O2 Summary

THE SHORT VERSION: By popular demand, here’s an “executive summary” and Q&A for the Objective₂ (O2) headphone amp. There are links sprinkled throughout leading to revised information in the previous three large articles and elsewhere.

QUESTIONS ANSWERED: Before you post a comment to one of the articles, forum threads, etc., please see if your question has already been answered below.

WHAT’S AN O2? The Objective₂ (O2) is my attempt at a “One-Size-Fits-Nearly-All” headphone amp. It can drive most any headphone from the most sensitive IEMs to some really power hungry full size cans from 16 to 600 ohms. It’s small, inexpensive, and runs on batteries or AC power. The design is freely available to everyone subject to the License terms. See: O2 Headphone Amp

WORRIED YOU’RE NOT GETTING THE BEST SOUND? Reading through the forums it’s obvious a huge number of headphone lovers are worried they’re not getting the best sound from their headphones. Their worry may be justified as most of the headphone amps (and headphone DACs) on the market have one or more significant limitations or problems. Many only work well with a limited range of headphones. See: Going Shopping

LESS WORRY: Very few manufactures provide anything resembling credible specifications for their headphone amps and very few are independently tested on proper equipment. So you rarely know what you’re getting. I’ve discovered significant problems or limitations with everything I’ve tested with a headphone jack on it. The O2 is designed to ease such concerns by working well with nearly all headphones and having fully documented performance. See: O2 Headphone Amp

WHY DID YOU CALL IT THE OBJECTIVE₂? I’m an electrical engineer, serious audio geek, and took a very objective approach to the design. It’s a minimalist amp focused on the best performance possible for the least amount of money. See: Objective Pitch

POWER TO SPARE: In the testing I’ve done, with more than a dozen different headphones, either my ears, or the headphones, reach their limits long before the O2 does. I suspect with some worst case headphones (like some of the planars from HIFiMan) the O2 will merely get genuinely loud at full output. But for more typical headphones, the O2 will be well under its full output. Compared to most battery powered amps that are often pushed to the edge of, or well into, clipping that’s a very audible difference. The O2 is loafing when most portable amps are straining. And, unlike dual battery Cmoy amps, the O2 not only has lots of voltage, but it also has lots of current for low impedance loads that require it—like AKG K701s, Audeze LCD-2s and HiFiMan planars. And the O2’s two stage design vastly improves noise and distortion over a single stage Cmoy.

HEADPHONE SAFETY: Unlike every two battery Cmoy or similar design I know of, including the $650 Emmeline SR-71 Blackbird, the O2 won’t try to destroy your headphones if you run the batteries low or one battery becomes disconnected. It has a power management circuit to shut it down if there’s anything wrong with the power preventing dangerous levels of DC at the output.

LOW OUTPUT IMPEDANCE = TIGHT BASS: If you’ve been using a headphone amp or source with a significant output impedance you may be in for a treat with the O2. A sufficiently low output impedance provides improved bass damping with many headphones and the most accurate frequency response instead of an exaggerated peak at resonance. With my Beyer DT770-Pro 80 headphones, for example, the difference in the bass performance is easily noticeable (and survives a blind test) when using a high output impedance source versus the O2. With the O2 the bass is tighter and more controlled. Lots of headphone source gear has a high enough high output impedance to cause audible problem. The O2 solves these problems. See: Amplifier Impedance

WHAT DOES IT COST? The O2 can be built in functional “bare board” form for $30 - $40. The total parts cost with everything, including a pre-made front panel, batteries, enclosure, and AC adapter is around $75 plus shipping. The DIYer can have a complete amp for $80 – $100 all inclusive. Even fully assembled complete with batteries and AC power adapter it can still be under $150. See: O2 Price Details.

WILL THE O2 DRIVE MY HEADPHONES? Short of a few discontinued models, like AKG K1000 “earspeakers”, the answer is very likely yes. The O2 can drive normally challenging headphones including the AKG K701 and even HiFiMan planars. See: More Power?

DOES THE O2 HAVE ANY HISS WITH SENSITIVE IEMs? The O2 is the first amp I’ve tested that’s dead silent with any headphones I know of at any volume setting, and any applicable gain, including with my ultra sensitive UE SuperFi Pros. The noise is –116 dBv or –133 dB referenced to full output.

WILL YOU BUILD ONE FOR ME? There are already individuals and businesses offering to build the O2 for others either as a completed circuit board or a fully assembled amp. See O2 Resources for various options. I’m staying out of the commercial aspect. I expect as the O2 becomes more established there will be additional purchasing options in the future and I’ll be updating the O2 Resources links.

