“Hey, I was just wondering what the major differences were between the ODAC and the OL DAC.”
This fine question continues to pop up in emails, on the phone, on reddit, on Head-Fi, etc.
I was excited to push OL DAC into the wild last November for a number of reasons. I’ve always placed great trust in JDS Labs customers, finding them to be knowledgeable value hunters, and OL DAC set a new bar. Alas, we omitted too many details at release, like why we created another transparent DAC in the first place. Rumors took off. My favorite assumptions include:
ODAC and OL DAC are the same circuit in different boxes (False)
OL DAC costs less, so performance must be lower (False)
In short, the DACs share few similarities, aside from comparable transparency.
OL DAC clearly has the upper hand in terms of performance.:
+/- 0.04 dB
+/- 0.15 dB
< 0.0029 %
< 0.0010 %
Dynamic Range (A-Weighted)
> 112 dB
> 114 dB
Crosstalk @ 1kHz, -10dBFS (RCA)
Sum of Jitter Components @ 11025 Hz
IMD CCIF, -6.03 dBFS, 19/20kHz, 24/96k
IMD SMPTE -2VRMS, 24/96k
Linearity @ -90dBFS
+/- 0.01 dB
THD+N Sweep (24/96kHz, 20-20kHz)
Frequency Response (24/96kHz, 20-20kHz)
USB Jitter @ 11025Hz
Why Another Transparent DAC?
ODAC was interesting five years ago for its claim of transparency at only $149. DAC performance and features improve every year; OL DAC is the logical successor. While there’s no need for “greater transparency”, few can argue with getting more for less.
There remains one potential advantage to choose a Standalone ODAC–running from USB power can be useful in certain scenarios. Thus, we decided to maintain ODAC and OL DAC concurrently. Confusing? Yes, sorry about that.
Today we’re announcing a chipset update to ODAC. Revision B improves general reliability, while meeting or exceeding the original performance criteria set forth by NwAvGuy.
This announcement will come as a surprise to many, considering ODAC was declared as the be-all and end-all of DAC transparency by a now absent engineer. This article explains who owns the ODAC design, why an update is prudent, and how ODAC Revision B’s objectivity has been exhaustively verified.
Scroll towards the end for benchmarks, or read on for the full story.
ODAC was released on May 9, 2012, shortly before NwAvGuy vanished from the community. While his name is closely tied to ODAC, it’s critical to understand that ODAC was jointly developed by NwAvGuy and Yoyodyne Consulting.
Yoyodyne generated ODAC’s schematic and PCB, and NwAvGuy provided prototyping feedback and performance analysis. Yoyodyne also generated the project title, “ODAC” in 2011 and has remained responsible for all production engineering and distribution of the project to end retailers like JDS Labs and our counterparts.
In other words, ODAC was benchmarked and certified Objective by NwAvGuy; Yoyodyne generated the design and controls its manufacturing to this day.
NwAvGuy’s name has been intentionally omitted from ODAC RevB, so as not to imply an ongoing collaboration.
Why Update ODAC?!
Our job is to deliver perfect audio performance to every user. We’ve hit this goal for 99.5% of ODAC users out of the box, and have found a way push ODAC’s reliability and objectivity to an even higher standard.
To better convey ODAC’s position, Yoyodyne has shared worldwide distribution data. ODAC’s popularity continues to grow. Over twice as many ODACs shipped in 2014 compared to 2012, with a total of 12,000 units in circulation:
Increasing demand over time is amazingly rare for electronic production, and is a testament to ODAC’s positive reception.
Although ODAC has proven itself in the audio community, JDS Labs and fellow retailers have observed lower than expected yield (<1% DOA units), higher than expected long-term failure rates (< 2%), and an ongoing USB hub issue that NwAvGuy did not have an opportunity to address before his 2012 departure.
One of the first bits of ODAC feedback we received in 2012 revealed odd behavior: severe distortion, completely resolved by a USB hub. This peculiarity would ultimately affect less than 0.5% of all users, and the simple USB hub solution became well known within the audio community (later published to ODAC’s operating instructions). We invested in a dScope Series III audio analyzer in 2012 and verified ODAC’s performance.
The behavior was later identified as a power supply regulation design choice made by NwAvGuy. ODAC performs consistently with all devices, unless the host USB bus has remarkably low ESR ceramic capacitors placed too closely to the USB 5V output pin (rare). When ODAC is connected to such a host computer, ODAC’s 3.6V linear regulator performance plummets from 100% stable operation to extreme oscillation, which turns the perfect audio signal into garbage (lots of very audible distortion). There is no in-between. The regulator is either 100% stable, or 0% stable. Consequently, we’ve offered support for this rare behavior since 2012.
