    Maybe some of the others can chime in with other commercial "Benchmark" level amps that are around in similar numbers to the Gilmore and AMB designs, to compare the O2 with, but I’m drawing a blank …………

  26. Thanks Dylan. The next revision of the PC board is due in the week of August 8th. I'll have to build a new amp on that board, test it, and if all goes well I'll publish the gerber and drill files. Before then, however, I plan to publish the schematic, a software generated image of the PCB, bill of materials, costs, etc.

    It's a lot to wade through but there are more details in the comments to the first O2 article. I also just added some new square scope shots to the measurements there.

    And thanks Maverick. The GS-1 would make an interesting comparison at $900. I can accuse Gilmore of overkill but the designs I've seen of his appear to be relatively solid--especially compared to a lot of his competition. The GS-1 even has roughly the same default gains I have spec'd for the O2. The question is how do I get my hands on one, or how do we get someone who's genuinely impartial with a serious audio analyzer to test both?

  27. If you can best a GS-1, you really have something !!!!!

  28. Even if they sounded the same, it's not going to hurt anyone because the GS-1 is going to be discontinued anyway.

  29. While outperforming the GS-1 on a test bench should earn the O2 some added respect, I'm sure many will still claim the GS-1 somehow sounds a lot better. So to complete the challenge, it would be great to sit a few well known members of the "community" down with the O2, GS-1, an ABX box, their favorite cans/source/music, and see how they do?

    Unless it turns out the GS-1 has audible flaws (but they would show up on the test bench) my prediction is you won't be able to tell the amps apart.

    And for what it's worth, I think the GS-1 appears to be very well made, has a better volume control than the O2, switchable inputs, great aesthetics, etc. For what it is, and how it's made, the price isn't unreasonable.

  30. The cynic in me says you should try benchmarking the O2 against a commercial headphone amp from Ray Samuels Audio, but I have a feeling I know which would measure better ;) Don't forget to don your flame resistant suit before posting the results :)

  31. How about a Burson Audio HA-160 amp as a comparison? The $699 USD price seems fair to go head-to-head with the O2 contender, and with its fully discrete circuitry would make an interesting "battle of the amp design philosophies".

  32. Which has more credibility among the serious headphone enthusiasts? Burson, GS-1 or something else that hasn't been discussed yet?

    It needs to be an accurate amp with low noise, flat frequency response, etc. If it has any obvious flaws it will lose on the test bench but many will argue that doesn't matter and the challenger still sounds better. And because of the flaws, it probably will stand out in a blind test as well and the fans will still be able to say it sounds better.

  33. The Gilmore is a very clean amp. It's been around for a long time. The Dynahi was considered "the" reference amp in its day. I'd go with the GS-1.

    I don't know about the Burson, it just got here.... who's design is it anyway?

    If we're staying away from DIY, more people have had and respect Gilmore designed commercial amps than any other amp.

  34. Speaking of Ray Samuels amps, he had a dual battery opamp portable, the SR71, that was pretty highly regarded... as a portable.

  35. What about the ALO Audio Rx MK2 or the Headamp Pico amp? Both of those have some good looking technical specs (according to the websites) and are both portable. However, they are severely underpowered compared to the O2 (2.63Vrms and 2.8Vrms, respectively)

  36. I'm so far favoring the GS-1. You might as well aim high right? The hard part is getting one and especially arranging a credible blind test or two.

  37. IMHO, ALO is snake oil and the Pico is no GS-1.

  38. Why don't you buy one and sell it after testing.... (if you still want to) ;-)

  39. I must of been one of the first to view this new one as I saw it with 0 comments last night (my time here in New Zealand) :P

    Anyway, WOW, that's all, I'd say. This can easily be as big in popularity as the cMoy design in years to come. The anticipation of the arrival of the boards and thus first builds is killing me. I'm not very patient with great things like this! But of course, as they say, great things take time. Fantastic blog really. It won't do your CV too badly either ;)

    Great to see Tyll on here!

    I won't be surprised if there are Chinese carbon copies / variations of this appearing on eBay a year from now.

  40. nwavguy, I appreciate what you're doing and wish you the best of luck. Going forward, I encourage you to try to condense your posts. Editing is a wonderful skill and will make your posts that much better.

  41. Thanks Karl. I won't be surprised to see eBay clones of the O2 either. I just hope it's not hopelessly screwed up or compromised in the process. But if so, I'll be putting the word out if someone gets it very wrong to help everyone avoid flawed versions.

    You're not the first to suggest the O2 could be the "new Cmoy" but with around 80 components it's not quite as elegantly simple and it won't fit in a mint tin. But it does a much better (and safer) job driving headphones.

  42. I agree Anon. My articles are getting longer instead of shorter. It's easiest and quickest to just stream my thoughts via the keyboard onto the page. It's a lot harder to wordsmith everything down to be concise. If I was making money from this it would be easier to justify spending more time on it.

    But I'll work at it. I just cut some redundant stuff out of this article. The writing in this article is not one of my better efforts.

  43. Keep at it, it will come with time. At least you have substantive things to say unlike so many others in the hobby.

  44. If you want to prune it down, please keep the original version (director's cut?) around somewhere. Most of it is words that are worth reading, at least for some people.

    It's articles like the objective vs. subjective debate that could be a bit shorter.

    This one is more like an informal report or guidebook, so people should be expecting lots of details. It's good to be thorough to promote the design and help get it going. Anyway, I appreciate getting the behind-the-scenes look at such a nice project.

    You (or somebody else) could write more of a summary/advertisement page later on that would be much more concise, for those who aren't interested in all the technical details or the thought process. i.e. those who just want an amp.