HOW CAN I GET ONE? Right now the O2 is mainly a DIY project and there will likely be more options in the near future. For what’s available today, see O2 Resources.

WHERE ARE THE LATEST UPDATES? Check out: O2 Important Information.

WHY DID YOU DESIGN IT? I shopped around for a decent headphone amp that didn’t have significant problems or limitations and couldn’t find any that were reasonably priced. Sadly, I mostly found heavily flawed designs unable to deliver what the recording engineer intended. I also wanted to show what can be done by properly implementing about $30 worth of parts. See: O2 Motivation

DO YOU HAVE A COMMERCIAL INTEREST IN THE O2? No. I don’t get any money, revenue or income of any kind from the O2. It’s open source hardware and I’m not selling anything. I don’t have a commercial interest in anything audio related at the moment. My day job is in another field.

WHY ARE YOU GIVING IT AWAY? I’m mainly trying to prove proper design goes a lot further than using expensive designer parts, following audiophile myths, trying to re-invent the wheel, resurrecting ancient single-ended designs, etc. The idea is to raise the bar in the industry as a whole. There are just too many marginal and/or overpriced headphone amps—both DIY and commercial. See: O2 Motivation

WHERE ARE THE MEASUREMENTS? The O2’s performance has been thoroughly documented using industry standard measurement practices. For a summary see: O2 Review. For the full measurements see: O2 Measurements 

WHAT ABOUT THE SOUND QUALITY? The O2 compares very favorably in listening tests against the critically acclaimed Benchmark DAC1 Pre’s headphone output. An O2 user has compared it to a $1000+ AMB beta22 (b22) and wasn’t sure he could hear any difference. As more amps get evaluated more and more favorable reviews are coming in. I have encouraged blind listening tests against other gear as well. See: O2 Subjective Pitch

COMPETITIVE CHALLENGES: Is the Camaro faster than the Mustang? Let’s take them both to the track and find out. First across the finish line wins! I’ve challenged others to beat the O2’s objective measured performance with amps up to three times the assembled price. And for the subjectivists, I’m willing to put the O2 up against suitable amps of any price in a proper blind listening test. See An Open Challenge for the details.

DOES THE O2 HAVE ADJUSTABLE GAIN? The O2 has a gain switch on the front panel. You might use one setting with IEMs and the other with power hungry full size cans. Or one gain for portable use and the other for home use. The standard gains are 2.5X and 6.5X but can be internally changed to anything from 1X to 12X. See: Gain Settings and All About Gain.

HOW LONG IS THE BATTERY LIFE? The regular version runs for about 8 hours and, by swapping out 3 ICs, it increases to about 25 hours. See: O2 Low Power Option

ARE THERE FORUM DISCUSSIONS? Yes, you can find the links in the O2 Resources section.

WHERE’S THE SCHEMATIC AND PARTS LIST? See: O2 Resources

WHERE CAN I GET THE PARTS? See: Obtaining The Components

IS THERE CONSTRUCTION INFORMATION? Anyone building the O2 should read the O2 Cautions and O2 Circuit Board Construction sections. I would also suggest: Using The O2

IS THE O2 A GOOD FIRST DIY PROJECT? Novice DIYers might want to wait until at least several others have built the O2 before deciding if they want to try building one.

HOW DID YOU COME UP WITH THE DESIGN? The high level objectives can be found in the Design Highlights section and the O2 design itself is explained in the Circuit Description section. The entire design process including the Requirements, Circuit Design, and Prototyping/Testing is explained in the O2 Design Process article.

CAN I UPGRADE THE OP AMPS? You could, but it would be a waste of money and might degrade the performance. I tested nearly two dozen op amps and the O2 is optimized for the op amps that proved best. See: Op Amp Myths and Op Amp Measurements. I have offered $500 for charity to anyone who can hear differences between op amps that measure reasonably well. Also check out: Designer Components.

