Objective Reviews & Commentary - An Engineer's Perspective

March 1, 2012

ODAC Update

nwavguy-dscope-benchmarkODAC PROGRESS: As my articles have slowed to a trickle lately, hopefully these 5000+ words will be welcome news. After lots of professional and other distractions, I’m once again devoting serious time to the Objective DAC (aka ODAC). There’s been some good progress and I’m feeling better then ever about the ODAC’s performance. I’ve been comparing it with my Benchmark DAC1 Pre and it’s been eye opening in some areas. Keep reading for more details and further background on the ODAC design.

QUESTIONS (FAQ): Before you post questions or suggestions in the comments please have a look at the FAQ section below to see if your issue has already been addressed. I’ve tried to cover the popular questions and issues.

NOT FOR PROFIT: Please note I’m not selling the ODAC, just helping design it. Just as with my other projects, like the O2 portable amp, any profit from sales will be to whatever companies decide to have them made and sell them.

ODAC + ODA: The ODA, for those not following the history, is the Objective Desktop Amp. It’s essentially a future desktop version of the popular O2 portable amp with a few added features and upgrades. ODAC development is taking priority for now because the ODA will be partially designed around it. The ODAC can also be used with the O2 and as a standalone device.

FILLING A VOID: There are simply not many reasonably priced DACs supporting 24 bit operation over USB under Windows without needing proprietary drivers. While there are some low cost options on the market, at least some of them, such as the FiiO E10 and NuForce uDAC-2, offer barely more than 16 bit performance. There are even fewer options if you want a high quality 24 bit DAC with a low impedance output that can drive most popular headphones well (this leaves virtually all pro audio DACs out in the cold). The least expensive option I know of that’s been properly measured and performs well is the Centrance DacPort with the 1 ohm output upgrade. But it’s close to $500. Most DIY DACs, such as those from Twisted Pear and AMB, only support 16 bit operation over USB or they require proprietary drivers under Windows.

odac in o2 protoODAC + O2 RETROFIT: The ODAC fits neatly inside the O2 portable amp using the standard B2-080 enclosure by simply removing the batteries as shown to the right. One side sits in an enclosure slot while a mounting hole lines up with the central mounting hole on the O2 PCB anchoring the opposite side of the board. The battery terminals act as a “back stop” to absorb the force of plugging in a USB cable and the rear panel does the same when removing the plug. It worked out surprisingly well and I can’t take credit for this feat—my commercial counterpart came up with the idea and we mutually optimized the PCB layout to work with the O2. The retrofit requires a new back panel with an opening for the USB jack. At least one person has volunteered to design and hopefully sell a laser cut plastic panel for those wanting the O2/ODAC combo. Stay tuned for more but it’s a pretty simple retrofit that only requires soldering 3 wires at each end.

o2 off board input jack modificationO2 INPUT SWITCHING: Those who only want a USB headphone DAC (like the FiiO E10 or NuForce uDAC-2) can simply wire from the output header of the ODAC to the P1 input header on the O2 board. But for those who still want to use other sources (like an iPod, etc.) with their O2, you can have your cake and eat it too. Once you cut the traces shown to the right you can wire the ODAC to terminals 3 and 4 of the input jack. These lead to internal switches in the jack and will connect the ODAC only when nothing is plugged into the O2.

ODAC STANDS ALONE: If you just want a DAC with no preamp or headphone amp, the ODAC can be used by itself as a USB powered 24/96 DAC. Because it supports 24 bit operation over USB there’s little or no penalty using only a software volume control or you can control the volume downstream of the ODAC. The board measures about 49mm x 58mm (1.9 x 2.2 inches) and will fit in many small enclosures such as the Box Enclosures B1-080. There are also several inexpensive eBay DIY enclosures that would work. Ignoring the output jack, it’s only about 4mm in total thickness as it’s entirely a surface mount design This allows it to easily “piggy back” on top of other boards, etc. In the O2 tradition, it has three mounting holes (geeks know 3 points determine a plane and 4 just mess things up ;). The components are also held back from two opposing edges for slot mounting. There’s a provision on the board for a 3.5mm stereo output jack that’s on the same edge as the USB connector requiring only one machined (or laser cut) panel. DIYers can also panel mount 3.5mm or RCA output jacks or simply build the ODAC into other gear. The ODAC can be connected to any input that accepts standard Redbook 2 Vrms line level audio and has at least a 10K input impedance.

