Headphone Amps/DACs Explained

INTRO: Prior to posting some headphone amp and DAC reviews I wanted to cover some common issues with amps and DACs. Some wonder if they need one at all. Others wonder which is best.

HEADPHONE DACs vs DACs: This article deals mainly with the headphone amp section of headphone DACs. I will be covering the digital aspects of DACs in more detail in a future article.  Many want to know if they can get better sounding by adding a dedicated headphone amp or headphone DAC That’s the main focus of this article.

IS AN AMP OR DAC NEEDED? The short answer is: "probably less than people think". A lot of people have no real complaints with the sound they're getting but they read others claims of huge improvement in the sound by adding often fairly expensive hardware. But sometimes the perceived improvement is mostly psychological--a well documented phenomena much like how a more expensive wine often tastes better than a much cheaper wine until you put them both in plain brown paper bags. See: Subjective vs Objective Debate

FIXING REAL PROBLEMS: Some have good reasons not to be happy with the sound they’re getting directly from their device. It might have audible noise, not get loud enough, or otherwise be a poor match for their headphones. For these folks an amp or headphone DAC may indeed be worth exploring. But they're not a cure-all. They can, in fact, create more problems than they solve so don’t just run out and spend your hard earned cash on some magic box without doing some research.

THE CONVENIENCE FACTOR: An external AMP or DAC creates more cabling, many need power (or to be recharged), may force you to use a different volume control, etc. This generally makes things less convenient than just plugging your headphones directly into the source—especially for portable use on the go. So the hassle needs to have some real payoff or it doesn’t make sense.

DO AMP/DACS SOUND DIFFERENT? Sometimes amps and headphone DACs do sound different but not to the extent, or as often, as many might think from reading all the subjective reviews and comments. Here are most of the more common reasons:

• Insufficient Power – When a headphone source runs out of power with typical compressed pop music you’ll typically either hear distortion or it simply won’t be loud enough with the volume all the way up. With more dynamic music, like classical or well recorded jazz, just the peaks may be clipped so it might not be as obvious there’s a power shortage. Power is a function of voltage and current. And some amps, with a given headphone, run out of current first and some run out of voltage first. See: More Power?
• Non-Zero Output Impedance – Output impedance is one of the more common audible differences between anything with a headphone jack. As a simple rule of thumb, if the headphones are less than eight times the output impedance, the amp can alter the sound. So if you have the FiiO E9 amp, which has an output impedance of 10 ohms, you can only use headphones that are at least 80 ohms, otherwise the amp might change the sound. Breaking the “eight times rule” can result in muddy bass and even audible frequency response changes—especially with balanced armature in ear monitors such as Shures, Etymotics, Ultimate Ears, etc. See the Impedance article for more.
• Inaccurate Frequency Response – Some amps roll off the bass and/or high frequencies. The bass can be rolled off due to capacitor coupled outputs which are generally used a cost saving measure in many portables, laptops, and PCs. And capacitor coupled outputs are usually required in single-ended amps and many tube amps. Other tube amps use an output transformer that also limits the bass response and can also roll off the highs. And some solid state amps roll off the highs on purpose just so they can sound “different” (some might say “warmer” others might say “dull”).
• High Distortion – There’s a lot of debate about what qualifies as “high”, but a fair amount of research has been done so there are at least some good clues. Most single-ended, tube, “low feedback (NFB)” or “zero feedback (NFB)” amps have relatively high levels of distortion and it may be easily audible. While some argue this distortion can “sweeten” or otherwise somehow improve the sound, there’s no denying you’re no longer listening to the just your music but you’re also forced to listen to your amp. It’s sort of like having someone softly murmuring in the background along with your music. That’s not my idea of a high quality amp, but to each their own.
• Noise – There are different kinds of noise. Computer headphone jacks, for example, can have whines, buzzes, clicks, chirps, etc. related to all the digital circuitry in the computer. Some portable gear can have similar noises. Headphone amp noise is usually hiss and/or hum. It’s generally more of a problem with sensitive headphones—especially balanced armature in-ear types. Noise can vary with the volume setting, types of connections, etc. See: Noise & Dynamic Range
• Insufficient Gain – This usually isn’t an issue with a USB headphone DAC. But with standalone amps it can be. For a given source the amp might not get loud enough even though, using the right source and the amp has enough maximum output capability. This can be due due to insufficient gain. See: All About Gain.
• Impedance Mismatch – This is a somewhat redundant category as an impedance mis-match will create one or more of the above problems. But it’s worth mentioning many headphone sources are optimize more for low or high impedance headphones but not both. For more see: Headphone Impedance
• The Snake Oil Effect – Some subjective audiophiles will tell you the above don’t matter as much as simply “trusting your ears” (or often you’re supposed to trust their ears even more). To be blunt, study after study has proven them wrong when you slip whatever they’re listening to under a brown paper bag or bedsheet so they don’t know which is which. Once they don’t know what they’re listening to, the Snake Oil Effect disappears and you’re left with all the parameters above determining the sound quality. If you’re skeptical, you might want to check out Subjective vs Objective Debate.

