Noise & Dynamic Range

INTRO: Noise is generally more obvious with headphones than speakers and a relatively common complaint among headphone aficionados. There’s a lot of confusion about sources of noise, specifications, and how to make valid comparisons.

NOISE DEFINED: Technically noise is anything present that’s not related to the desired audio signal. We usually only care about noise within the audible range of 20 hz to 20 Khz. And within that range, the ear is more sensitive to noise at some frequencies than others. The most common audible noise is relatively random in nature and heard as a broadband “hiss”. Low frequency hum at power line frequencies is also sometimes audible. And digital devices, especially computers and mobile phones, can generate noises at specific frequencies that are heard as whines, chirps, clicks, buzzes, etc.

SOURCES OF NOISE: Noise can, and often does, invade the signal chain in audible ways starting at the microphones used during recording. Here are some common sources:

• Recordings – Microphone preamps and other gear used during recording often have audible noise. But lots of techniques are used to reduce the audibility of such noise. Noise gating, for example, is used to cut noise when there’s no sound from a given microphone or instrument. Nearly all recordings before the early 80’s were mastered on analog tape which has significant amounts of tape hiss. And even digital recordings can have noise from all the electronics in the signal path. And, of course, vinyl has lots of noise.
• DAC – In theory a perfect 16 bit DAC has a 96 dB signal-to-noise ratio but some fall well short of full 16 bit performance. 24 bit DACs often only manage approximately 16 bit performance and the very best reach 21 bit (ENOB) performance. This is especially true of DACs inside a computer. Some DACs also produce significant amounts of their own noise such as as modulation and quantization noise (although these can also be considered forms of distortion as they are only present with a signal).
• Headphone Amp – Even a laptop or portable player has a headphone amp in it although it might be built into the same chip as the DAC. Any amplifier adds noise, it’s just a question of if it’s audible or not. Even some fairly expensive stand-alone headphone amps can have significant amounts of noise. They can also further amplify whatever noise is “upstream”.
• Noise is Cumulative – While sometimes there’s an obviously dominant source of noise it can just as easily be a little from here and little from there. Noise adds up.

NOISE MEASUREMENTS: There are two basic kinds of noise measurements. One is a an absolute measurement of just the noise and the other is a measurement of the noise relative to some known signal level. The decibel (dB) was partly developed as it more closely follows subjective human hearing. A one dB change in level is about the smallest change most people can detect. A 10 dB change is perceived as being roughly twice as loud (or soft). If Gear A has noise of -80 dBv and Gear B is -70 dBv the second one will have about twice the subjective noise:

• Absolute Noise – This is normally measured in microvolts and is the total output with no signal present. It indicates the noise “floor” which is useful from an analytical point of view but less useful for subjective noise evaluation where you’re more concerned about the noise compared to a given realistic listening level. All noise downstream of the volume control is absolute noise.
• Relative Noise – This is a more useful measurements as it correlates the noise relative to some known amount of signal. There are decibel units that are referenced to known standards. The most common are dBv and dBu. Noise given in dBv is referenced to a signal of 1 volt RMS and dBu is referenced to 0.775 volts. Both are reasonable listening levels for many full size headphones such as the Sennheiser HD600.
• Signal to Noise Ratio (SNR or S/N) – This is a more open ended method where both the noise figure and the reference signal level must be provided for it to be a meaningful number. The correct unit is dBr where the “r” means “relative” but it’s often just given in dB. Unfortunately, many manufactures don’t specify the reference level. When just SNR is specified with no reference you should assume it’s referenced to whatever the absolute maximum output level is for the device--the same as a Dynamic Range measurement. Sadly, that’s often not specified either (see: More Power).
• Volts vs dBv vs dBu vs dBr – Measuring noise in volts only works for absolute noise measurements. Measurements in dBv are referenced to 1 volt which makes the math much easier and they’re commonly used in professional audio. 0 dBv = 1 volt. In consumer equipment dBu is more common and referenced to 0.775 volts making the math more awkward. Measurements in dBr can be referenced to anything including each other.

DYNAMIC RANGE: As explained above, Dynamic Range is really the same as the Signal-to-Noise Ratio (SNR) using the maximum possible signal. It’s the ratio between the loudest undistorted output of the device and what’s left over when nothing is playing and is usually a positive number instead of a negative one. The theoretical dynamic range of 16 bit digital audio is 96 dB so that’s often used a benchmark for dynamic range—ideally you don’t want the playback hardware to be worse than the recording format. With higher output gear it’s not uncommon to see dynamic range measurements well above that value so it’s not an unrealistic target. Studies, such as the one conducted by Meyer and Moran, have shown 96+ dB of dynamic range is transparent for any normal listening conditions. The only way to expose the noise floor is to crank up the volume to unrealistic levels. Using a digital (software) volume ahead of a 16 bit DAC and leaving the volume after the DAC cranked way up may expose the 16 bit noise floor. In these applications 110 dB of dynamic range should be sufficient to keep the noise below ambient levels.

