February 17, 2011

Headphone Amps/DACs Explained

fiio-e5-boxedINTRO: 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):


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.


  1. Flippin' 'eck this blog is wonderful. Why aren't there more comments?

    This page alone just saved me a fortune, quite apart from being a great read.


    passing head-fi person.

  2. It really is pretty awesome, some very useful info. =)

    (another head-fi person)

  3. Time to review the Pure i-20, which to me, is alone among iPod docks for pure digital output. Check it out. Nice blog. Thanks.

  4. Hi, how much is it really worth in investing in a DAC coupled with a mediocre amp ?
    I have a pair of Dt880 (600 ohms), I was going crazy trying to decide which combo of DAC and amp to use
    1] HRT pro with Fiio E9(desktop amp)
    2] IPOD + HRT istreamer(for idevices) + Practical devices XM5 portable headphone amp

    I finally bought the HRT pro and Fiio E9 combo - now people are telling me I'm crazy to buy this, my main criteria was portability and hi end sound quality
    I can't buy anything which does not fit in a backpack/suitcase.. is this combo a good idea or should I return this and get the second combination of IPod,Istreamer and Xm5 ?
    Any advice will be appreciated ..:-)

  5. EvilEvo, I think it's overkill but if you're happy stay with what you have. And you don't need an iStreamer to feed your E9.

    All the FiiO products I've had experience with, besides the E3, work impressively well--especially for the price. The DT880 600 ohm cans probably would tax the voltage limits of the FiiO E7 if you like it loud or listen to really wide dynamic range music. But you still might want to try as it's cheap and very portable?

    You can drive the E9 directly with one of the recent generation iPod's (Touch/Classic) with a cheap eBay LOD cable and it should sound great. You don't need the iStreamer.

    The XM5 is basically a cheapo single op amp Cmoy with a bunch of useful extra features added including a low-end PCM2704 DAC. The DAC in the FiiO E7 and iPod Touch 3G is better than the XM5's. I also don't know if has enough voltage output to handle your 600 ohm Beyers properly.

    The XM6 is better as it has a buffered output, and like the FiiO E7, a Wolfson DAC. But it's really expensive for what it is. I've run a full set of measurements on the E7 so look for an article soon.

    1. i searched this blog but it seems you have said nothing else on FiiO E3
      it didn't impress you, no?

    2. i searched your blog and it seems you have not said anything else on FiiO E3
      it didn't impressed you, no?

  6. thanks !!!
    I knew this was the right forum/blog for this kind of stuff .:-)
    I was considering various combination of DAC and amp for powering the Dt880s - based on only 2 criteria Sound quality and portability.

    Also based on your inputs I'm listing the various combo's as per my understanding:
    I'm eliminating the DAC on the E7 and the XM5 from the equation as they are not as good as compared to the HRT DACs.(lets assume for now)
    So I'm keeping the HRT DACS as a constant in the following chain of combinations

    [Group A]Combinations with IPOD:
    1] IPod --> Dt880
    Ultra portable but inbuilt DAC and Amp of Ipod will not be anywhere near the dedicated DAC and Amp quality
    2] IPod --> Istreamer --> XM5(amp)-->Dt880
    Ultra portable, DAc is better than the "inbuilt IPod DAC" but the DAC of Istreamer will not be as good as that of the "HRT Pro"
    The XM5 amp may also not be that good as compared to the E9 or E7
    3] IPod-->Istreamer -->E7(amp)--Dt880
    In this chain the DAC is better than the inbuilt Ipod but not better than the Pro, and the amp is also better than inbuilt IPOd but not better than the E9 amp
    4]IPod-->Istreamer -->E9-->DT880
    In this chain the DAC is better than the inbuilt Ipod but not better than the Pro, and the amp is also better

    [Group B]Combinations with Laptop
    1] Laptop --> HRT Pro --> XM5(amp)--DT880
    DAC is excellent
    Amp not that great
    2]laptop -->HRT pro --> E7(amp)-->Dt880
    DAC is excellent
    Amp is now better
    3]laptop -->HRT pro --> E9(amp)-->Dt880
    DAC is excellent
    AMP is excellent considering its still quite portable

    If u have read so far ..thanks for the patience :-)

    So with all those combinations I narrowed it down to:

    (Ultrportable + good sound)category as:

    (Not so portable + awesome sound) category as:
    Laptop-->HRT pro-->E9(amp)--DT880

    Out of these 2 I narrowed it down to
    I know Im repeating this but just wanted to let u know what I was trying to do..and now based on this would you advise anything else or suggest anything else .I can still return some stuff and buy new stuff.

  7. Yeah, HRT>E9 is a great combo. And for portable use I'd go with iPod>E5 or iPod>E9.

    iPod Touch 3G into 600 Ohms = 0.5 mW power
    FiiO E5 into 600 ohms = 2.8 mW
    FiiO E7 into 600 ohms = 6.0 mW
    FiiO E9 into 600 ohms = 80 mW (claimed)

    The little $20 E5 might deliver enough volume with your Beyer's for when you're away from your laptop. The E7 will do even better. Neither comes close to the E9 but it's likely way more output than you need.

    I seriously doubt the iStreamer can better the iPod Touch 3G/4G internal DAC in any audible way (despite all the claims it does). Blind listening tests have proven it's nearly impossible to hear any meaningful differences between DACs that measure well. And the iPod Touch DAC and output circuitry measures better than a lot of home gear or supposed "audiophile" products like the NuForce uDAC-2.

    The only reason to add anything outboard to an iPod Touch is to get more output to drive difficult headphones. And for that, you just need a decent portable amp--like the E5, E7, etc.

  8. some more info

    HRT pro >
    Good for professional applications or very hi end gear which can make use of the balanced output. If the chain including the amp and headphones are balanced this is a good choice. It supports both balanced as well as unbalanced mode but if primary purpose is
    running it in an unbalanced chain then might as well go for the II+ version.

    The Pro only provides mini xlr outputs, to get output from the only option I could see was the "cardas mini femal xlr" to RCA or regualr size XLR, in unbalanced mode the "mini xlr to RCA" is the only option which costs about 100$
    + we will need the rca to mini cable to conect to the Fiio E9, all this will add additional 100- 120$ to the cost of the Pro even if running in unbalanced.
    (there maybe cheaper cables out there but I am not aware of any for the mini xlr to rca etc)

    HRT II+ >
    Same as the Pro version but not having balanced outputs.
    Even the DAC is the same as that of the Pro version "TI PCM 1794"

    HRT II + will be a good option as it will give the same performace plus its cheaper ..
    I got confirmation from Kevin Halverson himself as I had lots of queries over this confusion of Pro vs II+ version..running in balanced/unbalanced mode.

  9. Hello NwAvGuy!

    After reading your review of the FiiO E7 I have been thinking about getting one. However, I'm not sure if it's a waste of money for my purpose so I would like to ask for your opinion if you don't mind. ^_^

    I use a pair of DT880 600 ohm headphones with a laptop and foobar2000. Everything is fine except for one thing. When doing other things (e.g. scrolling webpages, etc.) while listening to music I get sporadic "disturbances" (clicky noise) from time to time.