CAN I USE A DIFFERENT ENCLOSURE? Yes. Several have already made some amazing enclosures including an elaborate one out of laminated wood. See: Enclosure Options

WHAT ABOUT CLAIMS ON HEAD-FI THE O2 IS FLAWED? As far as I know, the criticism was started by those with financial interests in more expensive products. And it’s mostly been put to rest. Please consider where the O2’s critics obtain their income (selling parts, amps, ads to sponsors, etc.) and/or their personal investment in far more expensive gear. I’ve requested the critics participate in either of the two official O2 forum threads on ABI and diyAudio but it’s telling they have refused. Instead, they seem to be hiding behind the admins who banned me at Head-Fi. I welcome an open discussion on either of the official O2 threads, valid criticisms, proper blind comparisons, and professional measurements. See Competitive Challenges above. The O2 isn’t perfect, but when you consider its size, price, battery operation, etc. I believe it’s hard to beat and any criticisms should be kept in proper perspective for a portable amp that costs $25 – $150 and can drive most any headphone.

DOES THE INPUT OVERLOAD? The O2 will not overload in any sort of normal use. Here are the facts (all input voltages are Vrms):

• Understanding Gain – Please see All About Gain.
• On AC Power - At 2.5X gain the O2 can handle 2.8V and at 6.5X gain it can handle over 1V.
• On Battery Power – At 2.5X gain the O2 can handle 1.8V and at 6.5X it can handle 0.7V (most LODs are 0.5V).
• The Competition – The very popular battery powered FiiO E7 overloads at 1.2V vs 1.8V for the O2. The equally popular AC powered FiiO E9 desktop amp overloads at 2.1V vs 2.8V for the O2. The Mini3 has about 6dB less overall headroom than the O2 with both running on battery power. The Mini3 also lacks a gain switch making it less flexible.
• The Math – The maximum gain on AC is 7 / Vin and on battery it’s 4.5 / Vin. So with a 0.5V iPOD LOD cable it’s 7/0.5 = 14X on AC and 4.5/0.5 = 9X on battery.
• Home Sources – The Redbook standard for home digital audio gear is 2V. Anything without a volume control above 2.8V would be extremely rare. But if you have such a device, just clip two resistor leads and the O2 will drop to 1X for one of the two gain settings. Or you can optimize the gain. See: All About Gain
• Portable Sources – Nearly all battery powered players have a maximum output of 0.5 – 1.0V and nearly all USB powered portable DACs are 1.4V or less. All of these are 100% compatible with the O2 without changing anything.
• Excess Gain – Some have claimed the O2 may not have enough “excess gain”—that is extra volume control range to “boost” improperly recorded/ripped music. Apple designed only a few dB of excess gain into their iPods because more would have been a serious compromise for 95% of the music people listen to. Too much excess gain is a problem because it renders a large portion of the volume control’s range unusable with most music. There are other negative side effects as well. Plus anyone using a PC or laptop as their source can add extra gain on the PC side to compensate for oddball recordings. They can also normalize any “quiet” tracks. With any headphones that work with the Mini3, for example, the O2 can provide at least 6 dB of excess gain even when operating on battery power. 6 dB of excess gain is plenty for more applications. With more sensitive headphones it can provide even more. See: All About Gain
• More Details – For anyone still concerned there are more details, including scope shots of the O2’s actual performance, under Maximum Input and Gain Stage Limitations. See also: All About Gain

WHAT ABOUT A DESKTOP ONLY VERSION? It’s possible to use the O2 board in a variety of ways in a desktop amp. This is discussed in the Enclosure Options and Circuit Board Construction sections. I’m also planning a different amp, based on the O2 design principals, for desktop-only use with the following possible benefits:

• Easier to construct as a desktop amp with everything self contained on the PC board rather than having to panel mount jacks, the volume control, etc. and use point-to-point wiring.
• Built-in 1/4" headphone jack (in addition to a 3.5mm jack)
• RCA input jacks (in addition to a 3.5mm jack)
• Most of the connections on the back for more tidy desktop use
• Higher input voltage range to handle even “very hot” sources at higher gains
• A more flexible gain structure allowing better gain matching
• A headphone protection relay preventing turn/off transients and offering DC protection
• Perhaps a few other enhancements as well such as an improved power supply

WILL THE DESKTOP AMP COST MORE? The desktop amp will require two machined panels instead of one adding about $20 and the other upgrades will add another $10 – $20 but you also save $11 as you don’t need the batteries. So if you buy both panels it might be around $30 more.

WILL THE DESKTOP AMP SOUND BETTER? It’s hard to say. The desktop amp will have even more gain flexibility than the O2 and there can be benefits to using a 1/4” headphone jack and RCA input jacks. The power supply may also have lower noise.

WHEN WILL THE DESKTOP BOARD BE AVAILABLE? Hopefully in the first quarter of 2012.