So, ODAC performs as described for about 99.5% of users. As demand grows, that USB bug becomes increasingly pronounced. Add in 1-2% DOA and long-term ES9023 failures, and ODAC retailers have growing collections of bad ICs. DOA boards are easy to catch via quality control, but long-term failures require frustrating warranty service.
Meanwhile, JDS Labs and Yoyodyne have engineered solutions to each reliability concern, meaning we can make ODAC reliable and objective for virtually 100% of users.
Yoyodyne produced a series of ODAC RevB variants in 2014 with reliability fixes. JDS Labs benchmarked each prototype to ensure equal or better performance compared to the original ODAC. Although the update was ready in late 2014, ODAC production runs occur about once annually. This long production cycle is best for the project, as it minimizes supply constraints and keeps distribution flowing smoothly to several O2/ODAC manufacturers.
I think the community hoped NwAvGuy would return and publish necessary updates to O2/ODAC/ODA himself. At this point, a reliability update is the best judgment we can make for ODAC’s long-term success. Keep in mind that O2 is protected from derivatives by its license; ODAC is coordinated by Yoyodyne and nondisclosure agreements with its IC suppliers. Even so, we do not want to modify ODAC. Subjective bias is not trivial in the audio business.
All of that being said, we’re confident ODAC RevB is a perfect reliability update. The newer DAC IC has proven reliable in other projects. In addition to thorough benchmarks, we’ve shipped ODAC RevB to a few users seeking support for their original ODACs. Feedback is perfect. We also shared ODAC RevB at the 2015 AXPONA tradeshow and allowed some random visitors to perform A/B tests. No one could differentiate.
ODAC RevB resolves all reliability inadequacies of the original ODAC, while meeting or exceeding original transparency requirements.
ODAC RevB utilizes the same PCB footprint and is a physical drop-in replacement to all existing ODAC and O2+ODAC assemblies. Revision B’s stronger output voltage of 2.10VRMS must also be accompanied by a slight DAC volume or gain adjustment when used in O2+ODAC; optimal gain is now 1.0/3.33x.
Analog filters and power supply passive components remain identical to the original board. The new chipset consists of an SA9023+PCM5102A, and the LDO has been updated to a ceramic stable Analog Devices ADP151 equivalent part. Fixes include:
Added 16x vias to USB support pads to improve mechanical strength of mini-USB jack
New chipset and locked EEPROM to prevent IC failures
Fixed USB supply stability, affecting < 0.5% of systems
Minor performance improvements (audibly equivalent)
ObjectiveDAC was designed for measurable and audible perfection. Reduced performance from ODAC RevB would be absolutely unacceptable, so we took great care in checking our work.
Engineering test methods impact test results. While THD+N, frequency response, and crosstalk are straightforward, even these basic tests are impacted by audio analyzer setup parameters and real world hardware setup. Certain ferrites on the mini-USB cable improve dynamic range by up to 10dB versus an ordinary USB cable. More complex tests like Jitter and IMD produce surprisingly different results based on signal strength, averaging, etc.. As Yoyodyne and I analyzed performance of the original ODAC through a TDK ZCAT2035-0930 ferrite equipped USB cable via dScope audio analyzers, it was clear that NwAvGuy had utilized averaging and custom dScope routines. We would never be able to definitively duplicate his work due to unknown averaging, scripting variables, and exact ferrite type.
To ensure a fair comparison, we measured a randomly selected ODAC production unit to establish baseline requirements. Measurements were repeated with two additional, randomly selected units to confirm consistency. The exact same cable and test scripts were then repeated with ODAC revB. All tests are performed under a standard 100k load.
In particular, please note that many of our measurements are taken at different signal strengths and sampling rates than used by NwAvGuy. Our table results are also taken without averaging; instead, we observe worst case performance over the course of 5 seconds of data collection.
So do not be surprised that our baseline ODAC measurements reflect lower performance than NwAvGuy’s nicely averaged 2012 results!
