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发表于 2011-9-18 01:09:37
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O2 Headphone Amp
SOUND QUALITY: The O2 is a small unique headphone amp that compares well in listening tests with much more expensive gear. Are you tired of worrying if you’re getting the best sound from your headphones? Does your iPod or Clip+ fall short with your full size cans? Perhaps the portable amps you’ve tried can’t properly drive your AKG K701s? You might want to keep reading.
THE PROBLEM: I go into more details later but if you look at what’s available in headphone amps it’s not ideal. Very few have credible performance measurements, nearly all the portable models lack enough output for many full size headphones, the Cmoy designs are deeply flawed, many have too high of an output impedance, and the desktop amps that get it right are seriously expensive.
THE (nearly) PERFECT HEADPHONE AMP: Wouldn’t it be nice to have a headphone amp that was so quiet you could use it with the most sensitive In Ear Monitors but it also had enough power to drive virtually any full size cans including 600 ohm and current hungry planar models? And, while we’re at it, what if the output impedance was low enough you didn’t have to worry about frequency response or bass damping problems? And, for maximum versatility, what if worked at home on AC power or on-the-go from rechargeable batteries? If you combine all of this with audiophile approved sound quality, switchable gain, great measurements, and a reasonable price, you have something very attractive but also highly elusive unless you’re willing to spend lots of money.
Objective Pitch
WHAT MATTERS MOST: Most of us want our gear to get out of the way so we can listen to the music as the recording engineer intended. We don’t want to listen to our headphone amp, we want to listen to the music. And if that’s the goal, the path to getting there can be summed up with one word (photo: Jeff Keyzer):
Accuracy - ak-yer-uh-see: “The condition or quality of being true, correct, or exact”
THE ROAD TO AUDIO NIRVANA: Accuracy, and hence the best sound quality, in a headphone amp is almost entirely determined by (the non-geeks might want to skip this):
Output Impedance – It should be less than 2 ohms to provide the correct bass damping even with 16 ohm headphones and avoid frequency response problems. Output Impedance differences account for most of the variations in sound quality people hear between different headphone amps and sources.
Power – Sufficient voltage, current, and gain must be available to drive the desired headphones to realistic levels without strain. Many amps fall short with many headphones.
Noise – The amp should not contribute any audible noise in real world use. Most, unfortunately have audible hiss—at least when using sensitive IEMs.
Channel Balance – The stereo image should not audibly shift left or right with any realistic volume setting. This is another common problem.
Frequency Response – The output should be flat to within +/- 0.1 dB from 20 hz to 20 Khz and not slew rate limited at full power. But many headphone amps, including some fairly big names like Creek and Pro-Ject, use capacitor coupled outputs that roll off the bass into low impedance loads and add distortion. Tube and single ended designs can be even worse.
Distortion – All non-linearities (unwanted garbage added by the amp) should be under 0.01% (–80 dB) at midrange frequencies where the ear is most sensitive, and under 0.05% (-66 dB) across the rest of the spectrum when operating with the desired headphones and output levels. Singled-ended and tube amps nearly always fail this by a wide margin as do many conventional designs.
Transient Response & Stability – Ringing and overshoot should be tightly controlled with all realistic headphone loads. I’ve seen several amps get this wrong in their quest for needless excess bandwidth and/or excessive slew rates.
Phase response – Less than +/- 2 degrees phase error from 100 hz to 20 Khz assures the best possible imaging and spatial perception.
VERIFIED PERFORMANCE: Unlike 99% of other amps and designs, the O2 design comes with very detailed, and verifiable, performance measurements made using professional instrumentation. If you believe in accuracy, and listening to your music instead of your amp, the right measurements help assure you don’t have to worry about the O2 getting in the way of the music.
Subjective Pitch
PASSION, ART & EMOTION: Music is an emotional art form and some argue music reproduction is also an art. They argue there’s more to it than just numbers and science. And, even in my virtual lab coat with virtual pocket protector, I agree. But I think 99% of the art is at the very end of the signal chain—in this case the headphones. There are massive differences in the sound of different headphones. I’ve written about some of them in my HD650 Review. Much like a vintner leaves their personal signature on the wines they produce, so do headphone designers. There’s no such thing as a perfect headphone. They all measure significantly differently. And they all balance the various trade-offs differently. But in a headphone amp, you can get close enough to perfection to where the differences that remain have been demonstrated as entirely inaudible. The O2 is just such an amplifier.