WHICH ONE SHOULD I CHOOSE? There’s a lot to be said for a portable amp. I’m using my O2 on battery power far more than I would have guessed. Do you want a BMW coupe or convertible? Both are great cars with their own benefits and disadvantages. There will be more details on the desktop version in the next several weeks.

O2 LICENSE: See: O2 Creative Commons CC-BY-ND License

WHERE ARE THE OTHER ARTICLES? There are three main O2 articles plus the op amp measurements:

• O2 Headphone Amp – The first article covering the design premise, review and measurements.
• O2 Design Process – The second article covering the requirements and design process.
• O2 Details – The third article with mostly practical information about buying/building an O2 as well as a detailed circuit description.
• Op Amp Measurements – Many different op amps tested in the O2.

BOTTOM LINE: I’ve tried to raise the price/performance bar for headphone amps with the O2. I’ve provided detailed measurements and the theory behind it. I hope the O2 encourages others to make, and publish, more accurate and complete measurements. I also hope it helps people smile more and worry less when listening to their music!

Banned at Head-Fi!

MOVING UP! I’m moving up in the world. First I found a number of inconsistencies with the AMB Mini3’s claimed specs, but rather than have a rational discussion about the discrepancies Ti Kan apparently found it easier to just ban me from the AMB forum (see the Mini3 Review). And now I get banned from a much bigger forum—Head-Fi! This time it’s apparently for questioning the safety of a Head-Fi sponsor’s product.

JUST TRYING TO HELP: I’ve spent hardly any time on Head-Fi since March when Jude started censoring my contributions there. But someone considering a Schiit Asgard headphone amp (yes, the company really is named Schiit Audio and the amp is the Asgard) wanted my opinion regarding a problem reported on Head-Fi. The problem appeared serious so I joined the discussion and here’s a rough break down of how it went (I have copies of the entire thread should it get censored and anyone is interested):

• There’s a rather shocking video of an expensive AKG headphone driver being literally deformed by a large DC current when the Asgard is shut off. I’ve never seen anything like it nor had several other people who commented on it.
• Jason Stoddard from Schiit Audio (a paying Head-Fi sponsor) responded claiming the power down transient of the Asgard is only 0.10 – 0.15 volts and supposedly similar to a half dozen other headphone amps. Therefore, he argued, the glitch was quite harmless. Several seemed satisfied with that response but it didn’t make sense to me. Driving a headphone diaphragm that hard against its limits should require much more voltage. So I continued to push for more answers.
• Several owners of Schiit products chimed in defensively to support the Asgard and some even started attacking me. In their eyes Jason had to be right and I was the bad guy. It was a classic case of “shoot the messenger” (see Subjective vs Objective). 
• Another Schiit employee, Mike Moffit, joined the discussion by first attacking my credibility and then singing the praises of Schiit in general. Not very helpful.
• Eventually, with some prodding, kwkarth who owns an Asgard reported his amp has the same problem and he measured the power down glitch at 2.2 volts—about 15 times worse than claimed.
• AKG reported even 0.25 volts of DC during power down might damage their headphones. Based on this, and the above bombshell, the original poster decided to return his Schiit Asgard.
• So I was correct all along and the glitch was far more serious than Jason had claimed. But, despite the glitch being well beyond what AKG considers risky, kwkarth continued to try and downplay the problem telling everyone the Asgard was still safe. This defied all common sense to me. With people’s headphones at stake, I and others tried to reason with him and got nowhere. Kwkarth is a Head-Fi admin, and claims to be an “audio engineer” in his profile. Yet, when challenged, he didn’t respond with much applicable engineering, just mostly hand waving, evasive answers, and silly animated pictures. It’s interesting to me Head-Fi would have someone like kwkarth who seemingly is willing to put people’s headphones at risk for some very questionable reasons. But then again, Schiit Audio is a sponsor.
• About 48 hours after my first post, I was banned from Head-Fi—for trying to protect people’s headphones from damage and not agreeing with a Head-Fi admin who would apparently prefer to see the same headphones harmed.

THE BIGGER STORY: The above drama points to a bigger potential problem at Head-Fi. Here’s a product that deforms headphone drivers and, according to AKG risks damage, but Schiit’s initial response was to dismiss it. And, more alarming, several Head-Fi members simply accepted Schiit’s response and a Head-Fi admin also insisted there was no issue. My impression was they were trying to say “nothing to see here folks” and wanted the whole thing to be forgotten.