  45. I appreciate all the feedback. Please keep it coming. This blog isn't for me, it's for you all of you. So I'm all for whatever makes it easier to digest, more understandable, etc.

  46. I like long and detailed articles. Probably because I have a tendency to go on an on as well.

    Prefacing it with an executive summary is probably the best of both worlds though.

  47. Great article as always, thanks

  48. The article seems slightly redundant but you guys are just nitpicking, leave the poor guy alone. :P

    Keep the great info coming!


  49. I enjoy reading all of it, this is a blog, not a novel. As far as I'm concerned feel free to just keep pouring it all out.

  50. UPDATE: I've revised the article quite a bit. I cut out some "fat" and added a few sections on component selection and purchasing.

  51. I liked the session on PCB placement and routing. Perhaps a future article(s) on this subject?

  52. I would love to see front and back shots of the populated rev 1 board in the next post. Thanks Santa.

  53. So here I am reading the revised article, all stoked about the O2, when I realize that it’s still a portable amp.

    I’m going to build one, but how many of you here are really interested in a portable amp? To steal nwavguy’s analogy, the circumstances surrounding the use of a portable amp are similar to the use of a Behringer amp onstage. You don’t really need super “audiophile” quality for most, if not all “portable” applications for exactly the same reasons.

    Too much design effort has gone into the O2 for it to remain just a portable….. it has too much potential. What I would like to see is a tweaked O2 board with a good PS sitting inside a GS-1 case, with the same 2 inputs, loop out, preamp out and gain switch, for 1/3 the price of the GS-1. Now that would really be something to get the headphone world to sit up and take notice.

    My suggestion is to stay the course with the portable and at the same time, partner with someone to make the tweaked upscale desktop model.

    You know, this is America, where profit is not a dirty word and besides, it might be the only way for us to induce nwavguy to tweak the O2 into a deluxe desktop version. I certainly would be interested in a nicely cased commercially finished version.

  54. DESKTOP VERSION - As discussed in both articles, and especially the comments for the first article, you can easily build the O2 into a very nice desktop amp. The B3-080 taller case allows room for 1/4" jacks and RCA inputs. You simply leave the batteries out. Want an even nicer look? Buy one of the ~$50 heavy DIY chassis on eBay and panel mount everything.

    POWER SUPPLY - As for the power supply, or other supposed "upgrades", if the O2 can already match the objective and subjective performance of something like the $900 GS-1 (for the sake of this discussion) please explain why the amp itself or power supply needs "upgrading" when there's zero real world benefit? I can only think of two reasons.

    VOLUME CONTROL - The first area I can see improving is perhaps the volume control. But there are not any practical options I'm aware of that don't cost around as much or more than the rest of the amp combined. See my revised volume control comments above in section 2-11. It's certainly not wise to lay out the PCB for an expensive DACT stepped attenuator when few will ever benefit from using one or want the added cost. Even the GS-1 doesn't have one as standard.

    RARE HEADPHONES - The other possibility is there may be some headphones in real world use the O2 doesn't quite have enough voltage and/or current to drive to suitably loud levels for some tastes. That might be reason for an even more powerful desktop amp but you would have to really look long and hard at how popular those headphones are and how close the O2 comes with them as-is. You can never please all the people all the time. The five owners of 1945 vintage Klobigohrlautsprecher cans might just have to use a different amp.

    DELUXE VERSION - Anyone is free to build the O2 into a GS-1 like chassis with similar build quality. That would be great and would go a long way towards pleasing the guys really into aesthetics and the "touchy feely" aspects of high-end gear.

    MARKETING - I suspect it might be difficult to market an expensive O2 as the audio topology is rather boring. It's not needlessly class A, degraded by a 3rd channel, single-ended nostaligic, tube distorted, fully balanced, diamond buffered, or snake oil injected. And even if it does really sound as good as the GS-1 in blind tests, many are going to subjectively brand it as "clinical", "dry" or "sterile" because, although it might measure well, it's not based on some over-hyped but under-performing trendy bit of psuedoscience.

    SIGHTED BIASED LISTENING - Regardless of what I publish here, those with expensive amps are going to listen to the O2 and hear what they want to hear. And most don't want a simple cheap amp to sound as good, or better, than their expensive gear. Even Tyll is likely to have that reaction no matter how good the measurements might be on his Audio Precision. If he were to say otherwise, it would undermine all those much more expensive amps he's endorsed that bring ad revenue to his employer and the industry as a whole.

  55. Any thoughts on digital volume control like the CS3310? I am aware it might degrade SQ, though I am not sure to what degree. But then it improves on crosstalk and channel balance.

  56. Deluxe Desktop Version.

    I can understand why you would seek to avoid items that raise the end cost, to no real world benefit, but here a few ideas.

    Given the fact that a desktop amp might come in a larger case - to match the form factor / aesthetics of other components, then we are not space constrained and could add other features/improvements

    VOLUME CONTROL - what about one of software/relay driven solutions (e.g. 256 steps). Is it easy enough to hook up the volume control to another board for the task.

    SOURCE SELECTION - select from multiple sources

    MULTIPLE OUTPUT / PRE AMP - use this to drive not only headphones , but a switchable feed to active speakers - why waste a decent volume control.

    IEC POWER SOCKET - just because.

    So just drop the board in a larger case!

  57. I think regardless of what most audiophiles say, they are biased towards price too. If a commercial version of the O2 were, say $400, then immediately it would be inferior to higher priced amps to some people.