LISTENING TEST: The O2 was stacked up against the well regarded $1600 Benchmark DAC1 Pre in a listening challenge. The DAC1 is something of a favorite in the audiophile headphone community. A lot of subjective critics, and those who have measured it, really love it. So it’s all the more satisfying nobody has yet been able to tell the O2 from the DAC1’s headphone amp. The DAC1 Pre earned Stereophile’s top class A rating in the headphone category, a product of the year award, and countless other accolades. It has been described as “stupefyingly good” and “a revelation”. The headphones used in the comparison include the Sennheiser HD650s, Denon AH-D2000s, Etymotic ER-4s, Ultimate Ears SuperFi Pro 5s, and Beyer DT770s. Collectively they cover a wide range of impedance curves, efficiencies, types and subjective tastes in sound. The Denons and Etys are ruthlessly revealing, the HD650s are incredibly musical, the SuperFis ultra efficient, and the Beyers show off performance in the deepest bass. I hope to conduct more listening comparisons involving the O2 and perhaps even a public challenge or two. In short, I’m willing to back up my claims for the O2. Perhaps an O2 listening challenge is in your future?
PRELIMINARY FEEDBACK: Initial feedback from others has been very positive. So far an O2 has been built in Australia and Hong Kong and both report good things. One compared the O2 to the $1000+ AMB beta22 using $1000 Audeze LCD-2 headphones and wasn’t sure he could hear any differences.
WHAT DID YOUR CABLES COST? Those who spent more on their cables than the total cost of the O2 may feel compelled to dislike it. It probably won’t matter the O2 likely outperforms whatever they’re listening to now. I’m sure many will expect more expensive, and more “esoteric”, amps to sound better. And, in the usual biased listening, their brain will likely deceive them into hearing just what they expect to hear. Some will probably brand the O2 as “sterile”, for example, because it measures well so their brain will serve up a “sterile sound” when they listen to it. This is an involuntary response (see the brain link above if you don’t believe me and check out Subjective vs Objective). So if you’re a fan of exotic cables and other expensive esoteric solutions to straightforward problems, you can probably stop reading here as you’ll be biased against the O2 from the start and only hear what you want to hear. The O2 is intended for those open to a more objective approach.
A NOTE TO BLING LOVERS: Clearly some people like bling. What their car looks like, or the name badge on the front, can be more important than how it drives. There are plenty of car, and audio manufactures, catering to such tastes. The O2, in basic form, is very big on performance but not so much on bling. To keep the price as low as possible it’s mostly function over form. DIYers, however, are free to make the O2 look as impressive as they like. They can have their high-bling cake and still feast on tasty performance. Slip the O2 into a fancy enclosure, with a big knob, expensive jacks, a blue LED, or whatever else suits your fancy, and nobody needs to know what’s inside. Just let them listen to it and they’ll likely be convinced whatever’s in there is made of unobtainium.
Going Shopping
FRUSTRATION: If you look at the headphone amps for sale, nearly all fit into one or more of these categories (photo: FreeAussieStock):
Missing Specifications - Very few amps have meaningful specs. Things like power output, frequency response, THD and crosstalk are meaningless if the load isn't specified and it almost never is. The output impedance is also a critical spec but is rarely provided. Vague specs are difficult to verify which likely means the manufacture has something to hide, or in some cases, never even properly tested their product. When such amps are properly tested they usually fall seriously short in one or more areas.
Inadequate Output - Lots of amps, especially portable ones just don't have enough power for many headphones. The realtively popular AKG K701 is a perfect example. Most portable amps, even supposedly "high-end" ones like the AMB Mini3, can't properly drive them.
High Output Impedance – Lots of otherwise decent amps have output impedances well above the 2 ohm limit. It’s a surprisingly common problem that causes audible frequency response errors and degraded bass performance. See the FiiO E9 review for more and also my Impedance article. Some manufactures raise the output impedance to provide short circuit protection, sometimes for stability reasons, and sometimes to try and mask other design limitations such as low frequency roll off due to capacitor coupled outputs.
Flavors of Cmoy – Nearly all variations of Cmoy amps suffer major limitations and/or serious problems. Most use op amps that don’t have anywhere near enough current capability for many headphones. And even into higher impedance headphones their distortion can be severe because they’re still not designed to drive anything less than 600 ohms. The dual battery versions can easily damage headphones with high levels of DC. And the single battery versions typically use very weak rail splitters or virtual grounds that further limit the output current and increase distortion.