    I tried the Behringer UCA202 interface as I thought an external device might solve this issue, however, with no luck. The disturbances still occur. So far the only way to "fix" this problem was using ASIO4all.

    Unfortunately, no other application can access/use the audio device when ASIO4all is active resulting in no sound in e.g. Skype, Firefox, etc. while listening to music. -_-

    So I thought that the E7 may be up to the task. What would you do in my case? Thanks a lot for your reply in advance.

  10. As long as it has enough power for your tastes, the E7 should solve the "noise" issue with your computer. 600 ohm headphones are not the E7's forte but it should do better than your laptop's headphone jack.

    A lot of laptops (and some desktops) suffer from assorted odd noises in the background due to the audio circuitry being crowded onto the same motherboard with all the high power computer circuitry.

  11. I recently discovered, after watching some movies/anime again after a long break, that I need a stronger amp for my modded Fostex T50RPs. The combination of subtractive EQ (-12db or so to everything but the deep bass) and volume eating surround virtualization mean that even at maximum volume my Maverick D1 DAC/amp combo isn't strong enough for movie soundtracks with lots of dynamic range.

    The D1 is rated by the manufacturer at 1W into 32 ohms. It doesn't say what the gain is and I'm not sure my cheapo multi meter could even measure it properly. The T50RPs have an impedance of about 50 ohms and are very optimistically rated at a sensitivity of 98dB/mW stock and modding further reduces it with extra distance between the driver and ear as well as more damping of both the driver and its enclosure. For comparison, my HD650s at around 12 o'clock on the dial are about as loud as my T50RPs at full tilt with the same EQ/DSP settings.

    To remedy this, I'm looking for a wire with lots of gain that won't start hissing or picking up noise when pushed to the limits of the volume dial. I've been following the Schiit Lyr and while it will have power to spare, I'm a bit concerned about the tubes adding too much "flavor". I have some tube gear already and for me, its coloration has turned out to be rather hit or miss. Schiit's Asgard doesn't have that problem but it leaves me a little more worried about headroom. Those are the only headamps I can think of that are both cheap-ish and stronger than what I already have.

    Everything else with that much power and gain that I've come across is either much more expensive or "balanced"/bridged without a phase spliter which would require me to get a balanced DAC too and make it just as expensive anyway.

    Would you happen to have any comments on Schiit's design philosophy or circuit topologies or have another suggestion?

  12. I don't know much about the D1 besides it's suspiciously cheap for a tube DAC, pre-amp, and headphone amp. If it can *really* put out 1000 mW that should be plenty for nearly any set of cans but I'm not familiar with your Fostex headphones.

    The main problem is likely your "subtractive" EQ and surround processing. If you can't drive the D1 with something close to a full scale signal (except perhaps in the deep bass with your particular EQ) you're not going to get full output. Do you have a way to add gain (higher than "0") in the digital domain?

    Another option is digital compression (often call "night mode") to bring up the level of the quiet soundtrack stuff and bring down the peaks.

    USB DACs typically have what I call "surplus gain". The uDAC-2, for example, clips just past half volume with a full digital input signal. The rest of the volume control's range is "surplus" and only useful for cranking up the volume during quiet passages, content ripped at too low of a level, or in other cases where you can't feed a properly scaled digital signal.

    But, in your case, at least the bass (with its 12 dB of relative boost) should push your D1 into clipping (or to its full output voltage, whichever happens first) during the loudest parts of the soundtracks.

    Even if you get a higher output amp, I'd strongly suspect your Fostex cans are going to complain if you really do manage to feed them 1 watt or more--especially in the bass where the excursion limits of the drivers are usually a problem. You might also cause thermal damage.

    As for Schiit, my opinion is they're experts at edgy marketing, their stuff *looks* great, but I know little about their design abilities. Single-ended minimal feedback designs usually have relatively huge amounts of distortion and higher output impedances. So even their solid state Asgard may have a fair amount of "flavor" as you put it.

    The Schiit distortion specs don't specify the load. And, especially with a single ended design, that's a significant thing to leave out. Into *no* load 0.1% THD at 1 volt is relatively easy. But into the reactive load of your 50 ohm cans, it could be far higher. And if the Asgard really consumes 35 watts of power, that's a toasty headphone amps.

    I also don't know how you can make a 5 pound headphone amp like the Asgard in the USA and sell it that cheap. I suspect the chassis and perhaps even circuit board are made elsewhere and only the final assembly is done in the USA?

    But you probably know more about Schiit than I do. They have a 15 day trial so you could always try one?

    The whole "tube thing" is an interesting topic and on my list for a potential future article. Some people like the euphonic distortion of tubes and there's nothing wrong with that. Tubes are better suited for headphone amp duty than driving speakers to respectable volumes. But *any* tube amp is going to be less accurate and true to the source than even some $100 solid state amps. But, to some ears, "accurate" isn't always better.

    And, in general, tube amps, low feedback designs, and single ended solid state amps like the Asgard, are going to have significant output impedance. So they'll interact a lot more with the headphones. And one person's "euphoria" might be the next person's flabby boomy bass.

    If you're looking to keep the price down, I'm going to be testing the FiiO E9 which is also rated at 1000 mW. I'll be measuring the gain as well. So that might be useful data if you're not in a hurry.

    But the biggest problem is your 12 dB of EQ. Even if you can get more power to the headphones, I'm not sure they can handle it. If you want louder overall levels, back off on the EQ, or use some compression.

  13. I guess it turns out I don't need a stronger amp.

    I was digging around in ffdshow and found that the normalizer can somehow pull about 20dB out of nowhere, without digital clipping, even before using "night mode" which is *very* surprising. I didn't even know the normalizer had a "night mode" and I assumed that a regular normalizer wouldn't do anything useful because the bass should have already been at 0dBFS. It turns out the Dolby prologic decode and surround virtualization really were eating that much of the volume. I just assumed it was adding some dynamic range or something which would have made more sense. (To me at least)

    I just never noticed it because it wasn't a problem with any other headphone I've owned because they were more efficient. The D1 had enough gain that a -20dBFS digital signal didn't matter with those headphones.

    In any event, it now goes way louder than I want to listen at with the movie/TV clips I've tried so far and the D1 will actually clip about 5 or 6 notches before full tilt now.

    I'm glad that worked out. I can try more headphones now that I don't need to spend some more money on another amp. I like my coloration to live in my transducers and I've already got too many amps as it is.

    Thanks for trying to help me though.

  14. I am currently using the iPhone 3GS as source, the 1964 Ears Quads (46Ohms / 118dB) and listens to only ALAC files. I am now wondering if I should bother with getting the Fiio E11 or direct the fund to getting the Moon Audio Silver Dragon cable. Will you suggest getting the amp, cable or none at all? Thank you.

  15. If you're happy with the volume you're getting I wouldn't buy an amp as the iPhone 3 already has a low output impedance. The cable is a total waste of money as nearly all cables are.

    See my Subjective vs Objective article for a link to a blind cable test, as in nearly all blind cables tests, nobody can hear the difference between cables. The article tries to explain why some think cable upgrades sound better.