    Regardless, there is enough scientific technobabble out there in the electronics/amp design arena that you could probably create a jazzed up marketing campaign that was still truthful.

  58. ELECTRONIC VOLUME - The flagship Crystal digital volume controls are the best ones I know of. They're great for a design that already has a microprocessor, remote control, DAC, etc. I'm not sure they really belong in a purist, minimalist headphone amp. They're also expensive and, I think, only surface mount.

    SOURCE SWITCHING - I agree this has some value and it can be done off board. Just put two pairs of RCA's on the back, a toggle switch on the front, and you have dual inputs. If you want more, use a multi-pole rotary switch. With decent wire/cable and proper grounding it will work as well as an "on board" solution but would be a lot more versatile for mounting in different enclosures.

    IEC POWER SOCKET - As long as it meets the audio requirements, I prefer an outboard power supply to get the transformer's magnetic field, and the higher E-fields of line voltage, away from the sensitive audio circuitry. The QRV09 has higher hum than the O2 for example.

    PRE-AMP OUT- This would ideally require a line-out buffer after the volume control. And you might want some way to shut it off with a switch or when the headphones are plugged in. So that would be best done "on board". I could see some wanting it, but where do you draw the line? Should it also have a DAC? USB? Balanced Inputs? Network music player? WiFi? Subwoofer output? Instead of a Mazda Miata it turns into a full bling Cadillac Escalade.

    ONE STEP AT A TIME - If the O2 takes off and actually gets some serious respect that opens the door to other possibilities in the future. But my main goal right now is to prove some key points. And once I hopefully accomplish that, to be honest, there's not a lot of incentive for me to invest a bunch more time in a fancy desktop amp and give that away for free as well. I can understand those who would like that, but please put yourself in my shoes.

  59. I couldn't agree more. The main reason I've been following this blog is because you're promising something that's *not* bloated with features to the point where it sacrifices the original goals. Keep up the good work, and I look forward to seeing the rest of the posts as they come.

  60. I'm just gonna ask this... Since Google came up with no adequate or acceptable results, did you design, construct and test the O2 amp yourself?
    If so, thank you very much for all this.
    If not, where can I get info on the kits (parts needed) or just plain buy one?

    Thank you very much and have a nice life.
    3602 of Head-Fi

  61. Soviet123: I know it's a lot to try and read, but yes I tried to make it clear in both articles this is my design, I built the prototypes (and will be building more), and I've done all the testing so far. There are multiple independent people who have also offered to test the O2 to verify my measurements.

    There's nothing to buy yet and I won't be selling anything. But several others have expressed an interest in offering the O2 in various forms.

  62. Whoa on the Tyl slamming.

    I am willing to give him the benefit of the doubt. Yes, he may have allegiances to Head-Fi, but that hasn't stooped him from honest assessments that run against the grain of the current love-boats of the foamer crowds. Witness his accurate coverage of the Ultrasone 10 headphones at over $3k.

    He certainly does not seem to be a Mikey Framer, where "the price is the product", and the more the cost, the more his praises of greatness flow.

    Now, Tyl may not *like* the sound of a "wire with gain", and that's his prerogative. But I don't expect that will cloud his measurements. He seems quite competent in that area.


  63. TYLL (heycarnut) - I wasn't trying to slam Tyll, just making a reference to his working for Stereophile and all that goes with that. I agree he's more honest than most in his reviews. But, to my knowledge, he's never said a really low end product is the equal of several that cost say ten times more.

    That's not so much Tyll's fault, it's just how the game is mostly played in this industry. The much more expensive stuff, as a category, has to be justified. To do otherwise is wading deep into dangerous waters--probably more dangerous than Tyll wants to risk.

  64. "Should it also have a DAC? "

    Dude, you're reading our minds! :)

  65. Personally I find the "clap on, clap off" functionality indispensable. Can we have that, please. Also, can you make it beep when I whistle so I can find it in amongst all my other audio gear. Ta.

  66. I think Tyll is the perfect guy to review it actually. Inner Fidelity is only a sister site to Stereophile, and he's the editor of IF. If you've seen his ED8 (iirc) review with the accompanied video I think you'll see he's probably the most trustworthy for secondary measurements - he will call bad gear just that. Basically I don't see him as the type to compromise honesty for money in the least.

    This is of course my opinion, but he is a well respected figure and for the right reasons IMO.

  67. I recommend some of you guys actually read the blog! You seem to be missing the point entirely.


  68. Alright guys, now let's be realistic about features and the scope of the project, particularly regarding things that would increase the cost, size, and design time considerably. This is a minimalistic design that sets out to do something the right way and do it well. The gain switch was the perfect addition already.

    Anyway, the extra desktop amp version features that involve extra jacks and such would be easy for somebody else to tack on to the existing O2 once the schematic comes out. Extra performance in any sense would be overkill, so not much can be reasonably improved there.

    I've got to say...once the dust finally starts to settle on the O2 and you have time for a much-deserved break, I'd be considerably more interested in DAC/source reviews than amp reviews for a while, especially for amps that cost more than the O2!

  69. That was Tyll's Ultrasone Edition 10 review that didn't earn him any favors among people who loved theirs and it was hilarious.

  70. Ah, it was the ED10. I knew it was one or the other. The video was absolutely wonderful of it to say the least.

  71. There are two people in the industry I follow closely recently, and that is Tyll and you. Tyll right now is actively looking for reviewers of Amps that could supplement the headphone reviews which he wants to continue doing himself (http://www.innerfidelity.com/content/innerfidelity-call-contributions). Tyll has set the rules for reviewers on his site, so everybody should have a look there instead of speculating on what he might be up to. From what I read there, he has no intention of measuring that stuff.