Single Battery or DC Wall Adapter Designs – To deliver the best performance a headphone amp needs a dual power supply but many of the more reasonably prices ones cut corners in this critical area. Headphone amps using a single battery, and/or DC wall adapter power supply are limited to using a virtual ground with all the compromises that entails, or capacitor coupled outputs which roll off the bass and add distortion, or a DC-DC converter which tends to add noise to the audio signal. While DC-DC can be made to work reasonably well, it’s mostly found in otherwise compromised amps. The other two options are typically much worse.
eBay Amp Roulette – Shopping headphone amps on eBay is a minefield of disappointments. A lot of poorly designed amps seem to be “liquidated” on eBay where they’re unlikely to be returned and can be sold anonymously. The Cmoy I tested was a good example—it had no voltage gain. Others have high distortion, poor volume tracking, etc. I’m sure there are some decent amps to be found on eBay but the odds are stacked against you. And eBay is full of knock off products. Even if someone finds a respectable eBay amp, 3 months later it might be gone, badly cloned, or using entirely different parts. This all makes it difficult to recommend most of what’s on eBay.
Single Ended and/or Limited NFB Amps – While these designs are hard to justify by any objective criteria, some people apparently like their added distortion, or at least buy into the hype. If you’re after the best accuracy, however, you won't find it with any single-ended amp. These amps are sort of like having someone softly murmuring in the background while you’re listening to your music. They often make their presence known rather than getting out of the way. You don’t just listen to the music, you may also forced to listen to the amp due to their higher output impedance, capacitor coupled outputs, and sometimes alarmingly high levels of distortion.
Tube Amps – Tube amps fit in the same category as Single Ended above but usually with even more obvious flaws. Some like them for nostalgic reasons, and some like to endlessly tweak and modify them, try different tubes, etc. But, ultimately, they’re the opposite of higher accuracy. They can get in the way of your music in obvious ways--especially with more challenging headphones.
3 Channel Designs – I’ve shown how the supposed advantages of 3 channel designs are yet another audiophile myth. So far in my testing I’ve only seen disadvantages to 3 channel amps—some rather significant. At best they’re a waste of money and put a lot of unnecessary electronics between you and your music. At their worse they seriously degrade the audio performance. They’re the complete opposite of the monoblock concept instead sharing lots of distortion inducing circuitry between both channels.
Expensive – The reputable headphone amps I know of with real bipolar power supplies, suitably low distortion, low output impedance, proper grounding, enough power, etc. tend to be rather expensive. You can buy a decent brand new laptop computer for less than the least expensive amps I know of that meet the criteria. Violectric has some of the least expensive options I know of that provide detailed performance specifications.
STALE AIR: In my experience most headphone amps (and headphone DACs) fail the accuracy, or cost, criteria above. Many have a 10 ohm or higher output impedance (QRV09, FiiO E9) while others lack enough output for popular headphones (FiiO E5, E7, Mini3) And some have excessive distortion (Mini3, NuForce uDAC-2). Some have poor transient response and/or are borderline unstable (QRV09, Mini3). Amazingly, a $39 Cmoy came closest to the above goals but it still had some serious fatal flaws—especially when configured with typical gain and into low impedance loads.
FRESH AIR: The Objective2 a conventional 2 channel amp, with 2 batteries, hence the “2” in the name. But O2 also represents oxygen, and in some ways, this amp is a breath of fresh air. It has none of the limitations listed above. It simply does a very credible job of disappearing from the signal chain leaving just the music as the recording engineering intended. Some might call it “straight wire with gain”. It’s all about accuracy.
Motivation
HOW HARD CAN IT BE? As they say on Top Gear: How hard can it be? Frustrated with commercial offerings that consistently fail with the flaws on the Going Shopping list, I wanted to prove it’s not rocket science to have your cake and eat it too. Unlike Top Gear’s comedic attempts at various challenges, I took a more objective engineering approach to designing my own headphone amp. (photo: anujpradhan.com)
ONE SIZE FITS NEARLY ALL: There are a lot of headphone amps that work well with some headphones but not others. This is especially true for portable amps most of which can’t even drive AKG K701’s properly let alone most of the planar cans. Others have output impedance issues, etc. Rather than take the typical expensive overkill approach, I put on my engineer’s hat and took an objective approach. I established a few worst case hard-to-drive headphones and worked backwards from there (see O2 Design Process). The result is the O2 should comfortably drive most any non-electrostatic headphone your average audiophile would want to use. That includes dynamic, planar/orthodynamic, or balanced armature, from 16 ohms to 600 ohms. The O2 has 2 gain settings to help match it to different sources and headphones. And if your headphones sound better with a higher output impedance, that’s easily accommodated too.