  16. @maverickronin
    The "normalizer" within the ffdshow codecs is "night mode" or (its correct name) dynamic compression.
    Besides I'm using it too, mainly for english movies with o-tone, which tend to stress my hearing with its silent voices. (hello from germany)

  17. Maybe it depends on the version. Mine has two different modes. One seems to be straight peak normalization and the other is called "night mode" in the tool tip.

  18. Your Blog is now my main source for finding a trustworthy view on Head-Fi. This blog is the greatest contribution to the Head-fi community, of all the sources I have read into. This Blog also educates people, so they are not that easy victims of the cheaters in this business. I see you write that the usb-input on the PC has a lot of noise. I have a Cambridge DacMagic, and I wonder if it is better to connect it to the PC via toslink? The DacMagic is connected to a Decware CSP2 tube amplifier. The last one will be
    out of business as soon as I can buy your O2.

  19. You are THE man. Awesome. Im waiting to build my own O2!

  20. Thanks for the typo catch anon. Fixed!

  21. What a great blog. I have recently upgraded my speakers to Audio Engine A5s. They are good performers compared to what I used before. I am plugging them into a 21" iMac. However I have recently thought about using an external DAC instead of the iMac line out. Do you know if this will add any true benefit? My hunch is that the Apple DAC is probably going to work just as well as any external DAC I'd want to spend money on (want to stay under $200). I also use headphones from time to time on this machine and a MacBook Pro. Will an external DAC add any benefit to these machines? I never thought Mac audio was a slouch so I'm not sure if I'd be wasting my money by using an external DAC or, even worse, make it worse by bypassing the DAC Apple is using internally in these machines.

  22. Glad you like the blog. Generally if you're not hearing any problems (odd noises, etc.) you're not likely to get much improvement with an external DAC. Apple's internal sound hardware, from what I know, has ranged from mediocre to quite good. I only have the last generation Mac Mini here and haven't yet put it on the dScope but plan to one of these days.

    To put it a different way, if you're not hearing any obvious problems you can attribute to your iMac that $200 would be better put towards a future speaker upgrade which would make a much bigger difference (I have nothing against the A5s but speakers are nearly always the weakest link by far).

  23. Thanks for the input. My instinct is the iMac probably is sufficient. I use it for editing audio and haven't heard any issues. I have however moved from using the DAC L/R output from my airport express directly to my Bryston audio processor with optical output and there was a noticeable increase in quality. So in that case it seemed to be a big difference but I am very wary of audiophile opinions on just about everything so I figured I'd ask in case you had some hard numbers on things. I have an oscope and may rig up my Mac to test the output. If you were to put up a blog to discuss to non audio engineers how to use an oscilloscope to test the critical parameters you track that would be helpful. I am sure I can replicate many of the tests you use, but perhaps a standard protocol you post so others can do testing and post results would be helpful to your readers.

  24. Sorry to say, but short of evaluating 10 Khz square wave response, looking for instability, or evaluating out of band (above 96 Khz) problems, oscilloscopes don't go very far towards evaluating all the subtle things that affect sound quality. Most digital scopes use 8 bit A/D converters with very limited resolution. And analog scopes are limited in other ways (they can't perform FFTs, etc.). Even a high-end digital scope with a six figure price tag can't measure THD, IMD, jitter, etc. to anywhere near the accuracy required for high quality audio analysis.

    Some have been misled by the name of the Prism "dScope", but it's not really a "scope" at all. It's a high-end audio analyzer much like those from Audio Precision. It uses 24 bit D/A and A/D and measures down to levels that no oscilloscope on the planet can even dream of. But that's because it's optimized for audio. Oscilloscopes are optimized for much faster signals and of limited use for high-end audio work.

  25. You said:

    "It also causes a loss of resolution in your music. Your 16 bit audio stream becomes 15 bit, 14 bit, or even less if you lower the PC volume from the maximum setting because the volume is being lowered in the digital domain".

    But people said that digital volume control affect only SNR:

  26. Bit resolution and SNR are directly related. See:

    And there's often a big difference between internal digital volume controls in devices and an external USB DAC. Most DAC chips/CODECs are 24 bit these days and the volume control in a device can operate at 24 (or even 32) bits. But most USB DACs only work at 16 bits (because of the USB link itself).

  27. I have internal card (Asus Xonar D1) with CS4398 that have internal "Volume Control":

    Do I lose resolution when I use volume control? If yes then how I can calculate it?

    As I understand, it's possible to do digital volume control with no resolution loss:

  28. It depends entirely on how your device is implemented. The Wadia paper you referenced explains exactly what I said in my previous post. And the link I provided answers your "how to calculate it" question. Internal volume controls can take advantage of higher bit depth. This article is mainly about USB DACs and headphone amps.

  29. OK, thank you. Last off-topic question: did you know what the dependence between volume in % and output voltage in volts? Is it linear? Sorry for off-topic.

  30. It's rarely linear but it's very device dependent. No two different devices are likely to be the same (digital or analog volume). Human hearing isn't linear so volume controls are usually psuedo-log based. Even some volume controls calibrated in dB are not really accurate because of the step sizes, etc. This is discussed in detail in my RMAA article as one of the huge limitations of using PC sound hardware for audio testing. You have no idea what the voltages are.

  31. Hey, great post!
    I was wondering how much jitter specifications (when measured in picoseconds) matter once the noise, distortion and frequency response variations are inaudible.
    BTW just out of curiosity, could the DAC linearity measurements change if you play both a 0dBFS signal and -90dBFS signal at the same time instead of just the -90dBFS signal?
    Thanks for your time!

  32. YTC, nobody that I know of can really answer your first question. There's a lot of debate over jitter, and there's more to quantifying it than just a single number in picoseconds. It has frequency/spectral components and more. Some claim it doesn't matter at all in any gear anyone likely cares about. Some say its responsible for audible differences between digital gear (but that's rarely been demonstrated with blind tests).

    The linearity question is an interesting one. High level signals, especially ones above about -1 dBFS, can raise the entire noise floor of some DACs. One variation is known as modulation noise. I've seen situations where that higher noise floor would pretty much "bury" a -90 dBFS signal. So the short answer to your question is yes it can. I hope to explore more "edge case" digital measurements, such as what you suggest, in a future article.

  33. Oh, and isn't phase performance a relatively important factor as well? how do typical stand alone dacs compare to computer sound cards or a sansa clip+ in this area?
    Thanks a lot! I have learned a ton from your blog.

  34. It's not really possible to measure the phase performance of most portable players and DACs via USB. Phase is a relative measurement between the input and output. But when they're in different domains (one a digital music file, the other an analog signal) it's not possible to make traditional phase measurements.

    It's not really possibly to generalize about portables, sound cards, stand alone DACs, etc. There are good and bad examples of all of them.

    As I mentioned above, I'm going to work on expanding my DAC/DAP measurements.

  35. Haha just wondering since this post's about DACs. Would you by any chance end up designing a DAC to pair with your O2 amp?