    In addition I would like to point you guys to the formum section of his site, where he started a thread with some sort of call for papers for a simple DIY headphone design back in march 2011. (http://www.innerfidelity.com/content/cheap-innerfidelity-branded-diy-mint-tin-headphone-amp).

    Also, keep in mind he created and developed the category high end headphone amplifiers for the industry with headroom.

    I guess he is looking at the O2 with a lot of sympathy.

  72. On the subject of finding a commercial competitor for the O2, the closest I can think of is the HeadAmp Pico Power.


    It's hard to say when it will be released, but if it is soon it would appear to be a more natural competitor than a larger stationary amplifier.

  73. Whoa, an amp design?! *drool* No wonder it was a bit quiet here for a while. Can't wait to see the schematic.

    Man, I do hope you have a good keyboard there (otherwise I'd have to send you over to Geekhack for getting clued in ;) ). This article here alone weighs in at about 80K of text. On a good day, I might be able to churn out 10K with a following wind (usually including ASCII art graphics). And that's not much different from before them tendons started complaining. OK, I've never been a speed typist, doing about 50 WPM max.

    If I had to pick a "reference" headphone amp, it would probably be a Lake People / Violectric one. Rock-solid pro gear with heaps of output power (up to 2 W), gain adjustment and pretty reasonable pricing. Built right here in Germany, too. They use ordinary 5532s with a discrete output stage in the big HPA V200 and IC-based ones in the others.

    So you got banned from Head-Fi? Oh well...

  74. “My perception having worked in and out of the audio business is you can only rock the boat so much before the big players respond to protect their sources of revenue.”

    You got that right.

    When confronted with an independent such as yourself, the industry will first try to publicly discredit you, if that fails they will try to absorb you (work for Tyll) and if that fails they will gang up and try to shut down what ever outlets you’re using to get the word out.

    It will be interesting to see how HeafFi handles discussions of the O2 and equally as interesting to see who the detractors will be….

  75. I have it on good authority that the management at Head-Fi have declared you suspect of having commercial interests (hidden agenda to boost sales of well measuring stuff) and thus mentioning you or anything you have done is liable to be deleted. I might try starting an O2 thread, should be fun.

  76. Hi:

    In addition to the Wired Wisdom article on speaker wire, don't forget the Behringer A500 shoot-out, which you have linked to previously:
    A cheap amp, cheaper transport, and even cheaper interconnects took down gear costing oh, so much more.

    Although I will not be able to construct one, I am very much anticipating the final design of the O2, as well as the many reviews as it is built and tested. Keep up the great work... please!

  77. They clearly just want to get rid of you. Actually, I don't think that's bad. If you were just some random troll it wouldn't matter and nobody would care but certainly you are not. People do care and some also turned their back on H-F as a reaction to the disgusting censorship going on there.
    Now with the O2 appearing and somebody else confirming your measurents even more people will see that you're not the bad guy. I can only hope that more and more random guys on H-F will post about your O2, measurements, and blog so that the mods/admins at H-F will be busy censoring for the rest of their lives.

  78. I'm a bit worried about the ease of use of the volume control. Doesn't putting the in/output jacks, volume control and DC power jack and gain switch all on one side make the front panel really crowded?

    I understand you're trying to keep costs low and with a bigger case one could choose to mount the input and DC power jack and gain switch in the back, for example. Still, I'm interested in your comment on the ease of use of the volume control if everything's on one side.

    The mini3 has in/output jacks and volume control in the front and I already find that to be a bit annoying when it's sitting on a desk, with non-angled plugs anyway.

  79. I don't the "commercial interest" concept holds much water.

    It seems very plain that HeadFi doesn’t want you to go around defrocking their audio scene with your measurements.

    What would happen to HeadFi if you exposed more than the uDAC and the Mini3? Whose wallet are they sorry about now?

    Suppose cables became a non-issue, or suppose by allowing you to continue, it encouraged other people with similar equipment to start measuring things?

    Someone posted that “you're not the bad guy”, but the truth is, viewed through their lens, YOU ARE.

  80. Per Anon, "...the mods/admins at H-F will be busy censoring for the rest of their lives."

    This will presumably be something of a quandary for the H-F admins. If at least one of the intended purposes of the H-F site is to provide information on great-sounding gear, and a great product is available for a modest outlay, at some point they either have to let the content through and speak for itself, or try to remove references to it and plainly expose their bias and intent. Neither option is particularly appealing, methinks. My guess is that the content will be allowed through, but loads of "yeah, but..." posts will attempt to soften and redirect the blow.

  81. VOLUME CONTROL LOCATION (anon) - I tried to leave as much room as possible around the volume control. If the cables on the left and right lead away from the amp on their respective sides, it works fine. The front panel is considerably bigger than the Mini3's which helps.

    You can build the taller desktop version and have rear mounted RCA inputs and a 1/4" jack located off in the corner on the front further away from the volume control.

    But there are many other options as well. You can mount the O2 PCB in the enclosure of your choosing and panel mount all the connectors if you like--back or front. The power switch could also be remotely located and the volume control as well with careful wiring. The only thing I would not remove from the PCB is the gain switch.

    The PCB could be mounted backwards so the gain switch, power jack, input jack, etc. were on the back panel. Then a remote mounted volume control and one or more headphone jacks could be remotely mounted on the front panel. Boom. You have a classic desktop headphone amp. And any or all of the above could be done by a commercial manufacture.

    So for the above reasons and more I'm not willing to change with the PCB at this point. I've tried to strike the best compromise between cost, ease of use, portable/home, and being DIY friendly.

