ODAC Released

CAREFREE ENJOYMENT: I’ve been listening almost exclusively to the ODAC for several weeks now and I’ve been smiling a lot—not so much at the ODAC but at the music. When you know your audio gear is genuinely transparent it opens a worry-free window into the music. And, while I suffer from expectation bias just like everyone else, I’ve run a second blind listening test and can report the O2+ODAC held its own against the $1600 Benchmark DAC1. They both are audibly transparent.

I’M NOT SELLING ANYTHING: Some have claimed because others may profit from the ODAC I’m profiting from it. But that’s not the case. My involvement was simply to help design and measure it--just like the DIY open source O2 amplifier. YoYoDyne and various other vendors are responsible manufacturing and selling the ODAC. They’re the ones taking the financial risks, so they’re also the only ones to profit. This is an entirely not-for-profit blog and will remain so. You can’t buy anything from me.

LIKE O2 LIKE ODAC: The O2 amplifier was created as a simple, low cost, minimalist design delivering 100% transparent performance into nearly any headphone. The ODAC takes the same minimalist approach to transparency. But it’s not quite as simple as it looks in the photo above. It has around 60 components and many of those were carefully chosen through a lot of detailed measurements and trial and error.

NO SNAKE OIL REQUIRED: Many audiophiles want to believe more elaborate or exotic DACs offer higher fidelity. The ODAC demonstrates you do NOT need any of these for 100% transparent performance:

• Asynchronous USB
• UAC2 (USB Audio Class 2) Support
• Asynchronous Sample Rate Conversion (ASRC),
• Minimum Phase Filtering (no pre-ringing)
• Non-oversampling NOS DAC chips
• Dual DAC chips
• Balanced Outputs
• Vacuum Tube Stages
• Elaborate and/or High Current Power Supplies

THE PROOF: I can confidently say none of the above are required for DAC audio nirvana—i.e. having your DAC disappear from the signal chain. Some of the above, like NOS designs and tubes, degrade fidelity. I’m confident because detailed measurements and blind listening tests verify the ODAC’s real world performance. I’ve even tried several different PCs and the ODAC’s performance is relatively consistent between them. So while some of the above might improve a few measurements, if they don’t improve the sound, it’s like taking 4 pills to get rid of your headache when 2 do the job nicely. Once the headache is no longer perceptible, more pain reliever doesn’t help anything. I encourage others to compare the ODAC blind against other DACs, at any price, that measure reasonably well.

ASSEMBLED BOARD: As explained in the ODA/ODAC and ODAC Update articles the ODAC is not DIY friendly. This is true for most 24 bit USB DACs. Neither the USB interface nor the DAC chips are available in small quantities. Both require signed OEM agreements. minimum purchase quantities, and the USB chip also requires custom programming. The ODAC has many extremely tiny 0603 surface mount components and uses a fine pitch 48 pin IC making it very tedious to build by hand. Given all that, the ODAC requires commercial automated assembled in fairly large batches. DIYers can add the ODAC board to the O2, build it into a stand-alone DAC, or add it to the upcoming Objective Desktop Amp (ODA).

NO S/PDIF: To hopefully avoid most of the “how come you didn’t add S/PDIF” or “when will you add S/PDIF” questions, please see the previous ODA/ODAC and ODAC Update articles. Sorry, but it’s not possible.

PARTNERS REVEALED: Because of the above requirements, the ODAC development has been a joint effort with YoYoDyne Consulting. George has supplied other open source USB DAC boards at reasonable prices and seems to be well respected on diyAudio. He’s taking considerable financial risk having a large batch of ODAC boards assembled and hence is coordinating all the manufacturing and distribution. I conducted all the measurements, refined the design, and helped optimize the PCB layout. Just to be clear, I’m not getting any money from the ODAC. Here are the ODAC resources so far:

• YoYoDyne Consulting ODAC Announcement (Canada)
• JDS Labs ODAC Announcement (USA)
• Epiphany Acoustics ODAC Announcement (UK)
• Head-N-HiFi ODAC Announcement (Switzerland)
• Audio Poutine ODAC Source (Canada Facebook Page)

AVAILABILITY: YoYoDyne is estimating boards will be available before the end of May assuming nothing goes wrong. It might take a few days longer to get boards to the UK and Europe. For other information please see the links above.

SCHEMATIC: YoYoDyne has received approval from Tenor to release the schematic once the production boards are verified and released to the various vendors above. YoYoDyne is also interested in a possible future version that’s more DIY friendly.

GUILT-FREE VOLUME ADJUSTMENT: The majority of USB DACs only support 16 bits over USB. That means when you turn down the volume in software you’re getting less than 16 bits of resolution. At background music levels you might only be listening to 11 or 12 bit audio. But the ODAC has a 24 bit USB interface and enough dynamic range to allow guilt-free use of software volume controls.

OTHER DETAILS: There’s a lot more in the previous ODAC articles, but in summary, the ODAC is designed to fit inside the standard O2 enclosure in place of the batteries. A few internal wires need to be soldered to connect the ODAC’s output to the O2’s input jack. Other line level sources can still be used with the O2. It will also fit inside the upcoming ODA. And it can be used standalone with either the on board 3.5mm output jack or panel mounted RCA output jacks. The USB connector is a standard USB-Mini-B as used on the FiiO products, the Sansa Clip, cameras, etc.

FOUR REVISIONS: The ODAC has been through four lengthy revisions—two of the earlier boards are shown to the right. Despite the fact we started with essentially the reference design from the datasheets, the devil was in the details. The  first version played music and sounded OK. Many companies and DIYers that “design by ear” would have stopped there. But that first version didn’t come close to delivering what the DAC chip is capable of. Each revision cycle took at least several weeks, cost hundreds of dollars, and involved countless hours of work. But, in the end, it resulted in much better performance compared to where we started..

REAL WORLD PERFORMANCE: Most DACs and PC sound interfaces priced under $200 fail to come close to the published specs for the chips they use. The FiiO E10’s Wolfson DAC chip is rated at 117 dB of dynamic range but the E10 only delivers a modest 98 dB. The power supply, PCB layout, grounding scheme, I2S waveform fidelity, clock quality, and more, often degrade the performance to well below the manufacturer’s carefully optimized reference design used for the datasheet specs. This is especially true for USB sourced signals and DACs running from USB power. But the ODAC, despite being USB powered, managed to come very close to the “Holy Grail” datasheet specs. See the Tech Section for more.

THE REAL NUMBERS: Here are the real numbers (versus the April Fool’s Day numbers I published two weeks ago) and they all meet the requirements for audible transparency. The letter following many of the results is the same letter grade (A-F) I’ve used in previous reviews with A being the best, and F being a “Fail”:

Measurement	ODAC	FiiO E10	DAC1 Pre
Freq. Response 10 hz – 19 Khz 24/44	+/- 0.1 dB A	+/- 0.1 dB A	+/- 0.1 dB
THD+N 100 hz 0 dBFS	0.0029% A	0.005% A	0.0009%
THD+N 20 hz –1 dBFS	0.003% A	0.004% A	0.0009%
THD+N 10 Khz –1 dBFS	0.003% A	0.004% A	0.0007%
IMD CCIF 19/20 Khz –3 dBFS	0.0011% A	0.013% B	0.0005%
IMD SMPTE –1 dBFS	0.0004% A	0.004% A	0.0004%
Noise A-Weighted dBu 24/44	--102.8 dBu A	-98.3 dBu C	-105.4 dBu
Dynamic Range –60 dBFS A-Wtd	--111.1 dBr A	-97.6 dBr C	-110.9 dBr
Linearity Error -90 dBFS 24/44	0.0 dB A	0.0 dB A	0.2 dB
Crosstalk 0 dBFS Line Out 100K	--93.5 dB A	N/A	-106 dB
USB Jitter 11025 hz J-test 24/44	Excellent	Very Good	Excellent
Maximum Output Line Out 100K	2.0 Vrms	1.65 Vrms	2.5 Vrms (1)

NOTE 1: Maximum output of DAC1 is configured with internal jumpers

BOTTOM LINE: The ODAC has been released to production and will hopefully be available by the end of May. I’ll soon be publishing more detailed measurements, results of listening tests, etc. I’m confident the ODAC is audibly transparent. And, especially when it’s installed inside the O2 or future ODA, it offers a level of performance that’s difficult to find without spending substantially more. It also offers detailed measurements and blind listening tests to back up its performance which is something very few other DACs offer at any price. And, paraphrasing from credit card commercials, worry free enjoyment of music can be priceless.