  36. Anon, I keep getting asked that question. The answer (at least for now): I think commercial DACs have a number of advantages and I hope to find one I can recommend for use with the O2. I'm going to be testing some DACs in the next few weeks. There are also some new DACs coming out soon that sound promising. Ideally I'd like to find a DAC that can do 24 bit audio over USB using native drivers. That rules out all the DIY DACs I know of.

  37. Ahh really sorry about that. Didn't look around in your blog enough. Thanks for the input. Will be looking forward to your future posts then.

  38. Can you list some dacs that you are testing/are planning to test?
    Sorry if this has been asked before.

  39. I'm looking mainly at USB DACs under $100 and hope to find something that can do 24/44 over USB and works well (24 bit has some advantages if you control the volume on the PC side). If the cost is low enough, even a 16/44 DAC would be useful if it works well. Some candidates:

    FiiO D5 and E10
    Creative X-Fi HD
    Asus Xonar U3
    Violectric USB 24/96 (for use inside the desktop O2 amp)

    There are several Tenor USB chips that allow high-resolution audio with native drivers and, until something better comes along, DACs using these chips are likely the most promising low cost solution. But I won't really know until I start testing some of them.

    If anyone has further suggestions for low cost DACs with 24 bit native USB support, please let me know?

  40. Hai nwav,

    Sorry if this isn't the best place to put this question.

    Let's say I have an mp3 player with only headphone out. I connect an inexpensive amp like the E7, E11, Cmoy amp from the source (via headphone out)to my Grados.

    Will this set-up degrade the sound quality? I keep hearing others say that it's best to bypass the internal amp to get a "cleaner" output and connect via the line out but is this scientifically true?

    By the way, Hifiman has a USB DAC, HM-101 but I'm not sure if it supports 24/96 playback.

  41. I don't think the HM-101 supports 24 bit playback but it might be fun to test one anyway. There are several similar products in that price range and I suspect HiFiMan just slapped their name on one of them but perhaps it's more than that.

    The "cleaner" output issue is only applicable for devices that even have a line out (usually via a "dock" connector as on iPods). It probably used to help more with older players but many newer players have very clean headphone outputs. The iPod Touch 3G is a good example.

    Headphone outputs also tend to perform better into the relatively high input impedance of most headphone amps. Unless the headphone output measures poorly in some obvious way (high noise, distortion, or frequency response problems) it shouldn't be an audible problem.

    One plus with line outs: On the iPod Touch (and I assume iPhone) using the dock connector automatically disables the internal volume control. But when using the headphone output you need to worry about setting the volume high enough to avoid noise problems but not so high as to cause clipping.

  42. First, what a fantastic website! Great information and such a refreshing change from audiophile sites.

    I came across the following paragraph on a headphone amp manufacturers site and wondered what you thought...
    "Some headphones are designed to be voltage driven (low amplifier output impedance); others are meant to be current driven (high amplifier output impedance). Depending on the headphone, how you drive them can affect their tonal balance. Many high-end headphones are designed and tuned around the IEC 61938 specification for 120 ohm amplifier output impedance, yet virtually no headphone amplifiers offer this standard output. Using the Sonett, you can now hear your headphones as their designers intended."
    Doesn't seem to take any account of the 8 times rule. I'm confused.

    I'm currently searching for my first set of quality headphones to use with my laptop and am drawn to the DT880 600ohm. To drive them I like the look of the NJC Audio Monitor 1 DAC/Amp which has an output impedance of 0.6 ohms and power output of 80mw at 600 ohms. It is £179 which seems like a bargain compared with some. Can I ask what you think of it? It has the added attraction for me of being made in England!

  43. See the Stereophile link in my Output Impedance article. The 120 ohm IEC standard is mostly laughed at these days including by the people designing headphones. It sounds like someone is trying to hype their high impedance amp. Such an amp will affect the "tonal balance" of near all headphones in ways that most probably won't consider an improvement.

    I don't know anything about the NJC DAC/amp but that's about 7 Vrms into 600 ohms which is (just) enough for the DT880-600.

  44. Hi everyone,

    My current portable rig consists of IPOD Classic 80Gb > HRT Istreamer (Battery powered) > Meier Corda Headsix. The headphones are Etymotic ER4P/S, Phonak Audeo PFE (grey filters) and AKG K500. I own a pair of AKG K701, but to drive them properly, I have a Heed Canamp (stationary use).
    So far, I am happy with the sound of the portable rig, but in search of improving the sound as a whole, my next goal is to get a new portable amp.
    I am wondering about the Justaudio uha-120 to drive the headphones (in-ear & AKG K500), but given the output voltage value of the istreamer (2,25 Volts RMS), I don't know if the Justaudio
    would be the correct choice (clipping & imbalance volume problems).
    I am currently using the headsix (hi gain mode) with the Etys & Phonaks at a volume around 9, hence my doubts regarding this matter.
    I have read about the 2Stepdance, but apart from the low battery duration, it seems to have issues regarding its digital volume (clicks & pops).
    I live in Spain, so I would really prefer to buy the amp in Europe (to avoid surprises with customs). The maximum price would be around 300 Euro, and both the 2Stepdance & Justaudio uha-120 are inside this range of price. However I'm totally opened to another brands and models.
    I'm not a basshead, I rather prefer an accurate sound in all the spectrum.
    I strongly prefer quality more than quantity, and I would really appreciate your advices and comments before making a decision.
    I have read lot of good things about O2 amplifier, but it's dificult for me to get it from Spain. I have contacted to epiphany audio, but they will have the amp in two or three months. Any other amps apart from the O2?

  45. Perhaps someone else will have suggestions? For European sourced amps I'm not sure what to suggest. There are not very many high quality portable amps with enough current and voltage made in any country to do a good job with the K701. I will be testing the FiiO E11 and E10 and they might come close depending on how loud you like to listen. But I'm not sure about availability in Spain.

  46. The listening volume I hear music is not very loud at all. The specs of three amps sold in Europe are the following:

    Just audio uHA-120
    Maximum Continuous Power output:
    60 mWrms/channel into 32 ohm @ nominal (3.6V) battery voltage;
    10 mWrms/channel into 300 ohm load @ nominal battery voltage;
    16 mWrms/channel into 300 ohm load @ fully charged (4.2V) battery voltage;
    28 mWrms/channel into 300 ohm load with 5.2V USB power supply;
    Distortion (THD): <0.005% @ 1KHz (-20dBu to 5dBu output level)[1];
    Dynamic range (dBA): >100dB
    Bandwidth (-1dB): 10Hz-40kHz
    Output impedance: < 1 ohm [2]
    Output current limit: ~= +/-80mA per channel
    Maximum Gain (REV A): 12dB;
    Maximum Gain (REV B): 10dB/6dB (based on jumper setting);
    Battery recharge time (max): 2.5 hour fast charge; 6 hour normal charge;
    Dimensions (approx): height=23mm; width=52mm; length=85mm
    Weight = 125g +/-1g
    [1] Both channels driven with 32 ohm headphones connected;
    [2] Actual measured output impedance < 0.1ohm; BW = 10Hz-40kHz with 2R2 load (100mV test level).