THE NEED TO BELIEVE:  If you read the thread it’s really obvious people want to believe in their gear and the company that made it. I can understand people wanting to believe in Schiit. But many don’t realize how harmful DC can be to headphones. They may not understand that wrinking up headphone drivers (as seen in the video) likely creates weak areas in the diaphragm changing the sound quality. And some don’t know volts from Volkswagens so when someone like Jason says it’s only 0.15 volts they’re not in a position to question him. Ultimately, with enough continued pressure to find out the truth, Schiit admitted there might be a problem and offered some resolutions. So the ending turned out reasonably OK but the path getting there is still rather disturbing to me.

ANOTHER OUTCOME? Given that most seemed to accept Schiit’s original assertion the glitch was nothing to worry about, what would have happened had I (or someone else familiar with the technical side of things) not been around to keep questioning Schiit’s numbers? Would everyone have gone back to business as usual with Schiit cranking out Asgards that were quietly trying to destroy headphones on power down?

GEEKS CAN BE USEFUL: Ignorance is not bliss here. It’s good to have someone around to keep manufactures honest and help those who might be less technically inclined understand what the issues are. But, to me, it seems Head-Fi would rather keep their members in the dark and protect their sponsors--even if it means expensive headphones being damaged. Consistently, from the threads I’ve seen, the first reaction to any problem with a sponsor’s product is denial. And not just from the manufacture. Before they even come along other members are jumping in defending the product. And the next reaction is to bury the factual information in the thread with dozens (or often hundreds) of useless posts—like kwkarth posting silly animations instead of discussing the problem at hand. It’s weird and rather disturbing.

WHY THE BAN? I didn’t violate Head-Fi’s public rules for conduct. I didn’t call anyone names or attack anyone personally (although several tried to attack me). I didn’t use any offensive language. So why ban me? Jude’s public excuse for censoring me last time was that I was using Head-Fi to promote this blog but he deleted benign posts that didn’t even mention this blog. Everything behind the scenes pointed to the fact NuForce was seriously unhappy with me. Now, once again, I find myself in the opposite corner from a Head-Fi sponsor. And I end up banned. I can’t help but believe Jude, in both cases, is trying to protect his sponsors. Head-Fi, from what I can tell, is not an open forum where everyone’s welcome as long as they follow the public rules. Back in March Jude made it clear he could censor or ban anyone for any reason if I kept pressing my point. And censor and ban he does.

BAN UPDATE 7/20: It came out in the comments Jude claims I threatened Head-Fi legally. For the record, I did not and that makes no sense. I can’t conceive of any legal basis as I can’t claim any sort of damages or harm. I have no income related to any of this and Jude’s certainly within his legal right to ban who he wants. So how could Jude feel threated by legal action from me? The only legal involvement I’ve ever had has been seeking legal advice to make sure I don’t cross any lines with this blog that would put me at legal risk.

HEAD-FI: FACTS OR FANTASY? If Head-Fi bans people who are just trying to be factual and help Head-Fi members and visitors understand technical issues, and perhaps save their headphones from destruction, what does that say about Jude’s priorities for the entire site? I’m certainly not the first person banned there for just trying to speak the truth (several came out of nowhere and contacted me over the NuForce drama). And why has he banned any discussion of blind testing everywhere at Head-Fi except the back-of-the-bus Sound Science forum? The factual element seems to be largely missing at Head-Fi. From all I’ve seen it’s rather hard to find under vast quantities of myth, smoke screens, snake oil, and misinformation. And it seems to be rather deliberate.

FOLLOW THE MONEY: It would seem Jude is trying to maintain an environment that supports people spending lots of money on products from Head-Fi sponsors—like $1000 cables from Qables, Asgards from Schiit, etc. While suppressing guys like me obviously benefits certain Head-Fi sponsors, and hence Jude’s bank account, it seems harmful to the thousands of active members. Imagine a $1000 Qables product going up against a $5 cable in a blind test and losing. This article, using blind testing in audiophile’s own homes, discusses how that’s more likely that most probably realize. Check out Sound & Vision Wired Wisdom. It seems, because of his “blind ban”, Jude would rather Head-Fi folks see only the subjective claims and have objective blind testing be extremely difficult to even find. Why not let both sides be presented and Head-Fi visitors can decide for themselves?