 

---

TECH SECTION

---

SPECIFICATIONS: Here are some ODAC basics:

• Audio Formats: 16/44, 16/48, 16/96, 24/44, 24/48, 24/96
• Interface: USB Audio Class 1
• Native Driver OS Support: Windows XP & Later, OS X x86, Linux
• Operating Systems Tested: XP, Vista, Win7 (32 & 64), OS X Snow Leopard, Ubuntu 9.1 32 bit
• Line Output: Approximately 2 Vrms into 5K ohms or higher
• Dynamic Range: > 110 dB A-Weighted
• Distortion: < 0.005%
• Dimensions: 49 x 58mm (see drawing to right)

BLIND EVIDENCE: So far I’ve run two relatively informal blind tests with the ODAC. The latest one used special software on the PC to play the same track on both my Benchmark DAC1 Pre and simultaneously on the ODAC plugged into the same PC (both connected via USB and running at 24/44). The ODAC was connected to an O2 headphone amp, and a switchbox allowed the headphones to be rapidly switched between the DAC1 and the O2+ODAC. The two sources were carefully level matched (using their respective volume controls) using a test signal and wideband DMM. I tried both my Sennheiser HD650 and Denon AH-D2000 headphones with a variety of well recorded favorite tracks. One other listener and I could not reliably tell which was playing.

FUTURE BLIND TESTS: I’ll hopefully be running a more comprehensive and rigorous blind test in the future. But, ultimately, it’s best to have listeners who expect to hear a difference, and someone else who understands the technical issues to supervise the test, perform level matching, sync the sources, etc. It would be ideal, for example, to have Mike and/or Lieven from Headfonia be a listener and George from YoYoDyne oversee the test. But sometimes geography and other factors restrict what’s realistic. If anyone is willing to help coordinate such a blind test, please contact me privately with the link in the right hand column.

USB INTERFACE: There are only a few USB interface chips capable of 24 bit operation. The best option we could find is the Tenor TE7022. It’s used in the Violectric USB 24/96 and several other commercial 24 bit USB DACs. Notably, it does not require proprietary drivers to work with any popular operating system including Windows because, like the Benchmark DAC1, it’s a USB Audio Class 1 interface. It also has respectably low jitter. The XMOS solution requires an expensive license and proprietary windows drivers and offers no audible benefit. The TAS1020B is being discontinued, requires extensive firmware, and also offers no audible benefit. A custom microcontroller creates even more hurdles with no audible benefit.

THE DAC CHIP: As I’ve explained elsewhere you can get transparent performance from most of the better DAC chips on the market from a half dozen chip vendors. What’s most important is choosing one that’s best suited to the particular application. In this case, that means running from a single 5 volt USB power supply, having a buffered voltage output (to avoid needing a single-supply op amp), and operating properly without using a microprocessor. Just those three requirements narrow the choices considerably. We chose the ESS Sabre ES9023 which is used in a lot of commercial designs (it’s an improved version of the popular ES9022). Unlike TI, Burr Brown, Analog Devices, etc, ESS specializes in audio chips and they did a nice job with the ES9023’s feature set and specs.

DIRTY LIES: Many popular “boutique” DACs (especially those being sold on eBay) mislead their fans by quoting only the chip specs for their entire DAC. Basically that amounts to cheating and lying. It further implies either the company is incapable of making the proper measurements or the real measurements were bad enough they didn’t want to share them. The implication is a DACs performance is solely determined by the chip used. But the opposite is usually more true. The implementation matters far more than the chip. the FiiO E10 is just one of many examples. The HA-Info I’ll soon be comparing to the ODAC is another.

CHIP ENVY: Any serious audio engineer designing DACs and making proper measurements knows the implementation matters more than the chip used. The first version of the ODAC, which closely followed the ESS datasheet, only managed about 98 dB of dynamic range. The distortion was also much higher than listed on the datasheet and the jitter was somewhat disappointing. Unless you exactly duplicate the chip manufacturer’s reference design, right down to the PC board layout (which I’ve yet to see any manufacturer do), you really don’t know what you’ll get. To measure all the important parameters you need more than just RMAA. You need a real audio analyzer with performance substantially better than the DAC being measured.

DESIGN BY EAR: Detailed and credible published measurements are missing with most “boutique” DACs being sold including those from Schiit Audio, Audio-GD, AMB, Twisted Pear, Burson, and NuForce. Where’s the credible evidence they got it right? A lot of these companies try to claim specs don’t matter, and they instead design by ear, but that method is seriously flawed (see: What We Hear). It would be like designing a car engine without a dynamometer and having no idea how much horsepower and torque it produced, how fuel efficient it was, etc. Given all the proven problems with sighted listening, and how our ears and brains work, those who claim to design by ear are very likely getting it wrong. Put another way, they’re often designing products with far lower fidelity than they’re otherwise capable of.

LESSONS LEARNED: If this project has taught me anything, it’s that getting much better than 16 bit (96 dB) performance can be challenging. The first version of the ODAC, despite following the reference design, only had about 98 dB DNR. That’s about the same as the FiiO E10. The photo to the right shows a few dozen assorted surface mount parts that were laboriously swapped out one at a time and measurements repeated dozens of times using the dScope. Some improvements were far from intuitive. Audiophile preferred polyphenylene capacitors performed worse than less expensive types. Additional filtering on the digital power supply dramatically increased jitter. Chasing down the last few dB of dynamic range the chip is capable of proved to be especially challenging. When it was said and done, the DNR went from 98 dB to over 111 dB. That’s a huge difference and something the design-by-ear crowd would have never achieved.

THE AUDIOPHILE WAY: All too many small or “boutique” audiophile manufactures and DIYers seem to just slap trendy chips on a board, listen to their creation expecting it to sound good (so that’s what they hear), and call it good. Many don’t even follow the reference design. Instead they include a bunch of “audiophile upgrades” expecting better performance—and they hear what they expect to hear even when it’s not true. But the ODAC demonstrated those upgrades often make things worse. So instead of getting even 98 dB DNR like the first ODAC revision, those following audiophile myths and designing-by-ear probably would have ended up with something even worse. Unless you’re making the right measurements, you really have no idea what you’re getting.

THE POWER SUPPLY: For reasons explained in the earlier ODAC articles, the ODAC is USB powered. This allows it to work standalone, as an internal add-on to the O2, and in the upcoming ODA. There are many obvious advantages to USB power but it often degrades performance due to noise. To get around this, the ODAC uses split digital and analog power supplies each with their own filtering and regulator. The analog supply has additional filtering and the critical reference voltages, and negative supply for the DAC chip, are further optimized. I literally tested more than 100 variations of components, including different brands of capacitors, to get the most out of the ES9023. This level of refinement would be impossible without a serious audio analyzer.

PUMP YOU UP: The ESS chip has the huge advantage of a built-in low noise charge pump. It generates its own regulated negative power supply allowing a Redbook standard 2 Vrms output from a single 5 volt USB power supply and a direct coupled output. This is an important distinction compared to a lot of USB powered DACs. Without the charge pump, or some other negative power supply, USB DACs can’t produce the Redbook standard 2 Vrms which reduces their effective dynamic range, lowers their ENOB, and creates level matching problems. It also requires an undesirable output coupling capacitor and usually results in loud transients on power up. The AMB Gamma, and most USB powered DACs I’ve tested, don’t meet the Redbook standard. The ODAC does. And it produces only a soft click on power up.

ADAPTIVE USB INTERFACE & LOCAL CLOCK: I’ve talked about this before, but just to be clear, the ODAC is NOT clocked by the USB port. So the quality of the audio clock, and any resulting jitter, is largely independent of the PC’s USB timing. It has its own low phase noise 12 Mhz crystal controlled oscillator that’s used to generate the MCLK and SCLK audio clocks.

A NOTE FOR 24/88 FANS: Some have asked about 24/88 high resolution audio support (popular for SACD rips). While the ODAC doesn’t support 24/88, it does support the audibly identical 24/44. It’s trivial to re-sample 24/88 audio to 24/44 with no artifacts as it’s a simple divide-by-two operation (and one the operating system will perform for you automatically). I know many audiophiles probably think they’re losing something, but nobody has proven they are. Meyer & Moran demonstrated in a very in-depth study that even 16/44 audio sounded identical to SACD. Another good read is 24/192 Music Downloads. And if you refuse to believe all that, try resampling some 24/88 audio to 24/44 and compare them yourself with Foobar and the ABX add-on. It’s been done at HydrogenAudio and elsewhere always with the same result: Unless you mess up the resampling somehow, or change the levels, you can’t tell them apart.