    Just audio AHA-120
    Maximum Continuous Class A Power output:
    60 mWrms/channel into 32 ohm @ nominal (3.6V) battery voltage;
    10 mWrms/channel into 300 ohm load @ nominal battery voltage;
    16 mWrms/channel into 300 ohm load @ fully charged (4.2V) battery voltage;
    28 mWrms/channel into 300 ohm load with 5.2V USB power supply;
    Distortion (THD): <0.002% @ 1KHz (11dBu output level)[1];
    Dynamic range (dBA): >100dB
    Bandwidth (-1dB): 10Hz-40kHz
    Output impedance: < 1 ohm [2]
    Output current limit: ~= +/-250mA per channel
    Maximum Gain: 12dB;
    Channel balance: <1.5dB (over entire electrical travel of volume setting);
    Battery recharge time (max): 9 hour fast charge; 21 hour normal charge;
    Dimensions(approx): height=26mm; width=80mm; length=126mm
    Weight = 332g +/-1g
    [1] Both channels driven with 32 ohm headphones connected;
    [2] Actual measured output impedance < 0.1ohm; BW = 10Hz-40kHz with 2R2 load (100mV test level).

    Corda Stepdance
    Mean current uptake: 35 mA / 20mA (high/low-current-consumption-mode)
    Discrete volume control. Typical stepsize 1.5 dB, 31 steps.
    Maximal amplification: 0 / 14 dB (low/high gain)
    Active balanced headphone ground.
    Star-grounding and ultra short signal paths.
    Input impedance: 120 / 17 kOhm
    Output impedance: < 1 Ohm

    The Justaudio amps use recheargable batteries. The 2stepdance uses one 9 Volt battery. My main use would be with the iems Ety ER4P(27 ohm)& Phonak PFE (32 ohm), and more sporadically the AKG K500 (120 Ohm). The AKG K701, I think is harder to drive with portable amps.

  47. Just Audio gets credit for doing a much better than average job with their specs. Those are great specs if those amps have enough power. The problem is they only produce about 1.7 Vrms which isn't even enough for the popular Sennheiser HD600/650 (see my More Power? article) and nowhere near enough for the AKG K701.

    The Corda amps are less clearly specified. And, to be honest, some of the Corda amps use designs that simply don't make any sense from an engineering perspective. My opinion is Corda tries to designs amps that are different just for the sake of being different.

  48. Well,
    In that case, it seems that I have only two options. One is to wait until the O2 is available from Epiphany shop, and the other one is buy one amp from another brand (continent).
    I have seen one Ibasso amp. It has a recheargable battery (40 hours play time) and the specs seem to be appealing to be paired with the AKG K500 & K701, apart from any iem.

    Ibasso P4 Warbler

    - ThreeChannel/ Four Channel compatible architecture
    - Totally 8 gold-plated DIP sockets for OPAMPs and Buffers rolling
    - 2-Setting Gain Jumper for impedance matching
    - 40 hours play time with stock opamps (TLE2141 Class A*2, BUF634U*2, dummy adapter*2)
    Power Source:Built-in 12.6V Li-polymer battery pack or
    16Vexternal power supply
    Frequency Response: 10Hz~200KHz /-0.3dB
    Signal to Noise Ratio:-110dB
    Crosstalk: -101dB
    Total Harmonic Distortion: 0.0015%
    Output power: 600mW+600mW into 32 ohm
    Gain: 0dB/+10.5dB
    Recommended Headphone Impedance: 8~300 ohm

    Do you yhink it has enough juice for the AKGs?

  49. Personally I'd find a way to get an O2. The Ibasso does have Li-poly batteries and longer run time, but the O2 is less expensive and has all the proven performance you need. You might want to email Ibasso and ask them why they don't publish full test results for their amps. Their specs are largely incomplete and questionable.

  50. Given the fact that I had to wait 4-6 weeks to get the O2 from Epiphany (UK), and reading the goods reviews in general about Fiio amps, I took the plunge and bought the Fiio E6, mainly because the "LO 2V", that in the case of using a high output voltage as the istreamer does, the few dBs of attenuation is a real need.
    I kept in mind the little size and price of the device (26 Euro shipped in Spain), so not a lot of money to lose if I disliked the amp.
    After charging the battery and turning the E6 on, I discovered how well I had chosen the amp.
    Regarding The Meier Headsix , the Fiio is cleaner and flatter in all the spectrum (with the EQ3 LO 2V activated). Apart from the Ety ER4P & Phonak Pfe (grey filters), the E6 is able to drive the AKG K500 to a moderate level of volume and transparency as well. I'm judging the E6 only with my ears , so if the amp is reviewed for you, I will be able to discover if I'm rigth or wrong regarding the SQ of this little amp.

    Thanks NwAvGuy.

  51. hi, great blog and here i come with a question. i've bought a Superlux HD-668B studio headphone, 98db SPL, 56ohms, 10-30.000Hz, 300mW and i now wondering if i need an amp or smth.. will be listening music from a laptop source (ntel 82801H HDA chipset).. so should i buy smth like FiiO E5 or PCM2704?

  52. @grey, sorry for the late response. It depends if that's 96 dB at 1 mW or 98 dB at 1 volt--both definitions are commonly used. If it's at 1 volt I'd strongly recommend an amp or headphone DAC with a decent amp. Something like the FiiO E10 might be ideal but FiiO doesn't seem to be able to actually ship any to the USA. With 56 ohm headphones you need an output impedance less than 56/8 or 7 ohms which rules out a lot of amps/DACs.

  53. Hi NwAvGuy,

    I've seen you stating in the comments that the JustAudio amps are not powerful enough to drive a K701, because they only output 1.7Vrms.

    I happen to own both a K702 and a JustAudio AHA, without any problem to drive them (I'm usually listening at 40% to 50% of the pot). So I'm just wondering what I'm missing here. Would you mind giving me some insights ?

  54. @Frenchbat, please see my More Power article and look up the K701/702 at InnerFidelity to see their real world sensitivity.

    The K701/702 are challenging because their voltage sensitivity is relatively low and so is their impedance. This means they need both more voltage and more current than most headphones. They need 3.2 volts to hit peaks of 110 dB SPL. The More Power article explains why it's desirable to hit 110 dB.

    The JustAudio AHA-120 is at an even bigger disadvantage because of how Li-Ion batteries work. Their voltage steadily drops from around 4.2 volts to 3.0 volts over their run time. Even with the battery at 3.6 volts, the AHA-120 can only manage about 1.3 Vrms and that's only good for about 102 dB peaks.

    The K701/702 also need about 75 mA of peak current to hit 110 dB. A lot of amps, especially those with virtual grounds, third channels, or op amp outputs other than the NJM4556, can't produce 75 mA at anything resembling low distortion if at all. So they distort badly and audibly at levels much lower than 110 dB SPL.

    We all have different ideas of "loud enough" and different types of music can also make a big difference. That's probably why you think your amp is loud enough hitting peaks of 102 - 104 dB SPL. That's all in the More Power article as well.

  55. Thanks for the answer NwAvGuy. I went back to your article and also the measurements of Innerfidelity (the impedance is correctly stated by AKG anyway, so no real surprise here) for reference, and I did my homework, or tried to at least.