SCHIIT AUDIO (revised 7/20): I have never tested a Schiit Audio product. It’s obvious they have plenty of loyal fans so it’s entirely possible they make some nice stuff. It’s also commendable they have stepped up to the problem.But I can’t avoid some NuForce déjà vu here. Small audiophile oriented companies, especially with headphone products, desktop products, and DACs, either seem to be skimping on measurements, perhaps because they design more by ear, or they’re seriously compromising their designs intentionally. This is often because they (usually falsely) think they need to in the interest of better sound. When it comes to the safety of expensive headphones, neither of those are acceptable to me. And I still don’t understand why the issue was initially dismissed given the description, video, etc. From my perspective, the best outcome here is to encourage Schiit, and all similar manufactures like NuForce etc., to do more testing before they ship their products and be more realistic and honest about their product’s performance. And if they don’t have the right equipment, expertise, etc. find someone who does. It’s likely cheap insurance in the long run. Otherwise, issues like this will just keep happening. So for those shopping, and not just from Schiit, caveat emptor.

BOTTOM LINE: I gave up on Head-Fi a long time ago so the ban isn’t a big deal to me. But you might want to ask yourself what Head-Fi’s real priorities are and remember they actively suppresses people and content. I personally believe it’s a very commercial environment focused mainly on the sponsors. Even Jude’s personal videos seem like paid infomercials for sponsor’s products. Ultimately, Jude wants you to buy stuff from his sponsors as that’s how he makes money. And, obviously, I’ve not been helping with that cause (although Benchmark is a sponsor and I have only good things to say about them).

ALTERNATIVES: If you’re not comfortable with the heavy commercial bias, irrational subjective bent, and censorship at Head-Fi, there are some great alternatives that are much less of a sponsorfest and much more manageable in size:

• AnythingButiPod (ABI) – This is a very friendly headphone/portable player centric site with some great content and a much better “signal to noise ratio” than Head-Fi. And most of their sponsors have nothing to do with the products being discussed.
• HydrogenAudio – This is a much more objective site than Head-Fi with lots of really smart people discussing often rather geeky topics. If you’re a geek, engineer, or more objective, it’s worth checking out.
• GearSlutz – This is more of a pro audio site but its loaded with some good discussions and reviews of headphone gear. Musicians and guys who record, mix and master music know a thing or two about headphones.
• diyAudio Headphones – For DIYers, this is the best place I know of.

DAC Listening Challenge Results

A BIG THANK YOU! I appreciate all the responses to both listening tests. I know listening for subtle differences requires some effort and I want to thank everyone for taking the time and sharing their results.

NOTE TO HEAD-FI MEMBERS (revised 3/28):  Unfortunately, before I posted this article, the administrators at Head-Fi.org started censoring links to this blog. So if you had some trouble finding the results here, I’m sorry. The links were apparently not a problem when I was posting at Head-Fi on several other products and topics. The censorship only started after a paying Head-Fi sponsors (NuForce) publically complained about my review of their product. Head-Fi has gone so far as even deleting posts from other Head-Fi members that reference this blog.

BACKGROUND: NuForce responded to their uDAC-2 measuring poorly by saying it was designed to sound good even if several measurements are notably bad. So I came up with the best way I knew how to judge the NuForce purely on sound quality. I also thought it would be interesting to compare the $29 Behringer UCA202 with a high-end product like the Benchmark DAC1 Pre. The DACs were recorded playing real music under as realistic and similar conditions as possible. Anyone could download the recordings and compare them without knowing which was which—sort of like a “brown bag” wine tasting. Would people like the $3 wine better than the $30 wine? I thought it would be fun to find out!

TEST METHODS: Not having conducted a public web-based listening test before, this was something of a learning experience for me. For the first round, to make the comparison as fair as possible, I used the line outputs of all three DACs . The second round of tests used the headphone outputs driving real headphones and included a modified version of the Behringer UCA202. The modifications were made using a few dollars worth of parts and improve the headphone output of the Behringer. The files were given names of US Presidents in the first test and common trees in the second. The original listening tests, and all the details, can be found here:

• NuForce uDAC-2 Listening Test
• DAC Listening Sequel

A NOTE ABOUT “MASKING” (revised 3/17): Some have questioned the validity of the tests because, as NuForce put it, the output of the DACs have been “re-digitized”.  And some also argue the equipment used to play back the files may not be high enough quality. The concern is these could mask the differences between the DACs. In reality this should not be much of an issue using playback hardware with reasonable fidelity. Here’s why:

• These trials are only about the differences between the files, not the absolute accuracy. It’s like shopping online for a shirt. You might look at 3 different blue shirts at the same online retailer. Even if your computer display can’t convey every color and detail perfectly, you can still easily tell most of the differences between the shirts if they’re good pictures, all taken the same way, and your have at least a reasonably decent display. For example if your computer distorts the exact shade of blue, you should still be able to tell which of the 3 shirts is the darkest blue. The same is true in comparing the sound files. Even if your headphones might exaggerate the bass, you can still tell differences between the bass in the various files. This “difference effect” has been well documented in research.
  