TRANSPARENCY GUIDELINES: The What We Hear article offers information and references outlining guidelines as to what’s required for a piece of audio gear to genuinely disappear from the signal path and not alter the sound in any audible way. Here are what I believe to be relatively conservative criteria for audible transparency and the ODAC passes all of them:

• Frequency Response 20hz – 19 Khz within +/- 0.1 dB (Most DACs, due to the Nyquist limit of 22 Khz, start to roll off past 19 Khz when operating at 44 Khz sampling rate—the ODAC is down about 0.4 dB at 20 Khz). The widely accepted, but less conservative standard is +/- 0.5 dB (1 dB total variation) from 20 hz to 20 Khz.
• All Harmonic, IMD, Alias, Modulation, & Crosstalk Components Below –90 dBFS and total sum below –80 dBFS (0.01%)
• All Noise Components below –110 dB and total sum below –100 dBFS
• All Jitter Components below –110 dB and total sum below -100 dBFS

GREEN GUIDE LINES: A few months ago I introduced green guide lines on several of my measurement graphs to help show the worst case ideal performance. Some of these are slightly more lenient than the above criteria or take into account more detailed thresholds (i.e. that power line hum can be slightly higher in level than midrange noise). For now I’m keeping the green guide lines consistent with earlier reviews. But please note the ODAC meets even the tougher criteria above.

PARTIAL MEASUREMENTS: I’ve made LOTS of ODAC measurements including some things I’ve never measured before—such as true latency. For this article I’ve only shown some of the more common measurements. In a future article I’ll cover additional measurements, 16 bit operation, 24/96, as well as several comparisons to the DAC1, FiiO E10, and an HA-Info eBay headphone DAC with a well respected DAC chip. So, in the interest of getting this article done sooner rather than later, and keeping it to a manageable size, only a sampling of 24/44 measurements are shown below.

DYNAMIC RANGE: A DAC’s noise floor impacts Dynamic Range (DNR), audible noise, THD+N, and can even exceed jitter-induced distortion. If you have to pick a single number to evaluate real world DAC performance –60 dBFS dynamic range (DNR) is one of the most revealing. The guys in the white lab coats have determined DNR greater than 100 dB results in transparency under realistic conditions. And, if you want to adjust the volume in software, it’s best to have at least 110 dB DNR to keep the noise floor inaudible even if the downstream gain is left cranked way up. Anything beyond 110 dB is past the point of diminishing returns—it looks nice on paper but doesn’t help the sound quality. The ODAC is a very substantial 14 dB better than the FiiO E10.  Here’s both channels of the ODAC referenced to the 2.03 Vrms at 0 dBFS. Note the channels are very symmetrical indicating a careful PCB layout:

ODAC VS ESS: As explained earlier, the DNR quoted on datasheets is often something of a Holy Grail. The chip specs are typically from a very high quality AES/EBU or I2S laboratory quality signal (as output by high-end audio analyzers like a Prism dScope or Audio Precision). And they’re typically running from expensive ultra low noise bench power supplies costing thousands of dollars. It’s safe to assume the datasheet numbers were not made with USB data while running on USB power. ESS rates the ES9023 DNR at –112 dB A Weighted. The ODAC delivers –111.1 dB A-Weighted under the same conditions. In other words, even using USB data and power, the ODAC comes within a fraction of a dB of achieving the datasheet spec! I’m fairly proud of this aspect of the ODAC. It wasn’t easy.

CCIF IMD: This 19+20 Khz twin tone is a difficult test for many DACs running at 44 Khz. Old style (NOS) non-oversampling DACs especially struggle due to aliasing problems. In addition the output buffer (or I-V stage) in many DACs contributes high frequency distortion because the RC filter can be a challenging reactive load at these frequencies. If you look back through my reviews, you’ll find lots of products struggle on this test. Even the E10 turned in a marginal result. The ODAC, however, due to careful optimization of the output filter, and the superior digital filtering of the ESS DAC, does very well here with everything in the audio band well below 100 dB (both channels shown). Note also the 19 and 20 Khz tones are visibly equal in level which is not the case for many DACs:

 

SMPTE IMD: This twin tone test is more revealing of low frequency problems including power supply interaction. Again, the ODAC does very well with everything well below 100 dB (both channels shown):

 

100hz THD+N @ 0 dBFS: This test checks for clipping of the DAC at 0 dBFS and also shows the maximum output and channel balance error. You can see the ODAC produces 2.03 Vrms which is within an in significant 0.03 volts of the Redbook standard for digital audio. And even at 0 dBFS, the distortion is still 3 times less than what’s required for 100% transparency. The channels are perfectly balanced to within 0.001 dB. This spectrum is shown all the way out to 96 Khz and you can see the ESS DAC is well behaved even above 20 Khz with all noise still below about –110 dBFS. This is excellent performance:

 

THD+N VS FREQUENCY: Here’s the distortion performance at –1 dBFS from 20 hz to 20 Khz into a more challenging 10K load with a measurement bandwidth of 22 Khz. At 1 Khz the distortion is only 0.0027% and it remains around 0.003% over most of the audio band with only a slight rise up to 0.0048% at 9 Khz before the harmonics fall above the audible range. This is excellent performance and both channels are very closely matched (yellow vs blue):

 

NEW JITTER FINDINGS: I did quite a bit more research on jitter during the ODAC’s development. I’m also using a new dScope method that shows the same spectrum as before but now the symmetrical jitter components are marked (with a white “X”) and summed to obtain a total numerical value (previously the dScope was just showing the total residual noise floor). Having the single number (-103.3 dB below) made it easier to optimize the ODAC for the lowest jitter. The objective evidence conservatively indicates  if you keep all related components below -110 dB, and the total below -100 dB, the jitter will be entirely inaudible. Jitter creates dissonant distortion products in the audible band. It’s reasonable to assume if the audible effects of jitter are kept at or below the inaudible noise floor, they too will be inaudible. So the same levels of –100 dB and –110 dB that apply to noise also apply to jitter contributions. This is also consistent with various professional reviewers and their anecdotal opinions on jitter performance as well as my blind testing against the Benchmark DAC1 which has even lower jitter.

ODAC JITTER: The ODAC passes the conservative criteria with several dB to spare even on the worst-case J-Test signal. And the spread at the base of the signal (very low frequency jitter) is extremely minimal being entirely below –130 dB. It’s also worth noting the jitter here looks subjectively worse because the noise floor is much lower than most of my jitter measurements which are done with a 16 bit test signal. A 16 bit noise floors masks most of the “spikes” seen below. The ODAC also has negligible inter-channel phase error and essentially perfect pitch accuracy:

 

CONCLUSION: Hopefully the above provides some good evidence the ODAC delivers transparent performance. The next ODAC article will compare the 16 and 24 bit performance and I’ll be comparing it to several other DACs. I’ll be publishing many more measurements such as modulation noise, channel separation, square wave/impulse response, latency, frequency response, absolute noise, and more.

ODAC Update

ODAC PROGRESS: As my articles have slowed to a trickle lately, hopefully these 5000+ words will be welcome news. After lots of professional and other distractions, I’m once again devoting serious time to the Objective DAC (aka ODAC). There’s been some good progress and I’m feeling better then ever about the ODAC’s performance. I’ve been comparing it with my Benchmark DAC1 Pre and it’s been eye opening in some areas. Keep reading for more details and further background on the ODAC design.

QUESTIONS (FAQ): Before you post questions or suggestions in the comments please have a look at the FAQ section below to see if your issue has already been addressed. I’ve tried to cover the popular questions and issues.

NOT FOR PROFIT: Please note I’m not selling the ODAC, just helping design it. Just as with my other projects, like the O2 portable amp, any profit from sales will be to whatever companies decide to have them made and sell them.

ODAC + ODA: The ODA, for those not following the history, is the Objective Desktop Amp. It’s essentially a future desktop version of the popular O2 portable amp with a few added features and upgrades. ODAC development is taking priority for now because the ODA will be partially designed around it. The ODAC can also be used with the O2 and as a standalone device.

FILLING A VOID: There are simply not many reasonably priced DACs supporting 24 bit operation over USB under Windows without needing proprietary drivers. While there are some low cost options on the market, at least some of them, such as the FiiO E10 and NuForce uDAC-2, offer barely more than 16 bit performance. There are even fewer options if you want a high quality 24 bit DAC with a low impedance output that can drive most popular headphones well (this leaves virtually all pro audio DACs out in the cold). The least expensive option I know of that’s been properly measured and performs well is the Centrance DacPort with the 1 ohm output upgrade. But it’s close to $500. Most DIY DACs, such as those from Twisted Pear and AMB, only support 16 bit operation over USB or they require proprietary drivers under Windows.

ODAC + O2 RETROFIT: The ODAC fits neatly inside the O2 portable amp using the standard B2-080 enclosure by simply removing the batteries as shown to the right. One side sits in an enclosure slot while a mounting hole lines up with the central mounting hole on the O2 PCB anchoring the opposite side of the board. The battery terminals act as a “back stop” to absorb the force of plugging in a USB cable and the rear panel does the same when removing the plug. It worked out surprisingly well and I can’t take credit for this feat—my commercial counterpart came up with the idea and we mutually optimized the PCB layout to work with the O2. The retrofit requires a new back panel with an opening for the USB jack. At least one person has volunteered to design and hopefully sell a laser cut plastic panel for those wanting the O2/ODAC combo. Stay tuned for more but it’s a pretty simple retrofit that only requires soldering 3 wires at each end.