  82. Hi! I really cant wait to get one!

    A question: I'm from a little country in the middle of Europe. Will I be able to plug the O2 into our power sockets? TIA!

  83. EURO POWER - Yes, I've already checked and there are suitable inexpensive power adapters for the O2 available with a Euro plug on them for 230 V power from companies like Farnell and others.

  84. Well, one thread has already disappeared:
    (I'm not the originator; Google still has it indexed.)

  85. I'm not sure that Lofft does soften the blow in that editorial. The only real 'but' is in this sentence:

    "I'm not sure that high-end audiophilia is a religion, although heaven knows the parallels to religion are all there."

    I recently did an extensive listening test of my own to compare a standard 'freebie' RCA cable with my own version of the 'ideal' interconnect (settled on after much reading ... I hesitate to call it research). You can probably guess the result. Then again, I was not invested either emotionally or financially. I was just very interested to know if there was any audible difference.

  86. “If at least one of the intended purposes of the H-F site is to provide information on great-sounding gear….”

    Uh, no!

    Huddler goes around buying forums in order to create “one stop shop web sites” for everything related to what ever your particular hobby is. Camouflaged by the forum they proceed to talk up a continuing array of products, (FOTM) so they can make money on their sale. Check out the Huddler web site. You’ll see their mission.

    What ever H-F used to be, it is no longer. Jude has been doing this for a long time and when H-F reached the tipping point, where there was a steady influx of new members in sufficient numbers, Huddler bought it. Check out Huddler’s other sites and you'll see what I mean.

    Another thing that is really annoying was the need for new forum software. Why was it needed, you may ask? Because the new Huddler software allows “deep data mining” (and to set up those one stop shops).

    What we really need is a new place to go. A place like the original H-F was when it split from HeadWize.

  87. We need Hydrogen-Fi. Something less annoying than hydrogenaudio but much less ridiculous than Head-Fi. :)


  88. Actually I vote for NwAvGuy-Fi ;)

  89. I am a member at the Head-Fi forum. I had a Heed CanAmp, which delivered radio signals when listening to music. I was not the only one writing this in the forum, but to my surprise these messages were deleted by moderator. I thought that Head-Fi were a forum where fellow headphone entusiasts should help each other exchanging usefull knowledge. But it seems now greediness rules the forum. Money talks. This blog is the most interesting thread i have found to date relating to the headphone business. I really hope that in a near future it will be possible to buy for us in Europe, living in a 230V environment. When it comes to dangerous amplifiers praised by the Head-fi forum, Singlepower certainly managed to survive for a lot more years than it deserved. I can imagine this were possible because of moderators deleting negative comments.

  90. "we should probably get this discussion focused back on the O2. The Banned at Head-Fi article is a better place to discuss Head-Fi drama."

    I didn't know anyone was still posting on that old thread of yours. There's no listings of current posts so it's hard to follow. I've just been posting on the latest Blog article.

    "I don't know if that means setting up camp at ABI, diyAudio"

    I think we should set up camp somewhere. You're blog articles are great but it's hard to follow the threads.

  91. I was talking about my "banned" blog article from just a week or so ago.

    As for setting up camp for the O2, I'm just waiting to get the third article done. I agree it's difficult to follow these comments (although I did turn off the Captcha verification to make posting here a bit easier).

    If I put this up on diyAudio without even a schematic many will look, not find complete info, lose interest, and probably not bother to check back later. So I'm just trying to have the complete project online before I start a thread on diyAudio.

  92. Perhaps start a thread on ABI first. Since it would make a great portable amp, I reckon at least some people there would be interested. It might make the forum a little bit more active too, not really a bad thing for ABI. :)


  93. I was planning on a review on head-fi, however, seems like they are taking your ban a little hard. Sad really.

    What about giving the O2 to a site like audioholics for a review? They seem to do a good job with the objective parts of reviews. Also, seeing as how they really don't seem to have any headphone equipment sponsors, maybe they would be more open to giving you a fair shake.

  94. ABI THREAD - Satellite,I took your advice. Done.

    ABI O2 Thread

  95. That was a comprehensive and exhaustive overview of your design. Wow! Too bad you won't be producing it but when people do, could you give us a heads up as to who is trustworthy?

    As you wrote regarding Chinese eBay amps:

    "But, out of probably 20+ products I’ve encountered, I’d say 90% had at least one significant flaw like this Cmoy does."

    Perhaps you can figure out a process to get third party QC done for end-user peace of mind.

  96. O2 SOURCES (traumerei) - I will be monitoring anyone offering the O2 in any form. So far, the vendors who have expressed an interest seem genuinely interested in following the original design and using quality components. If they don't, I'll quickly warn everyone to stay away.

    As for Chinese/Taiwan/Hong Kong quality control it seems companies either want to build up a solid long term reputation by doing the right thing, or they just want to want to make a quick sleazy profit selling anything they can on eBay even if it is defective.

    If I find, or hear about, any version of the O2 being sold I will check if they have honored the Creative Commons license, used the right PC board layout, the correct components, etc. If there's a problem, I will try to let as many people know as possible to avoid that vendor. The vendors who do it right will end up with the most business. The ones who do it wrong will be exposed and suffer reduced sales.

  97. Hey,
    Just to say, I started the thread that someone has mentioned that was deleted. The person who deleted it PMd me as to why it happened - after an exchange of messages, told me to take it up with Jude. As I see it at the moment, the rules prohibit posting *on behalf* of a banned member, but are not a blanket ban on mentioning any of their works. Currently seeking clarification. Regardless of any threats that may or may not have occurred ( I really can't presume to make any calls here, having not seen the original messages), I cannot see how this can have bearing on a discussion of a non-commercial design.