Frequency Response, 20-20kHz
THD+N 100 Hz, -0.15dBFS
THD+N 20 Hz -0.15dBFS
THD+N 10 kHz -0.15dBFS
– 103 dBu
Dynamic Range (A-Weighted)
> 111 dB
> 112 dB
Dynamic Range (Un-Weighted)
> 107 dB
> 109 dB
Crosstalk @ 1kHz, -10dBFS (3.5mm)
Sum of Jitter Components @ 11025 Hz, -1dBFS
IMD CCIF, -6.03 dBFS, 19/20kHz, 24/96k
IMD SMPTE -2 dBFS, 24/96k
Linearity @ -90dBFS
Frequency Response: NwAvGuy’s DAC Transparency Guideline calls for response of +/- 0.1 dB from 20 Hz – 19 kHz. RevB is slightly flatter than the original ODAC, exceeding the proposed transparency requirement for the complete audible range, 20 Hz – 20 kHz.
THD+N: The original ODAC measures 0.0056% at 10kHz -0.15dBFS using our worst case scenario measurements (see above table). RevB manages just 0.0024% under the same condition.
Shown below are THD+N -1dBFS, 8x averaged sweeps of each channel, directly comparing ODAC to ODAC RevB . The original ODAC’s right channel closely resembles NwAvGuy’s 2012 THD+N sweep, with a peak of 0.005% at 9kHz, and 0.004% at 10kHz. Note that the Left channel of the original ODAC differs from its Right channel in our sweeps. This observation is consistent across each unit tested from 2013 and 2014 production batches, despite no channel differences visible in 2012 benchmarks. RevB’s THD+N is consistent between Left and Right channels.
Revision B also cuts THD+N in half at 10kHz, and remains below 0.0030% across the entire audible spectrum for each channel. Both versions are well below NwAvGuy’s suggested transparency limits (green line).
Full-Scale Performance:Rumor suggests that PCM5102 clips at full scale. We first investigated this concern in 2013 with the then newly released PCM5102A. Empirical results show clean sine output at all frequencies. ODAC’s ES9023 reaches 1.99VRMS, and RevB’s PCM5102A generates 2.07VRMS at 0dBFS.
RevB’s full-scale performance is remarkably similar to the original ODAC. Notice that both DACs produce THD > 0.005% at 0dBFS due to FFT summing phenomenon at full-scale:
Elevated THD at 0dBFS is consistent for all DACs we’ve measured, and is the reason engineers (including NwAvGuy) typically conduct DAC benchmarks at -1dBFS or -3dBFS. Simply put, digital to analog conversion is less ideal at 0dBFS. Any reasonable recording should be free of such peaks. At any rate, it’s ideal to slightly reduce DAC volume when listening to recordings containing frequent 0dB peaks.
Noise: The newer PCM5102A DAC automatically enters a soft mute condition in the absence of an audio signal, pushing the measurable noise floor to an impressive -115dBu (near the dScope’s measurable limit). Therefore, noise was also measured with an applied -180dBFS, 20kHz signal, revealing the active state noise floor. RevB manages -103 dBu, slightly superior to the original ODAC’s -102 dBu. All noise components of RevB are well below the transparency requirement of -110dB for both mute conditions.
Dynamic Range: RevB improves A-weighted dynamic range by about 2dB, and achieves a cleaner noise floor.
Crosstalk: The PCM5102A’s soft mute function causes a standard crosstalk measurement to produce abnormally impressive results, as one channel is digitally muted. Thus, crosstalk looks substantially superior at all frequencies for RevB. Crosstalk measurements are similar between ODAC and ODAC RevB with a sufficiently small signal applied to the “muted” channel. Also note that the 20kHz “Ch A” test point is invalid for all four curves, as the dScope script conducts the test too quickly during relay initialization. “Ch B” curves at 20kHz are accurate.
Jitter: Testing is conducted using an 11025Hz, -1dBFS signal with 8x averaging.
Reliability fixes only necessitated a new power supply LDO and DAC IC. We swapped the USB controller for two reasons. First, the SA9023 provides 16/88.2kHz support. Second, its jitter performance is noticeably superior to the older TE7022L. We actually tested a TE7022L+PCM5102A prototype in effort to stay closer to the original ODAC. The SA9023 was ultimately a finer choice. Keep in mind that even the TE7022L produced audibly insignificant jitter (components below -110 dB). Hopefully 16/88.2kHz functionality adds value to some.
IMD SMPTE: The 60Hz/7kHz IMD test returns similar measurements for both DACs: 0.0008% using a -2dBFS signal referenced to 2VRMS.