O2 INPUT SWITCHING: Those who only want a USB headphone DAC (like the FiiO E10 or NuForce uDAC-2) can simply wire from the output header of the ODAC to the P1 input header on the O2 board. But for those who still want to use other sources (like an iPod, etc.) with their O2, you can have your cake and eat it too. Once you cut the traces shown to the right you can wire the ODAC to terminals 3 and 4 of the input jack. These lead to internal switches in the jack and will connect the ODAC only when nothing is plugged into the O2.

ODAC STANDS ALONE: If you just want a DAC with no preamp or headphone amp, the ODAC can be used by itself as a USB powered 24/96 DAC. Because it supports 24 bit operation over USB there’s little or no penalty using only a software volume control or you can control the volume downstream of the ODAC. The board measures about 49mm x 58mm (1.9 x 2.2 inches) and will fit in many small enclosures such as the Box Enclosures B1-080. There are also several inexpensive eBay DIY enclosures that would work. Ignoring the output jack, it’s only about 4mm in total thickness as it’s entirely a surface mount design This allows it to easily “piggy back” on top of other boards, etc. In the O2 tradition, it has three mounting holes (geeks know 3 points determine a plane and 4 just mess things up ;). The components are also held back from two opposing edges for slot mounting. There’s a provision on the board for a 3.5mm stereo output jack that’s on the same edge as the USB connector requiring only one machined (or laser cut) panel. DIYers can also panel mount 3.5mm or RCA output jacks or simply build the ODAC into other gear. The ODAC can be connected to any input that accepts standard Redbook 2 Vrms line level audio and has at least a 10K input impedance.

O2 DESIGN PRINCIPALS APPLIED – The O2 was implemented using well proven design techniques and countless rounds of measurements and refinements. The result is performance well beyond what most would expect from the O2’s modest and inexpensive components. The O2’s popularity has shown this design methodology is valid. Put simply, the O2 has proven far greater than the sum of its parts. I applied the same approach to the ODAC design. While it uses relatively modest components, it delivers performance well beyond what most would expect given the cost and components used. The ODAC closely follows the chip manufacturer’s reference design information and has been carefully optimized through several iterations. Using a professional mixed domain audio analyzer, like the Prism dScope, enables a level of performance refinement that would otherwise be all but impossible. This gives the ODAC a huge advantage over most DIY DACs and products from small manufacturers lacking proper test equipment.

THE AUDIO ANALYZER ADVANTAGE: To make meaningful measurements of a DAC you need an ADC with a noise floor significantly (at least several dB) lower than the DAC’s noise floor. Otherwise you’re measuring the ADC as much (or even more so) than the DAC. Such an ADC is increasingly hard to find once you get up to Benchmark DAC1 levels of performance. The Prism Sound dScope, however, has a noise floor of just over 1 microvolt or around 126 dB below the Redbook standard of 2 volts. Even my $1800 Benchmark ADC1 can only manage a best case 119 dB and less expensive audio interfaces are typically significantly worse. It’s also extremely useful to have an audio analyzer capable of real time mixed (analog and digital) domain generation and analysis. A product like the dScope, or a high-end Audio Precision analyzer, is about the only way you can properly measure and develop a high performance 24 bit DAC. Otherwise the true performance is masked by the ADC you’re using and the limitations of using PC software that cannot do anything in true real time because of the PC’s operating system. Input isolation is also a significant issue. Using RMAA typically requires sharing the DAC’s USB ground with the ADC’s ground which, in itself, can create all kinds of erroneous results and ground loops. Properly testing a high performance DAC requires more than just another PC audio interface. You need specialized hardware designed for the purpose.

THE OTHER WAY: Lots of DIY, and even some commercial DACs, seem to be little more than a few trendy popular chips slapped onto a nice looking PC board. If their designers conducted the proper measurements along the way, where are their published results? You generally have to look at expensive products like those from Centrance, Benchmark Media, and Anedio to find meaningful DAC measurements. Some designers and companies simply quote highly misleading specs from the DAC IC datasheet but, as I’ve shown in my reviews, the results often fall far short. A good example is the FiiO E10 that only delivers roughly 16 bit performance even running at 24 bits. Some companies, like NuForce, Audio-GD, and Twisted Pear, argue they “design by ear” but the unavoidable human bias associated with such sighted listening is well documented.

IT’S ABOUT WAY MORE THAN THE RIGHT PARTS: Using high-end parts is meaningless if the implementation is (often unknowingly) flawed. It’s like putting a Ferrari engine in a Yugo or using ultra fast RAM in a PC with a slow processor and chipset. It’s pointless. Just as cars are defined by far more than just their engine, and PC performance depends on several subsystems all working well together, the same is true of audio gear. The best products are from manufacturers that conduct proper testing and have the resources and desire to sweat all the little details rather than just using the latest FOTM parts and making it look nice.

FOLLOW THE PROFITS: Sadly a lot of “specialty” audio designers and companies seem to depend mainly on subjective hype and sighted listening bias, rather than proper design and objective performance, to sell their products. One can argue some are mainly trying to cash in on the latest FOTM craze rather than investing the time and money to design genuinely solid gear. It’s one of the goals of this blog to help not only expose half baked designs for what they are, but also demonstrate better alternatives don’t have to be be expensive or made with dual phase aligned unobtanium.

SMALL CHANGES CAN EQUAL BIG IMPROVEMENTS: I’ve sometimes been amazed how even small changes have made fairly large differences in the ODAC’s performance. Several of the ODAC’s optimizations run counter to typical DIY audiophile beliefs. Here are a few examples of typical design myths:

• Larger Value Capacitors Are Better – “Upgrading” certain power supply capacitors to larger values made the ODAC perform significantly worse compared to using the values specified in the chip manufacturer’s reference design. Bigger value caps often have higher ESR, more inductance, and much higher impedance at very high frequencies. They can also create other problems.
  
• Top & Bottom Ground Planes Should Be “Stitched” Together – Stitching is the practice of applying a board-wide grid of small vias (plated through holes) that connect the top and bottom ground planes. Some argue this lowers the ground impedance and keeps ground paths shorter. But they’re not true ground planes on a 2 layer board. Instead you have a bunch of ground fill areas isolated by signal and power traces. When you have these “ground islands” rather than a true continuous ground plane, the stitching can easily create undesirable ground loops and send ground currents where you don’t want them. I’ve seen several DIY and commercial designs ignore this issue and other proper grounding practices.
  
• Expensive Audiophile Dielectric Capacitors Work Best – I experimented with various types of poly film caps, including audiophile preferred SMT Polyphenylene Sulphide (PPS) types, and found they sometimes made things worse. I also discovered not all ceramic caps perform equally. There’s no simple rule of thumb that always works. You have to make the right measurements and sweat the details. It’s time consuming but proved worth it. Every capacitor associated with the DAC chip and analog circuitry of the ODAC has been carefully optimized using the dScope. Overall, my capacitor tweaking resulted in lowering the noise and distortion by more than 6 db. And, trust me, the final result is not what your average DIYer would intuitively think is best.
  
• Fully independent Analog & Digital Power Supplies Improve Performance – While there are probably a few high-end DACs that measure slightly better with fully independent digital and analog power supplies, the reality is the DAC chip itself and related digital noise is more likely the dominant limiting factor. I’ve shown, with multiple measurements and tests, the power supply is not holding the ODAC back in terms of performance. And while there’s some carefully designed filtering and ground routing between the analog and digital sections, they’re both derived from the same source. A DAC will only perform as well as its weakest link allows. In an optimized design that weak link is usually the DAC chip and/or a certain amount of unavoidable noise from the USB and I2S digital buses. When that’s true, adding more esoteric power supplies won’t help much if at all.
  

PCB LAYOUT IS EVERYTHING: I can’t stress this enough. Just as the O2 board was designed for “function over form” so was the ODAC. Many DIYers and audiophile manufacturers want to show off their PC boards. So they often lay them out at least partly to look nice. That almost always means the performance suffers. In the case of a 24 bit DAC I’ve learned it might suffer quite a bit. Keeping the noisy digital signals out of the analog side of a DAC becomes challenging once you get past about 17 bit performance. And for those trying to judge if they got it right by using RMAA and a sound card, or an 8 bit oscilloscope with only 40 dB of dynamic range, good luck with that (see The Audio Analyzer Advantage above and my RMAA article for why). And good luck using typical sighted listening tests to verify a PCB layout.