VOLUME SETTING: There are some interesting twists with volume settings some of which are not intuitive:

• Upstream Noise - Any noise that’s “upstream” of the volume control will be more audible as you turn the volume up assuming the music doesn’t mask it. The absolute noise is worse at higher volume settings but the SNR stays about the same because you’re also increasing the signal by the same amount as you turn up the volume.
• Amplifier Noise – Depending on where the volume control is located within the gear it may or may not significantly alter the noise. A digital volume control, for example, will only affect the noise in the recording itself (and not change SNR at all). Interestingly some devices with analog volume controls have the most noise at half volume—such as the FiiO E9. This is usually because you’re hearing the Johnson Noise of the volume control itself where half volume is the worst case situation. This is typical when the volume control is before the gain stage. When the volume is after the gain stage, most everything becomes Upstream Noise (see above) and is reduced at lower volume settings.
• Fixed Noise – Amps have a certain amount of noise that’s present at any volume setting. This is usually noise that’s from the circuitry after the volume control and, in a properly designed amp, it’s entirely possible to have it always be inaudible.

WORST CASE NOISE AUDIBILITY: Some define audible noise as anything you can hear under worst case conditions—i.e. nothing playing, the worst case volume and gain settings, a very quiet room, and using extremely sensitive headphones. An easily accepted guideline is –96 dB un-weighted referenced to a realistic maximum listening level (see Dynamic Range above) as that’s the maximum dynamic range of 16 bit digital audio. So whatever level produces around 110 dB peak SPL (see my Power article) should be the reference value and as long as the noise is about 96 dB below that it will be entirely inaudible. That’s achievable with less sensitive headphones but difficult with ultra sensitive IEMs. Some examples

• HD600 – 2.3 V for 110 dB gives 36 uV or –88 dBv of noise
• GRADO SR80 – 0.7 V for 110 dB gives 11 uV or –99 dBv of noise
• U.E. TripleFi 10 – 0.1 V for 110 dB gives 1.6 uV or –116 dBv of noise

PRACTICAL NOISE AUDIBILITY:  In reality, testing shows that 85 dB below 110 dB SPL is sufficiently quiet for most people (noise of 25 dB SPL). That puts the limit at –105 dBv, (-102.8 dBu) or 5.6 uV for sensitive IEMs. With the most sensitive IEMs in a really quiet room someone might still hear some noise at that level, but being realistic, it’s likely “good enough”. If you want to be assured of silence with even the most sensitive IEMs, aim for –110 dBv (-107.8 dBu).

NOISE & GAIN: Headphone amps have varying amounts of gain--the maximum amount they can amplify the input signal. Some have multiple gain settings. The higher the gain the more they will amplify upstream noise. And, typically, the higher the gain the higher their own noise. This is one reason you ideally want to use the lowest amount of gain required. See: All About Gain

AN EXAMPLE: The O2 Headphone Amp has the following measurements (see O2 Measurements):

• Noise dBv Volume 100% – The O2 measures –112 dBv un-weighted and –115 dBv A-Weighted. This is well below the –105 dBv guideline and means the O2 will be silent in use.
• SNR Referenced to Full Output – The O2 referenced to 7 volt RMS (full output) measures –130 dBr unweighted and –133 dBr A-Weighted. These numbers are extremely impressive but also unrealistic for most users who will never need even close to 7 Vrms of output.

HEADPHONE SENSITIVITY: Headphones vary widely in their sensitivity. Many assume a headphone that’s 10 dB more sensitive will make the SNR 10 dB worse but that’s often not true. As headphones become more sensitive, you need less gain, and/or use lower volume settings. Both of those typically lower noise So the ratio of the signal to the upstream noise, and hence the SNR, stays about the same. Only fixed noise (see above) is directly related to the headphone sensitivity. Johnson Noise from the volume control can complicate this a bit but as headphones become more sensitive the fixed noise becomes much more important. See Noise Audibility Worst Case above for examples of three different headphones.

NOISE SPECTRUMS: Sometimes you will see a spectrum graph for noise measurements. The approximate “noise floor” in these graphs is much lower than the actual noise specification. In the graph to the right the overall noise is about –112 dBv but the noise floor is down around –150 dBv in the graph. This huge difference is because the –112 dB number is the sum of all the noise from 20hz to 20 Khz. Think of spreading a cup of sugar out across the floor. It would barely change the height of the floor. But if you gather all the sugar up in a measuring cup, you can know how much total sugar there is—much like the noise measurements shown in the boxes in the graph. Click the graph for a larger version.

NOISE BANDWIDTH AND WEIGHTING: Typically noise is the sum of all energy within the audio band. Ideally the bandwidth is specified for un-weighted measurements. A-Weighting is often used which adjusts the measurement for the relative sensitivity of the ear at different frequencies and also limits the bandwidth. Another weighting standard is ITU-R 468. For gear that’s prone to a lot of out-of-band ultrasonic noise, such as Class-D amplifiers and digital equipment, a wideband noise measurement up to about 100 Khz can also sometimes be useful in addition to weighted/limited measurements.