    First, it's true that I'm a quiet listener and I usually don't need to push my amps very far. So it's likely that my concept of loudness isn't the same as yours. I'd like to add that Justin never stated that the K702 would be driven without problem from the battery. When I asked him, he told me that it would be fine with the USB plugged in, which isn't a problem for me since my amp has yet to get out of my house.

    Unfortunately there's only one power measurement for the 5.2V USB power, with an impedance of 300 Ohms. But comparing with the battery powered output, clearly the 5.2V USB (28mW) is driving quite some more power than 3.6V nominal battery power and 4.2V full battery power, respectively 2,8 times (10mW) and 1,75 times (16mW) more.

    It's obviously difficult to extrapolate the power value for and impedance of 32 Ohms, let alone the 65 Ohms impedance of the K702, but if the roughly 3 times scaling with USB power can be applied on the power delivered at 32 Ohms (60mW), then the results is in the neighbourhood of 180mW. Now from your article, we can calculate the needed power for the K702 to reach the 115dBSPL peak, which is 154mW. Provided that the scaling is applicable, it seems to me that the USB powered AHA 120 has at least some of the oomph needed to drive the K702 past 110dBSPL peak.

    I have a second question, most likely very simple, but it's been bugging me. My source is delivering 2.25Vrms to my amp, isn't it supposed to have some influence on the power delivered to the K702, say compaed to a standard 1Vrms source (typical DAP source) ?

  56. First of all, Just Audio's specs are rather suspect in at least a few ways. USB power is almost never over 5 volts and it's often under 5 volts (it can be as low as 4.5 and still be within the USB spec). PC's use fairly tightly regulated 5 volt power supplies from which the USB power is usually obtained and there's drop across the motherboard, through the USB power monitoring circuitry, at the connectors, and in the USB cable. So, with any kind of load, you're lucky to get 4.9 or 4.8 volts at the device.

    Just Audio either has a very odd PC or USB charger they used for testing or they're stretching the truth using 5.2 volts for their USB power value.

    The AHA-120 does not have the "oomph" needed to drive the K701 past 110 dB SPL under any conditions. If you extrapolate down from 300 ohms, and correct for 5V USB power, the AHA-120 would produce around 2.8 Vrms which is good for about 108 dB SPL.

    But, from the specs, voltage output clearly drops with load impedance. So it's much safer to scale up from the 32 ohm rating. Doing that you get 1.9 Vrms which is good for 105 dB SPL which is at the low end of the range of "loud enough" which should fit your "quiet" listening preference.

    For what it's worth, besides the suspicious 5.2V USB power, there's another glaring hole in Just Audio's specs. They claim 0.002% distortion at 32 ohms, both channels driven at 11 dBu. The problem is 11 dBu is 2.75 Vrms. And 2.75 Vrms into 32 ohms is a whopping 236 mW not the 60 mW maximum power they claim. I'm fairly certain there's no way the AHA-120 can produce 11 dBu into 32 ohms at 0.002%. That's either very creative marketing, whoever wrote the specs doesn't understand audio, or it's a serious mistake. It's very misleading regardless of the reason.

    See my All About Gain article for the answer to your last question. The AHA-120 has 12 dB of gain which is 4X. So it will produce it's absolute maximum on USB power into 300 ohms of 2.8 Vrms with 2.8/4 = 0.7 Vrms of input. So as long as your source can produce 0.7 Vrms the amp will produce its maximum output. A higher voltage source, like yours, just means you're forced to set the UHA-120's volume control lower to keep it from clipping. That's why in your first comment you said you never get past 50%.

  57. Thanks again for the insights, reading your article on gain is next on my todo list.

    You're most likely right on the USB voltage thing, and even giving the benefit of the doubt on the USB source in JustAudio, mine is clearly stating 5V, so unless its output is exceeding the specs, the 5.2V is rather unlikely.

    I cannot comment on the 11dBu spec, as I'm clearly not qualified.

    As for the voltage of my source, and the clipping limit of the amp, since my amp is set for a gain of 2X, I doubt the clipping is what makes me listening around 50%. With a gain of 2X, the amp needs 1.4Vrms, which is likely to push the clipping much higher than 50%. I genuinely don't need to go much higher than 50%, unless I'm listening to vintage records (jazz mainly) or quiet tracks.

    At this point I'm simply wondering why I don't have a problem driving them. On a typical track, anything past 60-70% on the pot is hear-bleeding loud to me. You'll probably say it's because of track compression, but this also includes supposedly well recorded tracks (I don't listen much pop anyway). Sonically I should be able to hear distortion or other defects.

  58. @Frenchbat, Lots of amps clip before they even get to half volume with a normal output source and properly recorded music. The entire upper half of the volume control in such cases is unusable with any headphones. People think that 50% volume equals 50% of the amp's maximum power, but that's rarely true. It's often closer to 100%. Turning the volume past 50% just results in massive clipping and, as you describe, is hard to listen to. It all depends on the gain, the source's output, and if they're playing properly ripped tracks that have a normal level.

    So, long story short, you're very likely using close to the maximum output of your amp. And with your particular tastes in loudness, that's loud enough. There's no mystery. The numbers in my last post show, on USB power, your amp gets into the 105 - 110 dB SPL range with your headphones.

    You really should read the All About Gain and More Power articles here. They will help further answer your question if the above isn't clear.

  59. @NwAvGuy, I have one question :

    On JustAudio's website, it's specified that the number are given per channel (there are 2 I think), and that all the numbers should be multiplied by 2, to give the final number. What do you think ? Plausible or marketing B.S ?

  60. @Frenchbat, all audio measurements are per channel. The only time you sum them together is if you're a marketing department advertising a "700 watt" audio/video receiver that's actually 100 watts/ch with 7 channels (and most of those can only put out about 25 watts/ch with all 7 channels driven at once).

  61. Fair enough, my question should have been more to the point.

    According to JustAudio's website, to find the Pmax output of the amp, it's necessary to multiply the results per channel by 2. I think it's because it's a Class A Push-Pull topology.

    Now scaling up from the 32 Ohms on battery, to USB power, you gave me a value of 1.9Vrms. Since this value is per channel, and following the reasoning of JustAudio, this gives us a result of 2.8Vrms on USB Power.

    This value is in the same ballpark as the 2.75Vrms that is implied by the 11dBu result from the specifications. So it seems like JustAudio is quoting the dBu value @32 Ohms with USB power, for both channels.

    So my question is the following : Does this make sense ?

    Additionally, I've been wondering if topology has an influence on the measurement methodology. I've done a bit of research but I couldn't find conclusive information yet.

  62. @Frenchbat, no you don't multiply by two. The topology, nor Class-A/B/etc., have anything to do with it. Watts, Amps, and Volts are the same regardless and follow the rules of Ohm's Law. You calculate the three values the same for a desk lamp as for a headphone driver.

  63. Massive knowledge-bomb! Thanks a lot.

    I've been a head-fi member for a little while as I've been looking for an amp/dac combo, though I have to say I don't understand what a lot of people there say like "This dac gives great soundstage". I recently tested a cambridge dacmagic plus and thought it didn't sound good enough for the money, but I never heard a difference in "soundstage", like I did when I picked up new headphones (Fischer FA-003).