BENCHMARK DAC1 REFERENCE TARGET: The Benchmark DAC1 models have been widely reviewed and praised—especially for their excellent measurements, sound quality, and hassle free 24 bit high resolution USB support. Hopefully many agree the DAC1 Pre is a worthy reference target to judge the ODAC against. While it’s a bit like putting a Mazda MX5 Miata up against a Porsche 911 it’s still a worthy goal. When I present the ODAC’s final measurements I’ll show many of the same results for the Benchmark and everyone can judge the end result for themselves. I’ll also be comparing the ODAC to the FiiO E10 as it’s probably the ODAC’s closest current 24 bit USB competitor in terms of cost and might even include a few pro audio interface results as well.

PRELIMINARY BLIND TESTING: In preliminary blind comparisons between the ODAC prototype and my DAC1 Pre, they sound the same. I’ll be doing more involved blind tests, but the initial results are very promising. If this trend continues I’ll be adding an “ODAC Public Blind Challenge” to my existing O2 and op amp blind challenges. If someone thinks they have a DAC that measures well and sounds better, let’s find out!

DESIGNED AND (for now) ASSEMBLED IN THE USA: This may not matter to everyone, but living in the USA myself, it’s something I take at least some pride in. Steve Jobs famously told President Obama “those jobs are not coming back” referring to Apple making nearly everything in China. And while Apple products are generally made to a high standard in China they’re still designed in the USA. The same cannot be said for a lot of reasonably priced “boutique” audiophile gear. NuForce, Audio-GD and FiiO are a few examples with well documented, and sometimes embarrassing, problems. And, based on what I’ve seen, a lot of the gear being sold mainly on eBay and direct out of Asia can be even worse.

USB POWER ADVANTAGES: A self powered DAC that operates entirely from USB power has a number of practical advantages:

• One Size Fits More – Because the ODAC needs to be commercially assembled in volume using automated equipment it’s much more cost effective to have a single version that works in the O2, ODA, standalone, and can be added to other DIY or commercial designs. USB power is virtually required to make this possible. Higher volume manufacturing of a single board brings the price down for everyone.
  
• O2 Compatibility - The O2 has a dual +/- 12 volt power supply delivering around 200 mA peak to each channel of the amplifier. The ODAC requires around 125 mA at a far lower voltage. The O2 cannot power a high quality DAC without adding a switching regulator or DC-DC converter which would add noise and still tax the O2’s power supply. There’s also no physical room for the added hardware. So USB power is the only viable option for the ODAC in an O2.
  
• Stand Alone Operation – For those wanting to use the ODAC by itself, USB power has a clear advantage. It eliminates the need for another power supply and is especially desirable for portable use.
  
• No Enumeration Problems – Some USB interface ICs are designed to be USB powered. They will not enumerate correctly with the host device if they’re already powered when the USB connection is made. This can necessitate using USB power for the interface chip while using a different power source for the DAC chip. This creates various other challenges including the next point.
  
• No Unpowered Inputs - USB power also simplifies problems associated with having unpowered IC’s connected to powered ICs. Digital ICs that are not powered don’t generally like being connected to ICs that are. It can result in a potentially destructive condition where the normally high impedance inputs of a powered IC represent a low impedance when the IC has no power. The powered IC pumps abnormal levels of current into the unpowered inputs. When power is applied with the IC in this invalid state it can “latchup” and draw large amounts of power supply current destroying itself. Some of the solutions to this problem can degrade jitter performance because the I2S bus is subject to this issue on a USB DAC with a split power scheme. Powering the entire DAC from USB power neatly solves these problems.
  

USB POWER CHALLENGES:  USB power also presents some challenges:

• Potentially Greater Noise -  USB power is more of an unknown compared to a dedicated power source. So it requires extra filtering and careful design to avoid degrading the performance of the DAC. This is especially true when you’re aiming for much better than 16 bit performance. The USB powered FiiO E10, Creative X-Fi Go, and NuForce uDAC-2 all promise 24 bit performance but only deliver around 16 bit performance. Likewise, when running from AC power, the latest MacBook Air also fails to deliver better than 16 bit performance from its 24 bit DAC due to extraneous power-related noise.
  
• Audio Output Voltage - USB power can be as low as 4.5 volts, and when you add in losses from power filtering and the DAC/op amp circuitry, you’re lucky to get 4 volts peak-to-peak of output swing without clipping. That works out to 1.4 Vrms which is a significant 3 dB shy of the 2.0 volt Redbook standard for digital devices. That’s 3 dB of potential dynamic range lost and a 3 db drop in level compared to normal home sources (like a CD player, network media player, etc). Indeed most of the USB DACs I’ve tested, and even popular DIY DACs like the AMB gamma, have this shortcoming.
  
• Capacitor Coupled Outputs - A single-ended DAC power supply usually requires an output coupling capacitor to block the 1/2 Vcc voltage at the DAC output. Such a capacitor, to drive a 10K load, needs to be a fairly large value to avoid low frequency roll off and excessive phase shift. It also should be a high quality film, rather than electrolytic, type. But many USB powered DACs, in the interest of saving money and space, use electrolytic or otherwise compromised output capacitors. This is even true of some audiophile DIY designs like the AMB gamma.
  
• Power Related Jitter - Power and ground “pollution” at various frequencies can have a significant impact on jitter performance. Noise from the power supply can, in effect, modulate the digital bit stream creating jitter.
  
• USB Maximum Current Limit - While the USB ports on any PC or laptop made in the last 6+ years can nearly always supply 500 mA of current there are some exceptions. There are a few ultra low power netbooks that have 100 mA USB ports and unpowered USB hubs are also, at least in theory, limited to 100 mA per port. I’m not sure about iPads and Android tablets but I suspect they may be rated for only 100 mA as well.
  

ODAC POWER DESIGN – The above challenges can be largely or entirely overcome with careful design and by using the right components. Here’s how the ODAC addresses them:

• Power Noise Below the DAC’s Noise Floor – It turns out ground and conducted (electromagnetic) noise are typically as significant as noise on the USB power line. Because a connection to the PC’s ground is required regardless, simply using an external power supply doesn’t automatically mean freedom from USB bus noise. With the ODAC the solution involved careful routing of ground currents, different power supply conditioning for the digital and analog sides, careful capacitor selection, and inductive filtering. The result is the latest prototype’s noise floor is mainly determined by the DAC IC itself not the USB power bus. Put another way, an independent power supply wouldn’t make much difference.
  
• Redbook Standard Audio Output Voltage – The ODAC delivers delivers the Redbook standard output of 2 Vrms without clipping by using a bipolar power supply. This alone adds roughly 3 dB of dynamic range to the ODAC’s performance compared to many USB powered DACs.
  
• Direct Coupled Output – The ODAC uses a ground referenced split supply so no virtual ground or output capacitors are required. This assures accurate low frequency amplitude and phase response with no bass roll off and avoids potential capacitor-induced distortion.
  
• Power Related Jitter – The ODAC’s power filtering was optimized not just for the best noise and distortion performance but also for the lowest jitter. Interestingly some of my attempts at filtering did help lower the power supply noise but increased the jitter. The dScope’s J-test proved invaluable in optimizing the PC board layout and power supply design for simultaneously low jitter and low noise.
  
• USB Current Limit – I’m not aware of any 24 bit USB interface and high performance DAC ICs that, combined, come in safely under 100 mA total. The bipolar power supply that allows a 2 volt output does require a bit more power. And the choices in 24 bit interface chips are very limited. The only way to keep the total budget under 100 mA would involve a much lower performance DAC and that’s not an acceptable trade off. The good news is I’ve tried the ODAC on many PCs, including two netbooks, and it works fine. It even worked on an unpowered USB hub despite being slightly over the 100 mA limit.
  
• Worst Case – If someone does encounter a USB port that is either extremely noisy or can’t provide 125 mA of current, the solution is simply to use a powered USB hub. They can be purchased for as little as $20 or so.
  

SO HOW QUIET IS IT? Using the industry standard A-weighted dynamic range test with a –60 dBFS signal, the current ODAC prototype has an impressive 112 dB of dynamic range. How good is that? My $1600 Benchmark DAC1 Pre, on the exact same test referenced to the same 2 volts, is slightly worse at 111 dB. CD quality audio, in comparison, has only 96 dB of dynamic range. I should note if you have an application where you can use the DAC1’s full 7+ volts of output, it can manage 116 dB of dynamic range referenced to it’s maximum output. So you do get something for your extra $1500.

ENOB: ENOB stands for Effective Number of Bits and is another measure of a DAC’s performance. No 24 (or 32) bit audio DAC can achieve true 24 bit performance, In fact, 20 ENOB is generally considered the “Holy Grail” of real world DAC performance. The ODAC is just under 19 ENOB and the Benchmark, even referenced to its full 7+ volt maximum output, is 19.3 ENOB. The FiiO E10, even in 24 bit mode, is only 16.2 ENOB.