• As to “re-digitized”, the Benchmark ADC1 used to convert the output of the DACs to a CD quality file is already better than most studio gear used to record the music we all listen to. Studio A/D converters typically cost about $100 – $400 per channel of conversion. The ADC1 is a reference-grade A/D with a $900 per channel price tag. It generally has better specifications than nearly all studio gear. If the less expensive studio gear is good to enough to capture the subtle difference between say a high-end Steinway and a high-end Yamaha grand piano, the ADC1 should also capture audible differences between DACs.
  
• Most recordings have already been through many more steps and kinds of digital processing yet are still very revealing of subtle differences. So adding one more relatively “pure” step isn’t going to make much difference. The recordings used in these tests are unusually pure and transparent.

PARTICIPANTS: This was a very informal survey. A total of 20 listeners picked their favorite (and sometimes least favorite) tracks. Many ranked them top to bottom. Some only participated in one of the tests and/or one of the songs. Some used ABX and some listened conventionally. More participated in the first (line out) trial than the second (headphone out) trial.

SCORING: Not everyone participated in every trial, some devices were offered in more trials than others, and different listeners provided different sorts of “votes”. Someone who’s better with statistical analysis than I am is reviewing the raw data. But, for now, to summarize the results here’s my attempt at a rough analysis:

• Top choices scored 2 points
  
• Second (“Runner Up”) choices scored 1 point
  
• Least favorite (worst) scored –1 point (several indicated only their least favorite and not a favorite)
  
• The results are summarized by category (line output vs headphone output)

RESULTS (corrected 2 minor errors 3/16): Here are the total points scored for each downloaded file ranked from most favored (highest score) to least favored using the above scoring method:

Taft - Benchmark Line Out Brick House	9
Harrison - NuForce Line Out Brick House	8
Juniper - Benchmark CX300 Brick House	7
Wilson - Behringer Line Out Brick House	6
Jefferson - Reference CD Just Dance	4
Lincoln - Benchmark Line Out Just Dance	3
Oak - Behringer CX300 Brick House	2
Spruce - Mod UCA202 CX300 Brick House	2
Acorn - Benchmark CX300 Tis of Thee	2
Maple - Mod UCA202 CX300 Tis of Thee	3
Monroe - Reference CD Brick House (!)	1
Hawthorn - NuForce CX300 Tis of Thee	1
Fig - Behringer CX300 Tis of Thee	1
Jackson - NuForce Line Out Just Dance	0
Cypress - NuForce CX300 Brick House	0
Adams - Behringer Line Out Just Dance	-2
Pine - NuForce UE SF5 Tis of Thee	-4
Total Benchmark Line Out	12
Total Benchmark Headphone Out	9
Total NuForce Line Out	8
Total Behringer Line Out	4
Total Modified Behringer Headphone Out	5
Total Behringer Headphone Out	3
Total NuForce Headphone Out CX300	1
Total NuForce Headphone Out Ultimate Ears SF5	-4

COMMENTS ON RESULTS (edited 3/16): Even with the small sample size a few things were fairly clear:

• The NuForce headphone output with the Ultimate Ears SuperFi 5 Pro headphones was an obvious “fail”. Several expressed a clear dislike of “Pine” and nobody favored it. I think this is due to the large frequency response variations caused by the relatively high output impedance of the NuForce uDAC-2 with these headphones. Several commented the high frequencies were rolled off or “dull”. This consistent with what the measurements would predict.
• The reference CD tracks didn’t score as well as expected. For “Monroe” it’s likely because the reference track stood out as being different. Early in the forums it was labeled “by far the worst”. And that public comment likely tainted others into also hearing it as the “worst”. Six people rated it as the worst of that group of tracks. I personally think this is a good example of subjective bias--the original piece of music was deemed the “worst”! The Just Dance reference faired much better with a positive score of 4 likely because it wasn’t labeled publically as being bad. And much fewer even bothered to vote on Just Dance so the score of 4 was close to “perfect”.
• In terms of total votes, the Benchmark stood out as a clear favorite. It was included mainly as a “reference” and not necessarily as fair competition to the other two, much cheaper, DACs. But, some might have expected all the DACs to sound roughly similar via the line outputs based on the measurements. The could be many reasons why this didn’t happen. The most likely is the same “peer bias” that caused a reference track to be rated poorly (see above). But it’s also possible the cheaper DACs do have audible problems.
• The NuForce did much better via the line outs than the headphone outputs. Given the output impedance problem and higher distortion of the headphone output, this is what the measurements would predict.
• Any channel balance error was corrected in all tests. So this removed the audible imbalance of the NuForce helping it score better.
• NuForce’s claim of “better sound” doesn’t seem to be true when using the headphones. It faired poorly even with the more common and “impedance friendly” Sennheiser CX300’s.