EXOTIC COMPONENT MYTH: The O2 proves you don’t need exotic parts or esoteric circuit designs for best-in-class sound, accuracy and performance. The O2 is a fairly minimalist amp designed around the solid objective goals listed above, not subjective hype or audiophile myths (see: Subjective vs Objective). It doesn’t aim to be “warm” by rolling off the highs, try to be “sweet” by adding a bunch of distortion, or alter the bass via a higher output impedance.
CREDENTIALS CHALLENGED: This article might seem a bit more, um, “enthusiastic” than my usual reviews. And some of that is no doubt personal bias creeping in. But, truth be told, I do have a point to prove. I’ve been attacked for my “lack of credentials” in reviewing other products. It’s funny; when I give something a favorable review my credentials are rarely challenged, but if I’m critical of gear someone owns, I’m suddenly a fraud and can’t be trusted. There’s more on this rather one sided phenomena in the Subjective vs Objective article.
An Open Challenge (two of them)
MY CRITICS: My critics have said things to the effect of “where’s your amp?” or “what have you designed?” I’ve designed plenty of things, but for various good reasons, they’re not disclosed on this blog. But the O2 is different. It was designed to fill a void in the headphone amp market driven by what I’ve learned about the audiophile headphone community since starting this blog. (photo: Ford Motor Company)
THE OBJECTIVE CHALLENGE: I’ve given my critics what they asked for and hit the ball over the net, so let’s turn the challenge around! Can anyone show me a portable headphone amp that overall objectively performs better for even triple the finished assembled price of the O2? If so, we’ll get someone independent with a real audio analyzer to test both. The O2 should be available for $150 or less fully assembled so I’ll put it up against anything up to $450. May the best amp win!
THE SUBJECTIVE CHALLENGE: Let’s raise the bar even further for all the subjective guys. For any amp that measures sufficiently well into the desired load (reasonably close to the specs outlined in the O2 Design Principals), regardless of cost, I’ll put the O2 up against it with any popular headphones within its drive capabilities. The challenger can pick the other amp, source, music, and headphones. The listening will be done blind using an A/B/X box and the comparison will be recorded on video for publication on YouTube. The test would be administered by an independent third party (I won’t even be present). The results, win or lose, will be published on this blog. And to sweeten the deal still further, if someone beats the O2 in a valid test, I’ll give $500 to the charity of their choice. If they lose, they give $500 to the charity of my choice.
COMPETITION & FAIRNESS: Raising the price/performance bar benefits consumers and that’s all I’m trying to do here. The primary goal of this blog is to get more objective information out there to help those interested better decide how to spend their money on gear. People want to buy things that live up to the manufacture‘s or designer’s claims. I’m trying to encourage manufactures to hopefully publish more and better specs and make sure their products meet those specs when properly measured..
WHERE ARE MANUFACTURE’S MEASUREMENTS? If you’re a manufacture producing a headphone amp, or even a DIY designer promoting your designs, it’s a really good idea to fully measure the performance of any amp to make sure it’s operating correctly, safe for headphones, and performs as intended. If you don’t make the right measurements, you have no idea if you got it right. So if that sort of testing is being done, why do so few manufactures publish any credible test results? It seems you have to spend $650 – $2000 for something like the Benchmark DAC1, Anedio D1, Violectric V90, etc. to get real measurements. If all the other amps perform as well as their creators claim, why can’t they publish some real evidence instead of a couple useless vague “specs”? In my opinion, they’re either not properly tested at all, or the measurements are so poor they don’t want to share them. Neither is very encouraging.