DISTORTION: The ODAC prototype on a standard –1 dBFS 1 Khz signal has about 0.003% THD+N while the Benchmark DAC1 is only slightly lower at 0.0025%. The ODAC, on every distortion test I’ve run, is well below my ideal 0.01% maximum THD+N. That’s true even at 0 dBFS which wrecks havoc with some DACs (like the NuForce uDAC-2). The ODAC’s performance does not noticeably degrade in any way at 0 dBFS so I can officially certify it Lady Gaga compatible (scroll down to Lady Gaga In Audacity here).

JITTER: You can’t put a single number on jitter performance but I’m quite happy with the ODAC’s results so far. It can’t match the expensive ASRC jitter reduction used in the Benchmark DAC1 but it’s significantly better than most DACs, including the FiiO E10, and several of my pro sound audio interfaces. The ultimate proof will be in the blind listening tests.

IT MAY GET EVEN BETTER: It’s a bit like peeling an onion. As you improve one area of the design, the lowered noise and distortion reveals other areas that can benefit from refinement.While there was some hope the version on my bench now would be close to the final production ODAC, we’ve decided to go one more prototype iteration and incorporate several more incremental improvements. Some of those changes may improve the performance still further but it’s hard to know by how much. When it’s all said and done there will be at least 4 generations of ODAC PC boards. Despite the more than acceptable performance of the current prototype we’re not done chasing the Benchmark DAC1 yet.

---

FAQ (please have a look here before posting questions!)

---

WHAT WILL THE ODAC COST?: The ODAC is still on track to hopefully come in under $100 for a completely assembled, programmed and operational board ready to slip into an O2 or the upcoming ODA. If you want a standalone DAC you have to add at least a few bucks for an enclosure, panel and output jack. We won’t know the final pricing until the production design is bid out for assembly.

WHEN WILL THE ODAC BE AVAILABLE? Given we have another prototype cycle ahead of us the best case is likely late April if all goes reasonably smoothly. But that could slip into May or even later if there are unforeseen problems.

IS THERE AN OFFICIAL ODAC FORUM THREAD? Not yet but I’ll post an update as soon as there is.

CAN I BUILD MY OWN ODAC? Unfortunately no. The ODAC will be offered only as an assembled board. Please see the next question. (PS – You probably wouldn’t want to anyway. It has a lot of 0603 components which are really tiny and a fine pitch quad IC package which are no fun to work with).

WHERE IS THE SCHEMATIC & PARTS LIST? As explained in the first ODAC article, there are no DIY-friendly 24 bit USB audio chips that meet the design criteria. So we’re forced to use components licensed for OEM use that are not available through normal distribution. Even the datasheets are marked confidential and access to the interface chip requires signing an agreement. Given the ICs have to be purchased direct from the manufacturer in substantial minimum quantities it’s not reasonable to expect someone to make a substantial initial investment only to undermine them by giving the same design to other commercial interests. It would be a bit like designing a new car for Ford and then saying “oh by the way we’re also giving this to the Volkswagen Group, General Motors, and anyone else who might want it. You better hope there’s enough sales volume to divide between all of you.”  Having the ODAC be an open source design would only increase the financial risk and the end result could easily be a “chicken and egg” situation. Everyone might choose to wait and see who else might choose to produce the DAC. And, regardless, individual DIYers can’t get the required parts anyway. Hopefully everyone can understand it’s only fair the person taking all the initial risk, and spending the money up front, should not have the rug potentially pulled out from underneath them by sharing the same design to anyone with the commercial means to produce it.

WHAT DAC AND INTERFACE CHIPS DOES THE ODAC USE? For the same reasons we’re not releasing the schematic, we’re not yet releasing certain other details yet either. Please see the question above.

IS THE ODAC BASED ON THE XMOS USB INTERFACE? No. The XMOS solution requires a proprietary Windows driver that must be licensed for a fee. We also don’t believe the XMOS chip offers any real world benefits in this application. See the next two questions.

IS THE ODAC USB AUDIO CLASS 1 OR CLASS 2 COMPLIANT? The ODAC is USB Audio Class 1 (UAC1) compliant because it’s the only standard that allows native 24/96 support in Windows, OS X and Linux without any special drivers. USB Audio Class 2 (UAC2), while newer, offers no meaningful advantages for 2 channel audio playback and is not yet natively supported in Windows. Its main benefits are for multi-channel audio and recording at 24/192. Some of the best performing DACs available at any price are UAC1 devices (including the Benchmark DAC1 series).

IS THE ODAC ASYNCHRONOUS There’s a lot of marketing and other hype lately surrounding asynchronous DACs. Much of it is myth. Just like op amps got a bad name from the old 741 released many decades ago, so did USB audio based on early synchronous designs. But most modern USB audio devices use an adaptive interface where a local clock controls the DAC and is only loosely coupled to the PC’s timing. Contrary to popular belief, with an adaptive interface the data is not directly clocked by the USB port. This method has been refined over the years and can work very well. It’s also natively supported by all major operating systems. Most methods of true asynchronous USB audio require proprietary drivers under Windows and proprietary drivers are rarely a good thing. Judging a USB DAC by whether it’s asynch or adaptive is a bit like judging a car by the engine configuration—i.e. an inline six in a BMW vs a V6 in a Nissan GT-R. Other aspects of the design are more likely to limit the performance and there are plenty of examples of outstanding USB DACs using adaptive interfaces.

WHAT DIGITAL FILTERING DOES THE ODAC USE? Another trendy DAC topic is the type of output filter. It’s mostly about chip and DAC marketing types dreaming up added ways to differentiate their products. But, ultimately, what matters most is the measured performance at audible frequencies and listening tests using proper blind techniques. Judged by those criteria, I’m not sure either filter has an overall advantage. I have not seen any credible blind listening tests that support one filter type sounding better than the other. The ODAC, like many great DACs, uses a linear phase filter. Just like with asynch vs adaptive, there are many other variables that determine the overall performance. You have to consider the entire product as a whole. It’s foolish to dismiss a BMW or Porsche because the engine cylinders are not arranged in a “V” because, ultimately, that’s only a small part of a much bigger picture.

WHAT AUDIO FORMATS DOES THE ODAC NATIVELY SUPPORT? The ODAC supports 44, 48, and 96 Khz each at 16 or 24 bits. 24/192 is not natively supported by Windows over USB, nor does it offer any audible advantages for audio playback (in fact most DACs that support 24/192 perform worse at 192 Khz than at 96 Khz).

ARE YOU PLANNING TO MAKE MONEY OFF THE ODAC? No. I will not get any money from ODAC sales. Someone else is taking all the financial risk and coordinating all the manufacturing and sales logistics. If the ODAC proves popular, any profit is rightfully all theirs. Frankly, given the 4+ design iterations, steep minimum order quantities, and all that’s gone into it so far, they might be lucky to break even. But it’s ultimately no different than JDS Labs and Epiphany making a profit selling the O2. For me this remains an entirely not-for-profit blog and endeavor.

WHERE ARE THE MEASUREMENTS? Given the performance may change (hopefully for the better) with the next iteration it would be a lot of wasted work to publish detailed measurements and graphs only to do it all over again once we have the final design done. I made that mistake with the O2.

WILL THE ODA BE OPEN SOURCE? Yes it will. The ODA will be just like the O2, and anyone who wants to sell the ODA + ODAC can likely negotiate buying assembled ODAC boards with some sort of quantity discount. They might even want to weigh in before the initial production quantity is determined.

WHY NO S/PDIF INPUT? I know there have been several requests for S/PDIF, but as was explained previously, it’s not a trivial or even logical addition. To summarize, here’s why:

• O2 Compatibility – Having the ODAC fit inside the standard O2 is a pretty cool thing. O2 compatibility seems to have more interest than S/PDIF support. It would be very difficult to add an option for S/PDIF and still have the ODAC fit in the O2. If it’s even possible with a 2 layer board it would require components on both sides which would substantially raise the price for everyone. And it would likely have inferior performance due to a less than optimal PCB layout. Likewise splitting the ODAC into two different boards would also raise the cost in multiple ways.
  
• S/PDIF Doesn’t Fill a Niche – There are lots of 24 bit capable S/PDIF DACs as it’s much easier and cheaper to implement compared to 24 bit over USB. Even the $25 FiiO D3 does 24 bit over S/PDIF. But there is a shortage of reasonably priced 24 bit USB DACs. See Filling A Void near the beginning of this article.
  
• No Native Support - There are very few suitable 24 bit USB audio compliant (i.e. driverless) interface ICs available. Likewise there are not many relatively high performance DAC chips that work well in a simple 24 bit USB DAC design without a microcontroller. The chips used in the ODAC lack native support for an alternate S/PDIF or second I2S input and the lack of a microcontroller complicates things further.
  