  98. About the voltage concerns, surely you can easily change the design so it incorporates all AC power voltages from 110v - 230v.....

    Even cheap Chinese designs do this....

    Basically: For those not on 110v, do we need a power adapter / transformer or just a plug adapter?

    Sorry if I've missed something but it doesn't seem clear to me.

  99. HEAD-FI - Thanks Willakan. We'll have to see how things go at Head-Fi. I know there have been many examples where they don't need to reference any rules at all for censorship. That happened to me back in March and Jude's response was "I can censor or ban anyone at any time for any reason". There's more at Banned at Head-Fi.

    110 - 230V POWER (Karl) Those cheap Chinese designs you're talking about use a DC power adapter. Those adapters just provide one voltage, but a proper headphone amp needs two DC power supplies. So amps that run on a single DC wall adapter have to use a virtual ground or have some kind of noisy power converter in them. For all the reasons I've mentioned elsewhere I didn't want either of those for the O2.

    The O2 runs on an AC/AC wall adapter. It outputs low voltage AC (not DC). And from that AC the power supply in the O2 generates two DC supplies. AC output adapters are just a simple transformer. They cannot auto adapt to different voltages like a DC switching supply can.

    The parts list of the O2 lists a 230 volt adapter with a Euro plug. You should be able to find one in most any country for the right voltage and with the correct plug on it. The output can anywhere from 12 VAC to 20 VAC and the current only needs to be at least 200 mA.

    Farnell Electronics is set up to ship to most of the world and they have AC adapters for the O2: Farnell Distributors

  100. It's AC/AC with presumably a diode or something similar facing one way for half-wave rectification to help generate the positive supply rail, and a diode facing the other way for half-wave rectification for the negative supply rail?

    I think some DIYers with an open mind looking at the design may be shocked at how such a dual power supply mechanism, using relatively small (physically, maybe in terms of electrical values too) capacitors and components, can achieve the performance measured in the first article. I guess it shows the PSRR of the chips used in the design, when implemented correctly.

    I'm not that familiar with any DIY designs, but my brief impression is that some or many desktop designs attempt to do way overkill power supply filtering.

    Or maybe I'm way off base guessing at what's going on. Can't wait to see the final design doc!

  101. POWER SUPPLY (Mike) - See the last item under 2-21 above in this article. You are mostly correct about the power supply. And while the O2's chips do have excellent PSRR, the regulated supply rails are already very quiet. Almost zero ripple makes it through the regulators.

    And you're correct a lot of DIY and audiophile commercial amps have overkill power supplies that do nothing to improve their performance but they look nice. One exception, however, are single-ended amps. They have horrible PSRR, and operate in Class-A, so they demand a lot from the power supply.

    I think a lot of beliefs in the audiophile world end up being wrongly "extrapolated" and applied in the wrong situations. While a better power supply can improve a single-ended design, it usually won't improve more modern designs. But the thinking seems to be "if it helps somewhere it should help everywhere." There are many other audiophile examples of similar flawed logic.

  102. "The O2 runs on an AC/AC wall adapter. It outputs low voltage AC (not DC). And from that AC the power supply in the O2 generates two DC supplies. AC output adapters are just a simple transformer. They cannot auto adapt to different voltages like a DC switching supply can."

    Is this similar to AMB's Tread PS which uses an AC wall adapter?

  103. I have no idea about the "Tread". Ti Kan has blocked my access to his entire site.

  104. "I have no idea about the "Tread". Ti Kan has blocked my access to his entire site. "

    Not a very nice thing to do.....

    but I was mistaken, I think it's one of Tangent's

    here are some pics of it anyway:


    and from Tangent's site:


    also: http://www.head-fi.org/forum/thread/124626/tread-completed

  105. That looks like just a single supply using a regular bridge rectifier. The O2 uses a true dual bipolar power -12V / 0V / +12V. It's like two half wave 12 volt DC power supplies with one of them "flipped over" so the positive side is connected to ground. And it's a real ground.

    A lot of people don't realize you can get a true bipolar supply from a single 2 wire AC wall transformer. Or even if they do they wrongly assume it can't perform well enough for a serious headphone amp. The O2 proves it's just an engineering problem and can work great.

  106. Do you have a rough as balls parts cost for this thing?

    I know it's still a work in progress but a couple of parts here and there wont exactly quadruple the cost. (hehe, unless you need z-foils and blackgates)

    For the people antsy about having to use an AC wall transformer (if it's hard for some to source or whatever), you'll still be able to use a normal transformer with only a little extra work, there's nothing to it.

    The dynalo is still available as a commercial product for about 300 bucks(Sheer Audio HA-006+), it should turn out to be an interesting comparison.

    Good luck with your ambitious project dude, I'm sure a lot of the DIY noobs will be very well chuffed to have the O2 as a cheap and cheerful option to build. The damn name keeps on reminding me of the Omega 2, oh well =P.

  107. Off Topic but you have to see this:


    check out the notes:
    "Note1. The uDAC-2 is designed for consumer playback of digital music. It has not been calibrated for professional recording. At the 9 o'clock volume indicator, there is hardly any signal; further, analog pots do not behave reliably. At normal listening levels, the volume tracking is within 1dB.

    Note2. The uDAC-2 volume max out at around 0dB input (maximum digital input) with THD+N around 1% is intentional, i.e., targeting the voltage rail limit in order to achieve the loudest possible, yet un-distorted, music playback. Short of 1% THD at 0dB input, we are not fully implementing our voltage limit, which translates in practical terms to a diminished volume range for music."