COMPARING NOISE MEASUREMENTS: You can only directly compare noise measurements given in dBu, dBv, or dBr at the same reference level. And they must use a similar bandwidth and all be either unweighted or weighted the same way. Otherwise, you can’t compare the numbers without at least doing some math and sometimes you can’t compare them at all. Here are some examples:

• RMAA – Unfortunately RMAA has no concept of absolute levels. So it can’t calculate noise levels referenced to any known value. It attempts to calculate dynamic range against ) dBFS (the clipping level of the DAC itself) but even that is subject to wide variations in the device settings (i.e. volume, “record” level, etc.), calibration settings, etc. Basically RMAA noise measurements are nearly worthless and the noise of the PC sound hardware might be worse than whatever you’re trying to measure anyway. Some RMAA results are comparatively arbitrary and this is one of them.
• dBv to dBr – If Gear A has a noise spec of –100 dBv and Gear B is –108 dBr (ref 10 Vrms) at first glance B looks to be a significant 8 dB quieter. But A is referenced to 1 V and B 10 V. The difference is 20*Log(10/1) = 20 dB. So B is really 20 dB worse reference to 1V or only –88 dBv. See Generic Conversions below.
• dBu to dBv – These are close. To convert from dBv to dBu the noise is 2.2 dB worse. To convert the other way it’s 2.2 dB better.
• dBr (400 mV) to dBv – I updated my own noise measurements from dBr referenced to 400 mV to dBv (referenced to 1 volt). To convert the old 400 mV measurement to dBv the noise improves by 8 dB. To convert the other way, it’s worse by 8 dB.
• Generic Conversions – The generic math for the amount to add or subtract is 20 * Log( Vref1 / Vref2). The lower the reference voltage the worse the noise figure. Noise can also be referenced to power instead of voltage. In that case it’s 10 * Log ( Pref1 / Pref2 ).
  • dBv to Volts = antilog( dBv / 20 )
  • -96 dB in Volts = antilog ( –96/20 ) = 16 uV ( 0.000016 volts)
  • Volts to dBv = 20 * log ( Vnoise )
• Weighting Comparisons – It’s impossible to accurately compare different weighting or weighted vs un-weighted as it depends on the frequency distribution of the noise. An amp with a lot of hum, for example, will have a proportionately lower weighted measurement than one with only uniform hiss. In general, however, expect an A-Weighted measurement to be about 3 to 6 dB better than an un-weighted measurement.

SOURCE IMPEDANCE: Johnson Noise is often a dominant source of noise in headphone amps and preamps. And it’s proportional to the impedance of the input circuitry which includes the source. The higher the source impedance, the higher the noise. So, for example, a given headphone amp might be dead silent when driven from a source with a 100 ohm impedance, but using a source with a 10K impedance could easily produce audible noise. In this case the noise you’re hearing is really coming from the upstream source not the amp.

MEASURING NOISE: Because noise measurements are a sum of the noise across the audio band, and extremely low in value, they’re tricky to measure accurately. The best high-end 24 bit PC sound hardware may have a low enough noise floor, but often cannot accept the full output of the device being tested. And more significant, PC sound hardware has no way to set or measure absolute levels—i.e. measurements in volts, dBv, etc. Very few Digital Multi Meters (DMMs) have the resolution and low enough internal noise to measure accurately down to a few microvolts of AC from 20 hz to 20 Khz. It is, in theory, possible to temporarily calibrate a 24 bit soundcard using a known accurate meter and suitable test tones. But it’s tricky to do accurately and apply to whatever software is being used. The source impedance is also an issue. Manufactures tend to short circuit the inputs for the best noise number, a more realistic test is to use a shunt resistance equal to the output impedance of a typical source. If you try to use a real source, its noise will also be included in the measurement (as with RMAA). Also, when measuring a source with a DAC it’s necessary to use a very low level test signal as DACs shut off completely giving an unrealistic noise value if there is nothing to play. A proper audio analyzer can remove the low level signal from the measurement leaving just the noise.

RMAA MEASUREMENTS: Even if you somehow calibrate the levels, you still don’t know what RMAA is doing internally. It’s a magic “black box” with no credible documentation about how it arrives at its final numbers. What bandwidth is being used? Is the result weighted or un-weighted? Plus the unknown output noise of the  RMAA sound hardware is included in the measurement by design. Ultimately, the best way to make noise measurements is with an audio analyzer such as those from Audio Precision and Prism Sound.

BOTTOM LINE: Noise of around –105 dBv (referenced to 1 volt) will nearly always be inaudible. Noise around -95 dBv is probably “good enough” for many. Noise referenced to other values must be converted to dBv or another consistent reference before it can be fairly compared. RMAA values are nearly useless because RMAA has no concept of absolute levels. It can only provide dynamic range and it often gets even that wrong because it’s difficult to set the levels properly without proper instrumentation.

OTHER RESOURCES: These articles may also be useful:

• Headphone Amps Explained
• Amplifier Power Requirements
• All About Gain 

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.