A ROUGH GUIDELINE: If you have headphones designed for portable consumer use that have an impedance of 80 ohms or less it's very likely the manufacture designed them to be driven directly from a typical device like an iPod. Using an amp or DAC with such headphones usually won't improve their sound much, and might even make them sound worse or create other problems like excess noise and channel balance issues. This is especially true if your headphones already get loud enough for your tastes. Generally, 16 – 32 ohm headphones with a sensitivity rating of at least 100 dB/mW do not need an amplifier. The exception might be balanced armature IEMs due to output impedance issues. See: Output Impedance

iPOD DACs: I think most of these are a waste of money as most modern iPods already have very respectable DACs in them. For example, the iPod Touch 3G’s DAC outperforms the one in the popular NuForce uDAC-2. I haven’t tested all the latest iPod’s, but in general, their DACs already approach the point of diminishing returns. So it’s rather difficult to significantly improve on them. See my Sansa Clip+ review for some iPod tests. And an external DAC can make jitter worse because to get the signal to the external DAC the critical digital audio clock (the very source of jitter in the first place) has to be embedded along with the audio data and then extracted at the other end. This introduces a new, and potentially significant, source of jitter that’s not present with the internal I2S interface inside the iPod. To be brutally honest, most external iPod DACs are just manufactures looking to make more money off the “iPod audiophile revolution” not products that make sense.

PC DAC? If your source is a PC or Mac, should you use an outboard DAC? It depends. If your PC is your main music source and you have a very high quality speakers (which excludes 95% of “desktop PC speakers”) and/or headphones in the $200+ category, then yes you might want to consider an outboard DAC—especially if you can hear any audible flaws when using the one in your PC. But a lot of PC’s have respectable DACs in them. If the problem is your headphones not getting loud enough, just an amp may be enough. Even something inexpensive like the $20 FiiO E5 might do the trick.

EXTERNAL USB DAC ADVANTAGE WITH WINDOWS: Because an external DAC is an added audio device, you can divide up your PC sound sources accordingly. Have you ever wanted to listen to just music and still have all the other sounds on your PC play through your desktop speakers (so you don’t get surprising blasts of Windows sounds in your headphones while listing to music)? This is easy to do in Windows 7. You can also do it in Vista and XP if your player supports assigning itself to a different audio interface (like Foobar 2000 and many others do). This also lets you set the volume for the external device to maximum for bit accurate audio streaming while keeping everything else set lower if you want. See the Computer Audio Setup guide by Benchmark for more tips.

GOING SHOPPING: I was shopping myself for a decent headphone amp that would work well with a variety of headphones and sources and didn’t cost more than a nice new laptop. You can read about what I found here: Going Shopping Ultimately, I couldn’t find a decent “one-size-fits-all” amp so I designed my own and have shared the design as open source hardware. It’s called the Objective2 or O2.

BOTTOM LINE: I’ve tried to present the basics above. Sometimes it’s obvious when an external amp or headphone DAC is needed. But often it’s much less obvious. Before buying anything I would strongly suggest also checking out the following articles (also linked in the text above):

• More Power?
• Headphone Impedance
• All About Gain
• Noise & Dynamic Range
• Subjective vs Objective Debate
  

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TECH SECTION:

AMP CATEGORIES: In terms of their design, I like to divide headphone amps (or the amp sections of headphone DACs) into five broad categories:

• CMOY Single Op Amp – There are many variations of these but they’re all at least somewhat similar to the original Chu Moy design. Grado offered the RA1 which has been frequently cloned. These amps were originally an inexpensive way to drive higher impedance headphones. They tend to have trouble with low impedance headphones as most op amps just can’t deliver the current required. And even for higher impedance loads a single op amp handling both gain and output duties involves some significant compromises. A lot of pro sound interfaces, and even dedicated headphone amps, use this sort of design. The Behringer UCA202 is one example. These either require 2 batteries, a dual power supply in a desktop amp, or they use capacitor coupled output or a rail splitter/virtual ground. All the single supply versions are typically flawed. See Virtual Grounds for more info.
• Multi-Stage IC Amps – These are typically an improvement on the single stage Cmoy-style amps because the gain stage can be isolated from the output stage. This can lower noise and distortion as well as improve stability.
• Discrete Designs – Some headphone amps either don’t use op amps at all, or they use them only for the input stages. The output circuits use discrete bipolar transistors or MOSFETs sometimes operating in Class A for lower distortion. These amps can typically deliver much more current and generally are better suited to lower impedance headphones as well as high impedance models (with the appropriate gain). But many are poorly designed and have no serious measurements, or often even specs, to back up their performance. Discrete design is tricky to get right. The Violectric V100 is an example of a well engineered amp with a discrete output stage.
• Buffer Designs – Companies like TI and National Semiconductor have developed output buffers well suited for driving headphones. They’re essentially small self-contained power output stages capable of relatively high power levels and very low distortion. One example is the National LME49600. These can offer extremely high performance—often higher than discrete designs. The Violectric V90 is an example of such an amp, as is the Headphone amp built into the Benchmark DAC1.
• Headphone Chip Amps – A lot of portable devices use self contained headphone chips. They tend to be optimized for low power consumptions and to work best with typical portable low impedance headphones. A good example is the TI TPA6130 used in the FiiO E5 and FiiO E7. The PA2V2 uses the LM4881 which is an inferior headphone chip amp that requires capacitor coupled outputs and has more limited performance.
• Esoteric Designs – The Nelson Pass “Zen” single ended MOSFET amps, single ended triode tube amps, tube amps, Schiit Audio amps, etc. fit into this category. These typically “minimalist” amps usually have rather poor measured performance often with audible distortion. But they have their own loyal following for people who seem more interested in listening to their amp’s contribution rather than the actual music.

MAC's & PC's CAN MESS THINGS UP: Even when you play a high quality FLAC file in say Foobar 2K you're still at the mercy of the operating system which has the ability to include all the other sound sources on your PC into the mix. And that might include things like the microphone input which tends to be very noisy. So, at the least, make sure you mute or disable as many unwanted sound sources as possible. If you have an outboard DAC you can set it up so your music source feeds it while everything else plays through the PC’s sound hardware. See the Computer Audio Playback Guide for more.

MAC's & PC's CAN DEGRADE RESOLUTION & ACCURACY: Depending on what computer, sound hardware, and operating system you have, your computer may not, by default, stream "bit accurate" audio even with an external DAC. On OS-X, before Leopard, the operating system took it upon itself to resample most audio. Resampling can have audible artifacts and isn't what you want if you're trying to play back that pristine FLAC file on your expensive outboard DAC. And before Vista and Win 7, Microsoft XP's built in "mixer" had some known problems delivering bit accurate streams. And there are often settings in the operating system or sound driver that let you configure various options like optimizing for small speakers, big speakers, headphones, etc. These settings amount to DSP and they alter the audio in ways that may degrade the sound of an external DAC. For more information about bit accurate streaming and how to assure you're getting it (when possible) check out: Computer Audio Playback Guide

USB POWER IS NOISY: Many of the reasonably priced USB DACs are USB powered. The USB power bus suffers from lots of noise. The very wires that deliver the DC power are right next to high speed noisy data signals bundled into the same cable. It's also power shared with other USB devices on the system which may even include things like RF Bluetooth or WiFi "dongles" which add RF noise.

VOLUME TRACKING: When you use efficient headphones on a high gain headphone amp, you will likely find yourself only using the first third or so of the volume control. This isn't good for several reasons. The obvious one is tiny movements of the control make relatively big changes in volume. And, due to the way exponential volume controls and voltage dividers work, the tracking between the right and left channels is usually much worse at the lowest settings. So using only the first third of the range means you're far more likely to encounter audible channel balance problems. Generally anything more than 1 dB of error can be audible. NuForce used an especially low quality pot in their uDAC-2. The one I tested had over 10 dB of error at the lowest audible settings!

BOTTOM LINE: If you’re happy with the volume and sound quality you have now, I’d strongly suggest simply enjoying the music and leaving well enough alone. If you think you need an amp, but are not sure, consider the $20 FiiO E5 as a toe in the water to see if you an amp will really help. For little more than the price of a CD or two you can try out a very decent little amp. If you want a more serious amp, I’m hoping my own O2 amp design will become commercially available offering a lot of performance at a very reasonable price. If you can’t wait, I’d suggest using products like the Violectric amps as your benchmark to compare others against. If whatever amp your looking at has limited or sketchy specs, you might want to do more research. If you’re after a DAC, the Benchmark DAC1, Grace Designs products, Anedio D1 and similar products can serve as excellent benchmarks to compare other DACs against. Again, if a manufacture can’t provide detailed specs, think twice. They probably either never even properly tested their product (as with the NuForce uDAC-2), or if they did, they’re embarrassed to share the results.