    Are they really trying to sell that crap for $400? I hope they didn't put there just some random specs there again...

    (I have paid for the normal version, thats why I'm waiting so much for your amp)

  108. How does the SR-71 stack up against the O2? I posted measurements link and pics in the ABI thread.

  109. Regarding the HA-006+ : If I'm not mistaken the Sheer Audio Dynalo has an modified circuit that uses a single fet where the Gilmore design uses a double, so it's not really a Dyanlo.

  110. Thank Anon... that pretty much rules the HA-006+ out. So we're back to the GS-1 or something else?

    There was an earlier suggestion for one of the Violectric amps and someone else privately suggested them as well. I'm impressed by their approach and specs. But, at least in the US, they don't seem very well known making it a poor benchmark.

    The Schiit Asgard will get its butt kicked on the test bench by the O2. But it would be fun to see what happens in blind tests? As I said earlier, if the Asgard's flaws are serious enough to stand out there would be an audible difference. If that's true, would people generally prefer "A" or "B" in a blind test when they don't which is which?

  111. eh, at this point, you should consider getting a wiki of some sort to organise everything. Blogs are wonderful for time sensitive information - the stuff you're doing is likely to be useful to people in future, and will be easier to find.

    Looking forward to the actual design, and PCBs, even if i'm a horrible procrastinator with DIY projects ;p

  112. A wiki is an excellent idea and if someone else wants to take the initiative I'll be happy to review it for accuracy, etc. I've already invested way more hours in this project than I'd planned and I'm not done yet.

    If the O2 proves worthy my hope all along has been others might want to help further support the design. Hopefully the O2 will end up being more than a one-man crusade.

  113. I've started a Wikia: http://o2amp.wikia.com

    I'll work on a logo and on the Design page layout later (it's past 2AM here where I live).

  114. Thanks Vittau! Please don't put too much work into this yet. But if the O2 proves popular a Wiki is a huge upgrade over my long, "linear" articles.

  115. Due to a misunderstanding, someone on Head-Fi messaged me thinking I was NwAvGuy, so I just thought I would pass on what he said:

    ...I had an opportunity to go to your blog site and look over your info and data.

    I've been fortunate to have found a number of sites on the internet, that explore a subject and go beyond the limit, for which many people may find to be too tedious and detailed. This then leaves these sites for the serious explorers. Your blog achieves this.

    By the way, you're the first individual, with whom I've communicated on the web, who has been 'banned' from a website. Way to go! I read your article about the issues with Head-Fi. A perfect example of why we (in free societies) must protect our freedom of speech. In your case, the monetary support was on the line, in spite of an advertisers reticence to evaluate their product.

    I will go over the information regarding the amp.

  116. Part 3!, Part 3!, Part 3!

  117. Part 3 with prices (might be just an estimate for the PC board) and a lot more should be posted sometime tomorrow (Wednesday).

  118. Incredible articles! Along with the other problems that will be avoided by the low output impedance, I assume that this will be immune to the hiss that low impedance cans exhibit, which is a bane of mine. I have never seen a decent explanation of how or why this hiss arises: have you explained this elsewhere?

  119. Thanks. No hiss to be found here. If you hear any from the O2 it's either your source or you have Superman's hearing.

    Hiss and noise is complicated. It has multiple possible sources including the topology (over all big picture design) of the circuitry and even the resistors used (including the volume control). And yes, I do go into more detail here and there. Check 2-3, 2-9, 2-10 and 2-11 in this article and the the noise measurements in the Tech Section of the first O2 article.

    Any amp will also amplify whatever hiss your source has. The amp's job is generally to make the music louder and it can't tell music from hiss. So amps make any hiss louder as well.

  120. I cant wait for the circuit diagram , inside photos and of course building one.
    Good work NwAvGuy , congratulations

  121. Holy Christ!
    I cannot help it but schedule each read of your juicy blog pages while eating some cheetos.

    It's Fantabulous!!

    Please, PLEASE, keep the good work.
    And THANKS A LOT for your time and sharing of your knowledge.

  122. Hi there,

    At resk of being found an anorak here, but the frequency compensation around U1A and U1B is not dominant pole compensation. That's built into the opamp by JRC, as you can see from the gain/phase plot in the datasheet. Your compensation scheme adds to this by including a pole-zero pair in the feedback loop, which has the net effect of moving another pole (not the dominant one, but a higher-frequency one) that's in the chip's frequency response a bit further out of harms way.

  123. Oh well, for the people who don't want to read the whole story, you've written some good summaries. But I am actually quite delighted about you sharing the whole story. It might be a stretch for some, but for me is was a pleasure to go through, not having to fill in the blanks, but to simply read all your considerations. It's what a good design report should be. There are very few of them on the internet.

  124. Hey Anon, I technically agree with you and made some changes to that paragraph. I was trying to simplify the discussion without getting into compensated vs uncompensated op amps, or worse, all the complex math and feedback theory. I'm accused of making too many eyes glaze over as it is!

    Yes there are multiple pole-zero pairs but the the 220 pF caps compensate the transient response by essentially the same mechanism I linked to at Wikipedia (at least for the sake of a simplified lay-person's argument).

    I welcome suggestions on how to better explain it without going into all the details? Please feel free to contact me via the link on the right if you want.

  125. Just stumbled on your blog. It's fantastic to find writing on audio equipment free of magical snake-oil baloney. Thanks for sharing your knowledge and hard work.