ODAC produces audibly negligible sidebands (below -120dB) within a few thousand Hz of 7kHz, whereas RevB’s distortion shows less jitter but higher amplitude components around the same tone. Note that all of these components are more than an order of magnitude below the audible transparency limit of -90dBFS (green line).
IMD CCIF: Twin tone amplitude is closely matched in the IMD CCIF 19/20kHz test. The test returns numerically superior measurements for RevB due to smaller 1kHz components.
Sidebands are slightly more pronounced from RevB. While sidebands are higher, NwAvGuy prescribed a maximum sideband limit of -90dBr with 2VRMS reference for frequencies below 19kHz, and -80dBr above 20kHz to achieve transparency. RevB meets expectations.
The dScope is internally limited to -6.03dBFS for the Twin-tone script. Yoyodyne points out that NwAvGuy displayed a sum of powers and utilized custom scripts when conducting IMD measurements (-6dB + -6dB = -3dB).
Linearity: Both versions demonstrate excellent linearity from -1dBFS, down to their respective noise floors.
Price and Availability
ODAC RevB begins shipping in all JDS Labs ODAC products ordered after 9:00AM CST on Monday, May 11 with unchanged pricing:
Lately we’ve received several phone calls which start like this:
Customer: “Hi, I’m new to high end audio. Does [insert product] have a DAC in it? Do I need a DAC?”
From these discussions, two observations are clear.
DACs are suddenly “hot” on the market.
The majority of prospective DAC buyers, especially those new to the scene, have almost no idea what a DAC does.
We sell one of the best reference grade DACs in the world, so this is concerning to me. I’d never shell out $100+ to buy something I didn’t fully understand, and the average customer is no different. He’s turning to Google to gain a better understanding of these mystical “Digital-to-Analog Converters”. Unfortunately, the same people who call keep telling me that they can only find bad/mixed information.
What is a DAC?
Put simply, a DAC is the chip which turns your digital music into something you can physically hear. Every digital audio player (laptop, cell phone, tablet, iPod, etc.) already has an internal DAC and a weak headphone amp.
In terms of the audio chain: [Digital audio source] –> [DAC] –> [Amp] –> Headphones
Why use an external DAC?
If you’re shopping for a DAC, your goal is either to fix poor audio performance, or you’re in the much greater pursuit of reference quality audio. Either way, you’re seeking peace of mind that your audio is audibly perfect.
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.
PC DACS: […] 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.
My advice to new users mirrors NwAvGuy’s recommendations. If you want better audio, start with nicer headphones and a headphone amp. An external DAC is a great investment when your source is noticeably poor (background hiss, device interference, etc.).
When you connect a headphone amplifier to an audio player, the amp relieves your audio source from its difficult job of driving headphones. The source now sees an easy load from the external amp. Assuming you’ve selected a high quality headphone amplifier, output power improves, output impedance issues are resolved, THD+N improves (due to the easier load), and background hiss is minimized through input attenuation at the amp. In other words, a headphone amplifier solves the most audible problems.
A high performance DAC ensures that your headphone amp receives a high quality input signal. Since signal quality of most DACs is already excellent, further improvements from an external DAC tend to be minimal.
Lifehacker polled users last month for the Best DAC. The ODAC was nominated, and a tube based amp+DAC won the poll. Considering the $1000 pricetag of the winning nomination, lifehacker correctly identified that they’d actually created a poll of aesthetics. Results are more correctly titled, Most Popular Digital-to-Analog Converter. So, what is the best DAC??
Over the years, I’ve grouped users into three categories:
Subjective audiophiles. Users who mainly ignore specifications and instead base their opinions on their own listening impressions, and/or impressions of other audiophiles.
Objective audiophiles. Users who primarily consider benchmark performance when evaluating an audio device.
Common users. New audio enthusiasts and everyone else, including non-audiophiles. These people aren’t sure what to expect from high end audio.
Jitter and DAC resolution these days are so excellent that the majority of common users and audiophiles we’ve talked to admit that they struggle to differentiate between one DAC and another. Unless your audio source delivers low quality to begin with, this is expected: You shouldn’t hear a reference grade DAC. That’s the point!
Therefore, there’s no such thing as the Best DAC on the market. Most modern DACs are equally capable of providing excellent audio. If you’re after higher quality sound, start with nice headphones and a headphone amp. If you’re after audio perfection, consider a reference quality DAC as well.