    Whilst I would love something like a benchmark, it's out of my price range. I was wondering, what do you think about things like the Asus Xonar Essence One? My intuition tells me it's probably quite good, as I've built computers with Asus components before and found them amazing, and they're a large reputable manufacturer.

    Also, I eagerly await the ODA/ODAC! My girlfriends Granddad does a lot of woodwork so I'm going to end up with a nice housing for it.

  64. @Deadeight, Stereophile has tested a couple Asus soundcards and I tested the Xonar U3 in November. The DACs perform fairly well but Asus has done a poor job on every headphone output so far.

    Internal soundcards are also at the mercy of the electrical noise levels of the PC they're in. That means Stereophile's test results might be far better than the next guy's. They also require proprietary drivers which can cause other problems, stability issues with your PC, etc.

    If you're wanting an ODA/ODAC anyway, I would just wait and skip the Asus. The ODAC won't need proprietary drivers and won't be at the mercy of the PC it's used with. And, best of all, the ODA headphone amp will outperform anything I'm aware of from Asus.

  65. @NwAvGuy
    The Essence One is the outboard one ( ), but has gained very little attention. I just wasn't sure if that was warranted or not. I guess the reason why I was interested is because Asus would possibly be a company who would rely more on testing because of their PC background. And the movement of a PC heavyweight into the category like asus might bring some competition and drive down some prices.

    Of course they always could have seen the audiophile market and want a piece of the mark up action. Economics isn't my area.

    The ODA/ODAC is providing me with a nice advantage too. By using it in an experiment in sept-dec on waves I'm going to get a lot of components off of my Uni. It's the best way I came up with to transfer waves from a computer down a wire, with the output oscillating a small magnet and I'm measuring things like dispersion down the wire. Most of the point of it is to come up with my own method, and I need marks for building apparatus. But it's also a nice way to know exactly what's happening with the signal (and a nice excuse to end up with a great amp/dac on a student budget).

    Assuming it turns out as well as the O2 it should be great. So thanks again for putting the time into designing it!

    P.S. You'll be a reference in my dissertation, congrats

  66. @silkroad, Blogger seems to be swallowing some posts. I approved yours but it hasn't shown up here yet. To answer your question, the FiiO E3 is the sort of thing that should be given away free in boxes of breakfast cereal. It's more a toy than a serious headphone amp. the E5/E6 is well worth a few more dollars.

  67. NwAvGuy, I came upon your blog this weekend and have spent most of the last 2 days devouring it. I just wish I had the technical knowledge to get the most out of your posts (most of the charts, for example, are lost on me). Clearly, you devote a lot of time and thought - and knowledge - to your blog. Thanks!

    I'm new to headphone amps (still haven't owned one). The reason I am looking for a headphone amp is that I recently purchased a new integrated amplifier - the Rega Brio-R. It lacks a headphone jack. Clearly, an opportunity to shop around and read up on the many handsome and clever headphone amplifiers out there!

    My setup is a Mac Mini > HRT MSII > Rega Brio-R. Headphones are Grado SR225. (For the record, speakers are JMLabs Cobalt 806S.) I have 2 questions for you and/or the community.

    1) Would a RCA (male) to headphone jack (female) adapter be all I need? Essentially plugging the headphones directly int my amp's line out.

    2) or do I really need an amplifier and if so are there special considerations when it will be taking it's signal from the tape out jacks on on the amp? I am leaning toward the O2 but is it necessary?

    Ok that's more than 2 questions.

    Any feedback would be much appreciated. Again, thanks for the trove of information and commentary.

    1. I have no idea what sort of headphone output the Rega has. So I can't say if you need an amp. The Grados are low imepdance and will benefit from a near zero ohm source. Such a source is unlikely, but possible, with your Rega. If it does have a decent built-in headphone amp they would probably advertise that fact as it's not something most integrated amps bother with.

      Using the tape out jacks is fine just as you suggest. If you don't want to drop the money on an O2, consider a FiiO E11 which will do a great job driving your Grados but, sadly, can only run from battery power while listening. An even less expensive choice would be the FiiO E5 or E6 which can be powered by a USB port on your Mac while you listen but are not as high quality as the E11 or O2.

    2. Thanks for the reply, though I'm not sure if you meant that option 1) above is fine or option 2) is fine. There is no headphone output or built-in headphone amp. So if I need a near zero ohm source, then I presume that I need a dedicated headphone amp.

      It's also not clear to me if the best way to connect the O2 or FiiO would be from the Mini's headphone jack or from the amp's tape out.

      I am also considering a Little Dot Mk II or III, if only as a relatively affordable way to see first-hand what effect a tube amp can have.

      The O2 and FiiO have 3.5mm inputs... is there any difference in audio quality between a 3.5mm input and RCA input? (or RCA-to-3.5mm cable VS RCA-to-RCA).

      Similarly, the Grados have a 1/4 inch plug. I would need a 3.5mm adaptor. Does degradation occur from using an adaptor? And, out of curiosity, is there a difference in sounf=d quality between a 1/4 inch and a 3.5 mm?

      Many Thanks.

  68. This is great stuff, thank you for taking the time to write this blog. I've spent months reading trying to grasp the buzz-words everyone was throwing around. It was really hard to get a basic idea of what was actually needed. People over there are more eager to justify spending more money to one-up each other than they are to help a new person who just wants to hear great sounds. Still a lot to read over here, I'll probably have to go over it a few more times, but I'm glad I didn't start throwing money around too much too early. I've decided to invest more in higher quality headphones before worrying about amps/DAC's just yet. I'll have to take another look at the O2 down the road.

  69. Hello, is the Fiio E6 amplifier an upgrade to the amp inside the 2011 macbook air?

    1. Not really. See my MacBook Air review. The E6 may have a few dB higher output but if your MacBook gets loud enough for your tastes, and you don't hear any objectionable noise, don't bother with the E6.

  70. Hey, would a fiio e7 or e17 be a significant upgrade to the internal amp and dac of the newest version of the iPod Touch?

    1. I can only speak for the E7 and the E17 doesn't seem to exist yet (at least in the USA). it won't upgrade the iPod's DAC and using it as a headphone amp depends on the headphones. It has a bit higher output level and a lower output impedance which could help with some headphones. But you might as well get an E5 or E6 which will perform similarly for a lot less money. Or, better still, the E11 if you're OK with battery-only operation.

  71. Hi, I've been frustrated ever since I started my head-fi ventures. Now I own Grado PS1000, Denon AH-D7000, Grace Design m903 and Xonar Essence STX. I'm even considering a tube amp from Woo Audio to find a good match, lol.

    I've learned more from the two hours spent on your site than I've learned in any forum post I've come across. Delightful that you can find objective people out there. I'm going to take this to a whole new level now and start making informed decisions.

    Thanks, hope you make a fortune with your amp design.

  72. Hey, would the DAC in the Headroom Total Bithead be and upgrade to the one in the 4th gen iPod Nano? What about the latest iPod Touch?

    1. Oh yeah and is the "crossfeed" feature that is on this amp and others marketing B.S. or can you actually make out a difference?