Design Highlights
A CASE FOR SIMPLE: Pointless excess is very 2007. Unlike that gold badged Cadillac Escalade that won’t fit in your neighbor’s garage, or some jacked up heatsinked monster headphone amp that looks impressive but falls over when you attempt a tricky corner at speed, the O2 was designed to be lean, mean and perform very well under real world conditions. Think of the O2 as a sort of Mazda MX-5 Miata. The Mazda is a classic design over 20 years old, that’s relatively minimalist, small, light, nimble, well engineered, amazing fun to drive, and very pure. Many find their way onto tracks on the weekend yet offer a comfortable car to drive to work during the day. There’s nothing excessive about it and it’s very reasonably priced for an open roof sports car. The current MX-5 is a lot like the original 1989 model. Mazda has admirably resisted the temptation to mess up near-perfection just trying to be trendy or be adding lots of weight and things people don’t need. It’s the same with the O2. It’s a very pure, honest, small, nimble, amp that is faithful to the music and amazing fun to listen to. (photo credit: Agath B)
SIMPLE O2: In the spirit of the MX-5 Miata, the O2 was designed to be as simple as possible. There are no tricky surface mount parts so no special tools or skills are required. To keep shipping costs much lower, and make things easier, all the components except the PCB and enclosure are available from Mouser Electronics in a single order. Most of the parts are very common and nearly all have Mouser substitutes if something is out of stock. There are also part numbers from worldwide distributors like Farnell. The board slips into an inexpensive all aluminum enclosure with no mounting hardware required. All the connectors and openings are along one side and require only round holes in standard drill sizes—no tricky machining required. And, to avoid any metal work completely, just send the supplied file to Front Panel Express for an inexpensive ready made front panel.
TWO BATTERIES BEAT ONE: As explained in my Virtual Grounds article, real grounds work best. The easiest, and most pure, way to a real ground in a portable headphone amp is to use two batteries. It makes the amp a bit bigger, but the single battery options are generally much less attractive and the reviews on this website back that up.
LOW BATTERY PROTECTION: A significant problem with dual battery designs is what happens if one battery dies first or becomes disconnected. Under those conditions the amp could destroy your headphones with DC. The O2 uses, as far as I know, a novel approach to solving this problem. It shuts the entire amplifier down when either or both batteries start to get low or if one battery becomes disconnected. This not only prevents harmful DC at the output, it also helps protect the rechargeable batteries from being damaged by cell reversal. So with the O2 there’s zero worry about the batteries. You can safely listen until it shuts itself off.
GAIN SWITCH (added 7/24): By popular demand, the O2 now sports a front panel gain switch. This allows more optimal use with different sources, and most important, headphones of widely different sensitivities. So you can use your IEMs on the go and your power hungry big cans at home with a flick of a switch. It’s part of the One-Size-Fits-All philosophy.
DESKTOP OR PORTABLE? The default configuration of the O2 is to be as small as possible. This means 3.5 mm mini jacks, everything mounted to the PC board (nothing panel mounted), and using the enclosure shown in the photos. However, for those who want a desktop amp, you can use the next taller case (same width and depth) and the PC board slides in with room to add high quality desktop 1/4” panel-mounted jacks. You can also add RCA input jacks if you want on the front or back panel in the same way. You could have multiple headphone jacks with different output impedances or even a 4 wire jack. All this can be done very easily with basic DIY skills.
OPTIONS: If you want to use panel mounted jacks, you can use the next taller enclosure from Box Enclosures (the B3-080) and easily add one or more panel mounted 1/4” headphone jacks above or below the O2 board. You can also add RCA inputs on the front or back (see Desktop or Portable above). Many more options are possible including customized gain, alternate output impedance, a lower power version, and more. See the O2 Details article.
The Project
DIY OR COMMERCIAL? For now the O2 is a DIY project. But see the O2 Details Resources for various options to get one. If there’s sufficient demand it may well be available in other forms with no soldering required—possibly even complete amps. And if you’re into DIY, you know the magic of listening to something you made with your own hands. The O2 can provide that magic.
OPEN SOURCE HARDWARE (please read this!): I don’t want to make any money from the O2. Ever. Like this blog, it’s free and here for those who want it. There are no ads, sponsors, hidden agendas, or online stores. I’m making the O2 design available to everyone under a Creative Commons License. I’m working with at least one vendor to sell the bare PC board at a price covering only their costs—no money for me. If there’s sufficient demand, you can expect kits, assembled boards and/or complete amps to become available. And it’s available for commercial use for by others (I have no interest). So AMB, for example, is free to publish the design and sell O2 components at their usual profit in the AMB store. And Mister-X can build the O2 for profit. The design can be used by anyone, personal or commercial, as long as the license terms are honored:
This work is licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.
THE BLOG TRILOGY: This is the first article in a series of three. There’s lot more in the later two articles:
O2 Design Process – This article covers the design methodology from setting the original goals and requirements through to arriving at more or less the final design.
O2 Details – This covers everything else including non-DIY options, where to get parts, tips for construction, build options, etc. |
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发表于 2011-9-17 23:26:47
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