** Scheduled Maintenance: We’re moving our server tomorrow night at 1:00AM CST, Saturday, July 14th. Anticipated downtime is 1-4 hours. JDSLabs.com may be difficult to access in some parts of the world for the next 24-48 hours while our new IP address propagates. **
Nick has spent the past three weeks making noise, breaking drill bits, and splashing super fun blue coolant all over the office. Most people don’t pick up mechanical engineering and machining as new “skills” in just three weeks, but we do what we have to here.
With the basics of CNC’ing out of the way, our attention has turned back to the reason we bought the CNC in the the first place. We want brilliant quality, and we want our parts now.
We’ve specified brushed aluminum on our endplates since they were first designed. It turns out that most aluminum suppliers do not offer brushing. The process is a time consuming and challenging task even for most machine shops. That was a disappointing discovery!
Our endplates will still be brushed, but this sourcing realization led us to experiment with the aluminum. The test pieces shown above were machined at our shop on Friday afternoon, then plated on Monday morning in downtown St. Louis. Clear alodine and black anodizing finishes both look fantastic. And it was all done in under 2 business days. 🙂
O2+ODAC Rear Panel with 3.5mm Output
Although we don’t recommend that you install a 3.5mm output jack on your O2+ODAC, some customers have requested to do so anyway. This is for you: Machined O2+ODAC Endplate w/3.5mm output. These are made from stock endplates included with the Box B2-080 cases, thus, the price is lower than other endplates.
We’ve avoided RCA outputs on the Standalone ODAC for several reasons. RCA jacks take longer to assemble and cost more than a 3.5mm jack, but this has been irrelevant to our decision.
We haven’t offered RCA jacks on the ODAC due to virtually identical performance: Why complicate a product with multiple variations and increased build cost at no benefit? The 3.5mm jack on the ODAC circuit board is wired to the same line output header as used by RCA jacks. Some customers have asked, “Aren’t 3.5mm cables more lossy than RCA?”. As NwAvGuy has stated, characteristic impedance is a non-issue in unbalanced audio systems. There are no impedance mismatches to be concerned with, and there are no significant losses resulting from the use of a 3.5mm cable. Crosstalk might change negligibly (1-3dB), but overall, it’s a myth that RCA cables are less lossy than 3.5mm cords. Case in point, why would NwAvGuy (an audio benchmark and measurement fanatic) have designed the ODAC with a 3.5mm jack if it were “lossy”? He wouldn’t have! I’m sure NwAvGuy can provide benchmarks if badgered.
Moreover, you can use a common 3.5mm to RCA cable for direct connection of a 3.5mm device to an RCA device.
With all of that said, it’s easier to build products customers want, than to convince everyone that what we’re building is the best solution.
So last week we set out to fit RCA jacks into our Standalone ODAC enclosure. I had briefly collaborated with Stefan of Head ‘n’ HiFi earlier this year on the Standalone ODAC case. We’d been asked by NwAvGuy’s contractor to lead the design and distribution of a standardized ODAC case for the DIY community. I wanted ultimate enclosure quality, and was willing to wait until late July for a custom case fabricated in the USA.
However, Stefan wanted cases immediately, and didn’t think he could fit RCA jacks into our thin case, with only 10mm to work with (a typical RCA jack is 9.5mm). Stefan’s a nice guy, but we had to agree to disagree on our design goals. Product excellence is more important than expedited delivery and cost, especially when a product will stick around for years to come.
Despite the tiny clearance, we’ve managed to design ODAC endplates that will accept 2x RCA jacks and still fit into our thin ODAC enclosure, without losing ground isolation. This is only possible due to a beautiful, black anodized finish. More pictures are coming soon. For now, imagine the gold RCA jack pictured above on our Standalone ODAC.
ODACs are scheduled to arrive July 19-25. All Standalone ODACs will ship by the end of July.
Those seeking to retrofit their O2’s with an ODAC can now preorder the O2+ODAC rear endplate for the Box B2-080 enclosure:
Not sure why this rendering is so dark (Nick?!). They’ll match our O2 Front Plates.
We’ve switched to a specialty machine shop based in Silicon Valley. Artwork on new endplates will be laser etched instead of silkscreened. Cost is slightly higher, but overall machining quality of our O2 and ODAC plates will be better than ever.
The assembled ODAC board is sold barely above wholesale price, at the request of NwAvGuy’s manufacturer. This is to keep the DIY project cost low. Longer explanation here.