• More Jitter – Adding an S/PDIF input option would mean “breaking” the I2S bus between the USB interface and the DAC to allow the insertion of an S/PDIF interface and/or switch. Ideally, for lowest jitter, you want to keep the I2S routing as short, clean, and direct as possible. With the current PCB size adding S/PDIF could easily compromise the jitter performance.
  
• Possible Future Version – There is more room in the ODA and it’s possible there may be a more expensive version of the ODAC someday as a “plug and play” replacement for the current ODAC. Such a DAC would likely use more expensive components and possibly a 4 layer PCB raising the performance bar even higher and could also more easily include an S/PDIF input. That way the cost is kept as low as possible for the mainstream version and those who want more can pay more. I also expect some better 24 bit USB audio interface ICs to come on the market compared to what’s available now. Some might even more DIY-friendly and suitable for an open source design.
  

Feb 1 Update

BLOGGER PROBLEMS: Just a quick update. I've been away from the blog for a couple of days only to find Google Blogger is having problems displaying more than 200 comments per article. Sometimes when you click “Load More” at the bottom of the comments it just gets stuck on “Loading…” and never refreshes the page with the latest comments. I’m not sure yet what the problem is, but if you can’t see comments posted in the last few days to articles with over 200 comments, that’s probably why. If I can’t find another fix, or Google doesn’t fix it, I need to find the time to go back and “clean up” the comments to keep the total number under 200 per article.

ODA/ODAC UPDATE: Many want to know when I’ll have the next ODA/ODAC article done. I’m working on it as time allows but that hasn’t been very often lately. I’ve been severely overloaded with other obligations, many of them unexpected, so progress has been much slower than I would like. Hopefully everyone can understand this not-for-profit blog, and related projects like the ODA/ODAC, are not my highest priority in life. I do plan on completing both the ODA and especially the ODAC. It’s likely the ODAC will be available first.

ODAC + O2 = USB DESKTOP AUDIO JOY: The current ODAC revision is designed to fit inside the O2 if you remove the batteries. This works even with the slim B2-080 standard enclosure. It’s not a plug-and-play modification, but anyone with a soldering iron and some DIY skills shouldn’t have any problem. The O2 will need to be powered from the AC wall adapter and the ODAC is self powered from USB. It will require a rear panel modification for the mini USB jack. Hopefully someone can design (perhaps with some key dimensions from me) a Front Panel Express CAD file for the rear panel. The ultra-high quality photo at right--the best image I could get with a flagship Nikon D4 DSLR ;)--clearly shows the ODAC PCB laying on top of the O2 PCB where the batteries usually hang out. Many are using their O2 for desktop duty so hopefully this might be a worthwhile option for some of them.

NEW SCOPE: Adding to my arsenal of test equipment is a new heavily optioned Agilent oscilloscope with one option that’s very applicable to this blog and the development of the ODAC—it has I2S audio decoding and triggering. So with the new scope I’ll be able to very precisely measure things like DAC chip latency and explore bit accuracy and other parameters in more detail. I’m not aware of anyone beyond a few of the chip makers publishing such data. The new scope can correlate digital audio bitstream data against the analog DAC output with nanosecond accuracy. Stay tuned for more.

A NOTE ON COMMENTS & QUESTIONS: Increasingly I’m receiving a lot of public and private comments that boil down to something like: “Would the UltraDAC XL or the UberDac 4000 be a better match for my TurboAmp and Mk II WonderPhones?” While I’ve tried to answer most such questions, the volume is becoming overwhelming. Often the questions have little to do with the topic of the article they’re posted under or even anything I’ve reviewed on the entire blog. Paid advice columnists typically only reply to a few such comments per week or per month but I’m getting several per day. Regrettably, I just don’t have the time to answer them all. I will still try to answer questions directly related to a specific article and those with broad appeal. So please don’t take offense if you don’t receive a reply to a question. I wish I could answer everyone but it’s turning into nearly a full time job and I have to pay the bills somehow.

Objective Desktop Amp (ODA) & DAC

INTRO: Ok, right up front you need to know this article is not one of my usual in-depth write ups. But here’s the deal. The battery powered O2 headphone amp has been more popular than I imagined with 2000+ PC boards already in circulation and more on the way. After thousands of O2 messages and comments I’ve learned many are interested in purely a desktop amp and many O2’s have gone into service sans batteries. Those considering an O2 for desktop use might want to read this “preview” article.

THE ODA: “ODA” simply stands for “Objective Desktop Amp”. The idea is to take the same objective principles behind the O2 and apply them to an amp optimized for desktop-only use (no battery power). Those principles include:

• Genuinely Transparent Performance – I wrote an article for InnerFidelity that describes what’s required of a headphone amp for transparent performance. I was even more stringent in defining the requirements for the O2. The ODA will meet or exceed the same requirements so you can listen to everything your music has to offer without your being forced to listen to your amplifier’s unwelcome contributions. This means no audible hiss, distortion well below audible levels, and a suitably low output impedance to maintain flat frequency response and proper damping with all headphones.
• Wide Headphone Compatibility – Music lovers shouldn’t have to worry about trying to figure out what headphones work well with what amp. The O2 has demonstrated it’s entirely possible to have one modestly priced amp that can accurately drive 98% of current headphones on the market.
• Focus On Design Rather Than Designer Components – The O2 demonstrates design and implementation is often far more important than using trendy expensive parts. The ODA places a similar emphasis on proper implementation to get the most from reasonably priced high quality components. As with the O2, I expect the ODA will outperform many other headphone amps using more exotic components and topologies.
• DIY Friendly Design – Several first time DIYers have already built an O2 and reported it worked the first time they powered it up. By avoiding surface mount parts, point-to-point wiring, and chassis mounted components, the ODA should be equally simple to build. Because DACs require surface mount components, and for other reasons, the optional DAC board will be available pre-assembled and tested (see below).
• Real Ground – The ODA will not have third channel masquerading as a virtual ground to mess up the performance. It uses a proper bipolar power supply referenced to true ground and direct coupled outputs.
• Maximum Value – Like the O2, the ODA is being carefully “value engineered” which takes into account everything from the size and complexity of the PC board to minimizing the number of different components on the parts list. Upgraded components are used where there’s a meaningful benefit or little price penalty. The ODA will use a low cost off-the-shelf enclosure.

ODA VS O2: So what makes the the ODA different and more desirable for desktop use? The main O2 thread on diyAudio is flirting with 1000 posts. And, collectively, there are another 1000+ posts and messages elsewhere. Some clear consensus stood out in what many wanted in a desktop headphone amp:

• Optional Internal 24/96 High Resolution USB DAC – The number one requested “upgrade” to the O2 is a DAC that’s designed and measured to similar standards of high performance. Just such a DAC is well under development for the ODA (see below).
• Higher Quality Inputs & Outputs – The O2, for size reasons, has 3.5mm input and output jacks. The ODA adds a 1/4 inch (6.5mm) Neutrik headphone jack for use with high-end full size headphones and RCA input jacks for higher quality connections and even lower crosstalk.
• Rear Panel Connections – The ODA has the power and input connections in back to keep the cables out of the way for a cleaner look and better ergonomics (the downside being another relatively expensive panel to buy).
• Wider Source Compatibility – The ODA should work with anything from an iPod Line Out Dock (LOD) to high output home DACs with a wide range of headphones. It has more flexible gain options compared to the O2.
• Higher Quality Power Supply – The half-wave power supply in the O2 is something of a compromise for size, cost, and battery charging reasons. The ODA power supply improves on the O2’s in several ways.
• Headphone Protection Relay – The O2, like many amps, produces a “click” in the headphones when you turn it on and a soft “thump” when you turn it off. The ODA uses a headphone relay to eliminate these noises and provide added protection for expensive headphones.
• Other Possible Upgrades – Depending on how the details sort out, the ODA may have some other upgrades as well such as a possible preamp/line output.