  126. On a forum, aside from conjecture and personal attacks, the following claims have been stated:

    "[...] the PCB design and layout is [expletive] poor to say the least [...]"
    "Look at the ground plane, it is a bad joke."
    "[...] a ground plane that doesn't work [...]"
    "The ground plane on the PCB is indeed a little messy, and there appear to be traces to nowhere."

    Since I lack the EE knowledge to assess these claims myself, I'd like you to comment on them. Are these objections (partially) valid? If yes, could this impair performance, or is it just a matter of aesthetics? Or perhaps just another audiophile myths? Thank you.

  127. As I've stated many times, the O2 is threatening to a lot of people and commercial interests. It's an inexpensive amp using inexpensive parts with a mostly conventional design that beats a lot of more exotic amps. The performance speaks for itself. If it had PCB layout problems, it wouldn't rival the performance limits of a $10,000 audio analyzer.

    The comments you quote are very subjective and rather immature. And they are much more "personal attack and conjecture" rather than valid criticisms. They don't say anything specific except "traces to nowhere". In other words, it's like looking at a Ferrari and saying "I think your car is ugly so it's a lousy car".

    The ground plane is intentionally divided into sections that serve different parts of the amp and terminate in a star ground. It's also intentionally kept away from some components. I've explained why in this article and in the O2 Details Circuit Description.

    Anyone who knows much about PC board layout would know a guard trace when they see one. That's what the "traces to nowhere" are. They close the loop area of the inputs to the 4556 op amps without creating a ground loop. They're basically an "individual ground plane" for just those signals.

    So, bottom line, what you read seems more like some jealous kid throwing rocks who doesn't really understand amplifier design rather than any sort of valid criticism. If they think they can do better, that's why I've put several open challenges out there. Not one person has stepped up yet.

  128. I applaud you for your hard work.

    Your blog is by far the most informative source of compiled knowledge(including all the big forums) i've found since I became interested in headphones and the like. the unbiased, objective research you provide for free has definitely influenced a lot of my purchase decisions.

    Thanks again.

    Hopefully one day I can make this thing but my hands grow out of my ass so probably not soon lol.

  129. Thanks for step by step process its very helpful post for me to get some idea about to design printed circuit board.

  130. Before I get totally flamed for asking my question, i'd like to say that

    1. I love the idea of a totally unbiased testing method. There should be ISO standards dedicated to testing sound equipment.
    2. I love the idea of bringing back quality and affordability.
    3. You seem to know a lot about this, and so keep at it.

    Here's my question: off all Op-Amps to test, why didn't you test a single one from http://tangentsoft.net/audio/opamps.html (tangent's website)? He seems to have the subjective side down pat, you might as well show him your measurements on all his Op-amps and see if he was correct from a scientific point of view.

    Also: could tangent be correct in saying they have different sonic characteristics? it seems to me that any slight change in purity of the metal/build quality/anything really could change slightly a certain frequency (or range).


    1. I'll happily challenge whoever wrote all those subjective comments at tangentsoft to a blind listening challenge. If they can hear a difference between op amps, including those they "rated", that measure well in a given application I'll give them $500. The catch is the challenge will be recorded on video and published on YouTube. Feel free to contact Tangentsoft and see if they're up for the challenge.

  131. Just curious how does your "active current limiting" work? Is it part of the power management circuit?

  132. @Yang Zhou, the current limiting is built into the 4556 op amp. Each op amp current limits at about 100 mA. With two in parallel you get 200 mA per stereo channel.

  133. Hello,

    Just got my Objective amp from JDSLabs...
    One word:


    Great amp..


  134. Wow great article very technicaly. I dont like the audiophile world myth about bla..bla...bla... expensive components = sound great. I think this article and your O2 can beat them all.

    cheers..... :)))

  135. I like this amp so much I ordered the parts from Mouser and the pc board from JDS...some of the parts are on backorder until mid to late August. Will be fun to get out the soldering iron again!!

    I also ordered one of the higher voltage/current AC adapters from mouser just to see how the amp will play with my LCD2's in the high gain setting vs the low gain setting...and yes even with only using the low gain setting the loudness gets VERY loud.

    Cant wait to pair this amp with the ODAC.

    Its nice to have a well designed amp that is transparent and doesnt get in the way so you can compare other things in the chain..

    All the best

  136. I have a question about the design volume control. Considering your background you might already know about the trick to make a linear potentiometer more logarithmic by adding a fixed resistance, as is described here: http://sound.westhost.com/project01.htm

    Did you consider such an implementation for this project? If not, do you think it might be a good idea to add? If you did consider it, why did you not use it?

    To me it seems like a great, simple and cheap idea to get better left/right tracking, but maybe there are drawbacks that I don't know about.

    // Anders

    1. It's a valid trick and has some merit. But the idea with the O2 is to set the gain properly (in the gain stage) so you don't have to worry about pot tracking issues in the first 5% of the pot rotation. The O2 is also about simplicity and I wanted it to fit in the B2-080 stock enclosure. There's not much room left on the PCB, even for a pair of resistors.

      The other issue is the availability of high quality reasonably priced volume pots in the values required. You can't add a resistor to the 10K pot, in the necessary ratio, without excessively loading the gain stage at some volume settings. That would increase distortion. The volume control resistance is important balance between loading the gain stage and johnson noise for the output stage.

      The Alps pot in the O2 is specifically designed for volume use and is available in two different tapers. It doesn't seem to subjectively suffer from a "pronounced discontinuity" as the author suggests especially in the preferred 15A taper. There's a curve of rotation vs resistance in the Obtaining The Components section of the O2 Details article.


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