Our case manufacturer asked for a favor earlier this week. He’s speaking today at a big corporate meeting and needed to show off a customer with “the most interesting application”. We provided a short summary:
Growing so quickly hasn’t been easy. We’ve streamlined most tasks to keep up with production and shipping, but now we’re out of space. So, we’re moving 1 mile down the road to a new office in Glen Carbon, Illinois.
ODAC absolutely requires a headphone amplifier for proper operation
I know many of you are looking forward to installing ODAC in your O2’s, or having it retrofitted at our factory. (For those new to this project, three wires from ODAC must be soldered to input jack terminals P1 on the Objective2, a couple traces must be cut, and the 2x9V batteries must be removed.) A retrofit is a nightmare of a design choice for us. Sorry, NwAvGUy! Soldering one DAC is no big deal for a do-it-yourself’er, but we’re faced with soldering hundreds. And that means hundreds of our past O2 customers might want to send their amps in for the upgrade. Cost for the O2+ODAC retrofit will be announced after ODAC is released. But we’ve devised a far more elegant solution.
Option 1: Standalone ODAC in Enclosure
NwAvGuy equipped ODAC with an optional 3.5mm line-output jack, for use with other headphone amps. It just so happens that ODAC fits into the same custom aluminum enclosure we built for c421. Nick sketched new end-plates, and voila:
This is only a preview! We’ve printed prototypes and tested fit. Design files are now under review by our machine shop, so things could change based on manufacturability.
Placing ODAC in a standalone enclosure means:
Objective2 owners can add an ODAC without modifying their perfectly good amp
All features of the O2 are retained (no need to remove 9V batteries)
Standalone ODACs can be paired with any headphone amp!
Same performance as coupled to the O2
This standalone ODAC edition will be added to the preorder page as soon production begins–any day now. Available approximately upon release of ODAC.
Option 2: O2+ODAC Combo
As NwAvGuy plans, we’ll offer ODAC preinstalled in new O2’s, which will look something like this:
These will also appear on our preorder page after production begins. Available approximately upon release of ODAC.
Option 3: O2+ODAC Retrofit/Upgrade
Identical to option 2, but for those who already own an Objective2. Available in late May.
Option 4: Assembled ODAC PCB
Consists only of an assembled ODAC circuit board–no amplifier or enclosure. This is the only offering on our preorder page for now. Ships mid May.
As you can see from the above images, the ODAC product lineup is well underway. All prices and preorder options will be available by the end of May.
A Note on Reviews
The first time I browsed to Headfonia.com, I couldn’t imagine seeing our products on Mike’s awesome review site. It was all high-end gear, way beyond anything we’d built at the time. So I was surprised when Mike e-mailed me for a cMoyBB sample in early 2011. The review went well.
Note: We are not a Headfonia sponsor. In fact, we don’t sponsor or endorse any audio publications. I’d rather see our great products spread through the audio community by word of mouth, and use cash-flow instead to increase product development and distribution.
Anyway, Mike posted a well received review of our c421 last month, with the OPA2227. But he was quite opposed to the AD8620, totally contradicting my opinion.
I’m not taking sides. I fully endorse NwAvGuy’s work and his rebuttal, and I’ve always been a fan of Headfonia.
I personally prefer to listen to c421 with bass boost on, with DT-880’s. I have nothing bad to say about the Objective2. It’s a technically brilliant amp. But I’m a bass boost guy. Hence, cMoyBB.
The fact is, audio is subjective. NwAvGuy can post measurements and discuss psychological theory all day long, but we’re humans. Our brains operate subjectively (“expectation bias”, as NwAvGuy wrote).
Case in point, Nick says O2 and c421 sound the same. But Nick isn’t an audiophile and doesn’t spend his free time reading audio forums and reviews. He just knows that JDS Labs sells nice headphone amps.
I don’t intend to discredit Mike or Headfonia. There’s a lot of truth to what NwAvGuy writes, especially in regards to expectation bias. Mike e-mailed us after the O2 review went live; I replied explaining that I wasn’t disappointed. He expresses his honest opinion in his reviews. NwAvGuy blogs about objective audio engineering. JDS Labs has to tie everything together. We build amps that we believe in, both in terms of design, and in terms of taste/expectation bias. We receive dozens of emails each week asking for comparison of one amp to another. While I could steer a customer one way or another, I sincerely avoid replying in depth to any of these messages.
Read reviews and blogs. Read specifications. Look at pictures. Borrow a friend’s amp, or go to an audio meet. In the end, pick an amp that meets your needs and expectations.