ODA + ODAC: What’s an “ODAC”? It’s a USB DAC designed the objective way—just like the O2 and ODA. It fits inside the ODA turning it into a desktop headphone DAC. Here’s some preliminary information:

• High Resolution USB – Relatively few reasonably priced commercial DACs, and almost no DIY DACs, support anything beyond 16 bits at 44 or 48 Khz over USB (despite most having 24/192 DAC chips). The problem is, until recently, there were no suitable options for high-resolution audio over USB without needing special drivers or spending lots of money. The ODAC will support up to 24/96 over USB.
• 111+ dB Documented Dynamic Range – The most dynamic range you can get from a DAC operating in 16 bit mode is around 96 dB with most falling several dB short of that. While that can be enough if you control the volume in the analog domain somewhere after the DAC, it’s often not enough if you want to control the volume at your PC in the digital domain. That’s where those extra high resolution bits show their stuff. Even with the PC volume turned down, you can still get 16+ bits worth of resolution and dynamic range. That can mean the difference between hearing noise or pure silence in your headphones. And, unlike nearly every other DAC out there, the Dynamic Range of the ODA will be fully documented. Many DACs just quote the spec for the chip off the datasheet which is usually nowhere near the entire DAC’s actual performance. See the Tech Section for a preview.
• USB Audio Class 1 Compliant – Unlike most pro-audio interfaces that support 24/96, the ODAC requires no problematic proprietary drivers for XP, Vista, Windows 7, OS X or Linux. It’s true Plug-And-Play. It also does not require UAC2 drivers, like some DACs, which are not provided in any current version of Windows.
• ODAC vs DAC1 – I’ve done some preliminary blind testing against my $1600 Benchmark DAC1 Pre with a variety of music and my best headphones, and so far, a least two different people cannot tell them apart. The DAC1 Pre has won a lot of professional accolades for being one of the better DACs money can buy at any price. I can see a formal ODAC NwAvGuy blind listening challenge coming up.
• Redbook Compliant Output – For many reasons, it’s good to have a DAC that complies with the Redbook standard for digital audio output levels. Many USB DACs, like the AMB gamma, can’t produce the required 2 volts RMS of output and suffer lower dynamic range. And some go way over the specification which risks overloading the inputs of a lot of equipment. The ODAC is Redbook compliant. Among other benefits, this makes it much easier to do level matched A/B comparisons with other DACs and digital sources that are also Redbook compliant.
• Pre-Assembled Board – Because USB and DAC chips are typically fine pitch surface mount devices they’re not very DIY friendly. To make matters worse, if you want 24 bits over USB with native Windows drivers, I’m not aware of any suitable chips that are available to DIYers. All the current solutions require licensing and/or some sort of contractual agreement. They’re not stocked by DigiKey, Mouser, Farnell, etc. And to further complicate things, they also require custom programming before they will operate correctly. That’s 3 strikes against DIY for high resolution USB DACs. The obvious solution was finding someone to co-develop the board with and handle all the contractual, assembly, programming and financial details so they could offer a pre-assembled board.
• Line Input Retained – The ODAC won’t defeat the line input on the ODA. You can use both sources or even plug another Redbook compliant DAC in for direct A/B comparisons (even better if a friend does the switching out of the listener’s sight!).
• Standalone Capable – The ODAC is designed so it’s also usable by itself for other applications while operating entirely from USB power. No ODA required.

NO MONEY FOR ME – There’s been some speculation that an assembled DAC board is a chance for me to finally make some money. But that’s not the case. There’s no money in the ODAC for me. As with the O2, and ODA, I’m leaving that part for others to sort out. This blog, and hence my reviews and commentary, remain entirely non-commercial.

WHAT THEY’RE NOT: A few things the ODA and ODAC are not:

• No S/PDIF – There’s little need for an S/PDIF output as it seems unlikely anyone would want to use the ODAC as a USB-S/PDIF device for another DAC. And while there are a few applications for an S/PDIF input it’s not something most will ever use. For everyone else, an S/PDIF input would add complexity, size, and substantial cost with zero benefit and may degrade jitter performance. That’s not keeping with the “best performance for the lowest cost” O2/ODA approach.
• No Balanced Outputs – Balanced audio is great for pro use with long cables and electrically hostile environments. But it’s generally more of a liability than an asset in home audio gear. All else being equal, balanced stages usually have higher overall noise and distortion than their unbalanced counterparts. Headphone drivers don’t magically somehow work better when driven by a balanced source. Most of the stuff you hear and read about balanced audio for home headphone gear is myth, hype and even snake oil.
• No Arc Welding – If you’re looking for an ultra high output amp for those vintage AKG K1000s, or to weld that broken light fixture, the ODA isn’t it. Just like with S/PDIF, it doesn’t make sense everyone should pay for power they can’t use (and that could also more easily damage their headphones) just to support a tiny minority of potential users with unusual requirements.
• No Battery or USB Only Power – The ODA isn’t intended to be portable. That’s what the O2 is for. The ODA needs a proper power supply for the best performance and should outperform any USB powered headphone DAC. The ODAC board by itself, however, can be powered from USB for use in other applications.
• No Recording – The ODAC is playback only. If you want a microphone input for Skype calls, check out the $12 Syba CM-119 I reviewed. If you want to record a string quartet, there are lots of great reasonably priced professional USB audio interfaces optimized for recording rather than playback.

DAC INTERPLAY: I’ve held back on some ODA design details waiting to see if the ODAC was going to be a reality. So in some ways, the ODAC has come first. The ODA PC board layout and other details are subject to change greatly depending on what form a DAC option might take. Because I want to remain non-commercial and not profit from any of this, I had to find someone interested in offering the ODAC for sale in pre-assembled form. It might be a bit premature, but so far the ODAC is looking viable and I’m very pleased with the performance and testing so far. It’s been a joint effort.

HOW MUCH? Many have asked about the cost. I’m guessing the ODA itself will be around $30 more than a complete O2. That would put the total DIY price somewhere around $130 without the DAC option. Just the board would be under $70 complete and fully functional if you build it yourself. And I’m told the optional assembled, programmed and tested ODAC board should be under $100. These are all just estimates and subject to change. The more popular the ODA and ODAC become, the lower the price will probably go.

WHAT’S NEXT & TIMING: My current plan is to publish another ODA/ODAC article sometime in December with more details, more test results, etc. Depending on how things go, the ODA documentation package (including PCB artwork), and even the ODAC board, could be available as early as late January. Being realistic, February is more likely and it might slip into March. It all depends on how many board revision cycles are required, what changes/problems come up, etc. The O2, for example, had a last minute re-design and delay because the main distributors ran out of volume controls and a few other critical parts. For now I just wanted to let everyone know there’s progress, and if you’re thinking about an O2 for desktop use, or perhaps another desktop headphone amp or headphone DAC, you might want to wait for the ODA.

 

---

TECH SECTION

---

DYNAMIC RANGE BACKGROUND : Probably the biggest spec thrown around by DAC chip makers for bragging rights is the dynamic range (DNR) performance of their chips. Some sleazy audio DAC marketing types just publish the number from the datasheet as the spec for their completed DAC. But that’s usually cheating in a big way and a bit like saying the tires on your car are rated for 155 MPH so that must be how fast the car can go. Uh, no. But then again some of those designing and selling USB DACs probably have no way to measure the actual DNR so perhaps that’s why they cheat. Getting even close to the chip’s ultimate performance requires great care with the power supply, PC board layout, grounding, and more. For 112 dB of dynamic range with the Redbook standard output of 2 V rms at 0 dBFS, there can be only 5 microvolts of total noise. That means all the noise in the audio band added together has to be less than 5 microvolts! That’s hard to do even if the DAC chip itself were somehow noiseless and that’s hardly the case with lots of noisy digital signals just a few millimeters from the analog pins and noisy USB signals not much farther away.

ODAC DYNAMIC RANGE: Shown below is the performance of an early prototype of the ODAC. The final design could be worse, but hopefully might be even better. Regardless, the results show nearly 112 dB of genuine dynamic range (A-weighted or “dBA” as is the industry standard). Most USB DACs are limited to 16 bits and around 90 dB plus or minus a few dB of dynamic range. I used a –60 dBFS signal instead of my usual –90 dBFS as the former seems to be more of an industry standard for this measurement:

 

WHY 24 BITS CAN MATTER: Many find it convenient to leave their headphone amp turned up and use the volume control in your PC’s operating system or player software. For one thing it allows locating the headphone DAC out of reach and/or out of sight. But when you do that with a typical DAC capable of only 16 bits over USB you get less than 16 bit audio delivered to the DAC at anything less than maximum volume on the PC. Turn it down only 6 dB and you have 15 bit audio. Another 6 dB and you’re getting only 14 bits. And so on. Here’s a graph of the ODAC running in 24 bit mode in blue. It’s playing a 6 Khz test track just below maximum at –1 dBFS but the volume control in Foobar was turned down about –45 dB. So the end result is a signal that’s about –46 dBFS being sent to the DAC. In 24 bit mode the THD+N was impressively low at only 0.01% relative to the –46 dB signal. Shown in yellow is the much higher noise floor in 16 bit mode without changing anything else. The dScope can’t show FFT readings from previous sweeps easily, but the 16 bit THD+N was nearly 20 dB worse at about 0.08%—or eight times as much noise and distortion in 16 mode (As has been pointed out in the comments, the readings in this graph may not be correct but the spectrum is correct. I plan to re-run the test):

 

MORE TO COME: Check back in December for more on the ODA and ODAC!