    2. It's a USB DAC so you can't connect it to an iPod. It's designed to be used from a PC. I can't speak for the Nano but the latest iPod Touch probably has a better DAC than the Bithead which is a fairly low-end entry-level product. That said, some PC's have really lousy headphone outputs, and for such a PC, the Bithead would be an upgrade. But the FiiO E10 is much cheaper and probably outperforms the Bithead. See my E10 review.

    3. Sorry I didn't know that . . . I'm a newbie.

    4. No worries. It's probably something I should address more clearly in this article (adding DACs to iPods, etc.). Crossfeed is real and some like it while others find it "artificial" sounding. Some crossfeeds are better than others. If you listen on your PC you can do much more advanced processing designed for headphone listening. Dolby, and others, have "surround headphone" processing that puts the simple crossfeed in headphone amps to shame. But, again, some like it and some dont/

    5. OTOH some people, like me, find listening to headphones without crossfeed artificial and fatiguing. I get headaches without it and the more separation and soundstage in the mix the quicker it happens.

      I might be able to listen to and hour or two of modern rock or pop without crossfeed but I usually don't even make it the first movement of a symphony without it.

      I seem to be in the minority though.

  73. Hi, NwAvGuy. I'm planning to buy the Beyer DT880 600ohm premium headphone. My question is can the HT Omega eclaro drive those set? HT Omega eclaro states that it can drive up to 600ohm headphones by using a "High fidelity stereo headphone driver by buffer circuit". I am wondering if this high buffer circuit is just a vanilla kind/type or all buffer circuits are designed to be high quality ones? There is no specs listed for the output impedance.

    I can buy the Beyer DT880 250 Ohm premium/Beyer DT880 600 Ohm premium for the same price.

    1. I don't know anything about your HT Omega. If the specs don't include the output impedance and power into 600 ohms, there's no way to know how suitable it is. The DT880/250 would be a safer bet than the DT880/600 unless you have some way to test the 600 ohm combination first or trust someone else who has.

  74. Hi,
    Based on your articles and some responses to comments I'm trying to determine the benefits of amping. To my understanding the need for amping is to pass the source signal at a voltage and current suited for a headphone based on its impedance and sensitivity. Low currents with a low sensitivity headphones will result in low listening levels, as will low output impedances with headphones with high impedances.

    My questions are:
    1) Are there benefits to using an amp with low impedance and high sensitivity headphones that I'm missing?
    2) Regarding impedance matching, are there benefits to approaching the 1/8th the headphone impedance as opposed to sticking with an output impedance less than one?

    Thanks very much

    1. 1 - Yes there are benefits. See Sonic Advantages of Low Impedance Headphone Amps and my Output Impedance article. Especially with balanced armature IEMs the benefit are substantial.

      2 - No, you want the output impedance to be under 2 ohms (under 1 ohm is even better) regardless unless you have one of the rare headphones around that was honestly designed for a higher output impedance.

  75. it seems that the consensus about K701s or the Q701s is that they're overly analytical (and hard to drive), and a little than desired bass response, But According to the Inner fidelity impedance graphs they're surprisingly flat from 10-1000Hz and the exponentially rises there onwards up to 20kHz finishing at 100 Ohms, and since most headphone amps have output impedance above the 1/8th rule with these cans, I'm really curious as to if these cans really deserve this rep, and would perform nicely on an O2 amp, and since they're half the price of the HD 650's on Amazon, they really seem like a bargain,

    To be more precise, would the change of impedance from 60 to 100 ohm on an average dac/amp (10 ohm?) create a rather large deviation in volume in frequencies to create this perception of the cans? especially compared to the rather huge peak near the 100Hz @ 300ohms on the similarly priced HD595 or HD 598? or is there something else I'm missing?

    1. The main reason the 701 is difficult to drive is they require unusually high amounts of both current and voltage due to their relatively low sensitivity and relatively low impedance. Many sources simply can't deliver enough current and/or voltage regardless of the output impedance.

      You're correct a flatter impedance curve might make the output impedance of the amp less important. But there's still the electrical damping of the headphones which is more of an unknown. Even with a completely flat impedance curve some headphones may have higher distortion and degraded bass performance when driven from a high impedance source (see the Benchmark article on output impedance).

      The biggest problem with the 701 is most amps will run out of current and/or voltage on musical peaks. Nearly all USB and battery powered amps don't have the beans to properly drive the 701--especially if you like to listen to wide dynamic range music at "live" levels (see my More Power article).

    2. "You're correct a flatter impedance curve might make the output impedance of the amp less important"

      My main concern is that it gets rather high above 1000Hz without the hump at 100hz, comparing it with another pair on a high impedance DAC/amp could create a perception of poor bass response and it's general lack of sensitivity to me sounds (figuratively) like it could create a consensus that the cans sound sterile or analytical when in reality their amp that isn't doing it's job, since their basis for comparison is overly bassy with a high output impedance.

      "The biggest problem with the 701 is most amps will run out of current and/or voltage on musical peaks."

      This would be more clipping and non-euphoric(well some people like the sound) distortion probably reinforcing the headphone's reputation.

  76. i am to get a dt880 ,now by june 2012 have options for a amp/dac fiio e17 or UHA-6S.MKII Leckerton Audio ,am eager for someone shed some light whether any of those or may something else be most suitable to drive the dt880 well

  77. I'm confused.
    You are saying most of the ipods and laptops are able to drive headphones below 100ohm well.
    But I remember you telling someone in a comment about Q701/K701 that the E7/E6 is barely scratching the potential of the Q701/K701.
    So which is which?

    Anyway, I have a Sennheiser HD518, Superlux 668b and coming q701.
    For the HP laptop(i've got dolby advanced audio on a IDT hi-codec chip), it sounds very loud even on 10/100(on the latop volume).
    I regret didn't reading yoour article and gone out to buy a Creative x-fi surround 5.1pro, well, the difference is not noticeable like you mentioned.

    So, i knew i didn't need an amp for the laptop.
    However, my ipad 3, the sennheiser and superlux had to be at 70/100 on the ipad volume. With a creative aurvana Live(another of my can, and the best sounding among them all), I can use it at 30/100 on the volume knot. So, I went out and bought a LOD cable and a Fiio E6 and the sennheiser and superlux immediately became more listenable at 30/100. So your initial comment about low impedance phones being easy to drive with ipad/ipods might not be accurate?

    Anyway, I thought I needed more power and I went out and bought another E11.. and another Bravo Audio V2 Tube... both are on the way.. and I thinnk this will be my last purchases! :(

  78. You need to read my More Power article. The Q/K701 requires way more current and voltage than most sources can manage. You can't just go by the impedance you also have to factor in the sensitivity (efficiency) of the headphones and the 701's are power hungry.

    The iPods have limited voltage and current but it's enough for most earbuds which are typically < 64 ohms. The problem is their approximately 7 ohm output impedance can cause frequency response, damping, and distortion problems with many low impedance heapdhones--especially the balanced armature IEM variety. See:

    Headphone Amp Impedance


    Sonic Advantages of Low Impedance Headphone Amps


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