Trying to draw other conclusions is a bit more difficult, but some things worth noting:

• Nearly half the people couldn’t hear any differences at all.
• The Just Dance track had a lot of inherent distortion that made hearing differences difficult for most. So those results are likely less valid (and there are fewer of them).
• A number of votes were somewhat randomly distributed between all the middle scoring test files. This may suggest more guessing rather than clear preferences.
• The line outputs of  the $29 Behringer didn’t do as well as its measurements would suggest. The NuForce and Benchmark were preferred. This could be “peer bias”.
• The differences among the individual files were more obvious via the headphone outputs rather than the line outputs. This is to be expected due to the impedance interaction.
• For those who argue tests like this mask differences, it’s interesting to note the test may not have masked any subtle advantages of the high-end Benchmark. This is despite the fact that few listened to the files on similarly high-end gear.

BOTTOM LINE: I think the most obvious thing is roughly half the participants couldn’t tell any difference and another group of the “middle scoring” results is almost random with no clear preferences. Comparing the headphone outputs, however, the differences seemed more obvious. And they were very obvious when using balanced armature headphones (the SuperFi’s) on the NuForce. It’s clear the frequency response variations created by the NuForce’s relatively high output impedance creates audible problems.

It’s apparent subjective opinions of differences are easily swayed by public comments. The reference track from the CD of Brick House was strongly disliked by six listeners. They heard something different and were easily swayed into thinking different was worse based on previous public comments. It just took one person to say something negative and several others followed. This is exactly the sort of subjective bias that affects the majority of online subjective reviews in forums. As further proof, others using ABX (which is blind) voted the same track their favorite.

There’s also significant evidence the NuForce uDAC-2 does not sound better—at least using 2 different types of headphones—as NuForce claims it does. But it seemed to do a respectable job via the line outputs—at least with the channel balance error removed.

Overall I think this has been an interesting experiment. I learned a lot about how to run (and not run!) a listening test. And there were some fairly clear results—some expected and some not. If there’s sufficient interest, I may try to build on what I’ve learned here and conduct future listening tests?

COMMENTS WELCOME: Please feel free to add comments to the end of this article on the results—especially if you participated. I’d also like to know how many are interested in future listening tests? They’re a fair amount of work to put together and only really valid if you get a reasonable number of votes. So it’s something I only want to do if there’s enough interest. Please feel free to make suggestions, etc?

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TECH SECTION:

IMPROVING THE RESULTS: I realize this wasn’t the best run study. It was more an informal experiment than anything. I think a larger scale listening test, that had more uniformity, would be needed to verify some of the closer results. If I do this again, I’ll research better methods, and I also welcome input from others with experience in this area?

DIFFERENT LOADS: It’s been suggested it would be also useful to include high impedance headphones and, except for the complexity that adds to the mix, I agree that would be interesting. I chose low impedance headphones as they’re, by far, the most popular—especially for use with a portable entry level DAC.

PASSWORD: An encrypted 7-Zip file was included with the file descriptions to prevent me from cheating or changing anything after people’s votes. The password is:

CY&YUMN5cZ9x2X8BhNj2t

PINE EXPLAINED: The graph below, in blue, shows the frequency response of the NuForce uDAC-2 using the Ultimate Ears headphones used in the trial:

The 4+ dB of response variation seen above is caused by the relatively high output impedance of the uDAC-2 interacting with the SuperFi headphones (typical of balanced armature designs from many manufactures). For more on this see Headphone & Amp Impedance.

OTHER DETAILS: The other details of how the test was run can be found in the original articles:

NuForce uDAC-2 Listening Test

DAC Listening Sequel