O2 DESIGN PRINCIPALS APPLIED – The O2 was implemented using well proven design techniques and countless rounds of measurements and refinements. The result is performance well beyond what most would expect from the O2’s modest and inexpensive components. The O2’s popularity has shown this design methodology is valid. Put simply, the O2 has proven far greater than the sum of its parts. I applied the same approach to the ODAC design. While it uses relatively modest components, it delivers performance well beyond what most would expect given the cost and components used. The ODAC closely follows the chip manufacturer’s reference design information and has been carefully optimized through several iterations. Using a professional mixed domain audio analyzer, like the Prism dScope, enables a level of performance refinement that would otherwise be all but impossible. This gives the ODAC a huge advantage over most DIY DACs and products from small manufacturers lacking proper test equipment.

THE AUDIO ANALYZER ADVANTAGE: To make meaningful measurements of a DAC you need an ADC with a noise floor significantly (at least several dB) lower than the DAC’s noise floor. Otherwise you’re measuring the ADC as much (or even more so) than the DAC. Such an ADC is increasingly hard to find once you get up to Benchmark DAC1 levels of performance. The Prism Sound dScope, however, has a noise floor of just over 1 microvolt or around 126 dB below the Redbook standard of 2 volts. Even my $1800 Benchmark ADC1 can only manage a best case 119 dB and less expensive audio interfaces are typically significantly worse. It’s also extremely useful to have an audio analyzer capable of real time mixed (analog and digital) domain generation and analysis. A product like the dScope, or a high-end Audio Precision analyzer, is about the only way you can properly measure and develop a high performance 24 bit DAC. Otherwise the true performance is masked by the ADC you’re using and the limitations of using PC software that cannot do anything in true real time because of the PC’s operating system. Input isolation is also a significant issue. Using RMAA typically requires sharing the DAC’s USB ground with the ADC’s ground which, in itself, can create all kinds of erroneous results and ground loops. Properly testing a high performance DAC requires more than just another PC audio interface. You need specialized hardware designed for the purpose.

THE OTHER WAY: Lots of DIY, and even some commercial DACs, seem to be little more than a few trendy popular chips slapped onto a nice looking PC board. If their designers conducted the proper measurements along the way, where are their published results? You generally have to look at expensive products like those from Centrance, Benchmark Media, and Anedio to find meaningful DAC measurements. Some designers and companies simply quote highly misleading specs from the DAC IC datasheet but, as I’ve shown in my reviews, the results often fall far short. A good example is the FiiO E10 that only delivers roughly 16 bit performance even running at 24 bits. Some companies, like NuForce, Audio-GD, and Twisted Pear, argue they “design by ear” but the unavoidable human bias associated with such sighted listening is well documented.

IT’S ABOUT WAY MORE THAN THE RIGHT PARTS: Using high-end parts is meaningless if the implementation is (often unknowingly) flawed. It’s like putting a Ferrari engine in a Yugo or using ultra fast RAM in a PC with a slow processor and chipset. It’s pointless. Just as cars are defined by far more than just their engine, and PC performance depends on several subsystems all working well together, the same is true of audio gear. The best products are from manufacturers that conduct proper testing and have the resources and desire to sweat all the little details rather than just using the latest FOTM parts and making it look nice.

FOLLOW THE PROFITS: Sadly a lot of “specialty” audio designers and companies seem to depend mainly on subjective hype and sighted listening bias, rather than proper design and objective performance, to sell their products. One can argue some are mainly trying to cash in on the latest FOTM craze rather than investing the time and money to design genuinely solid gear. It’s one of the goals of this blog to help not only expose half baked designs for what they are, but also demonstrate better alternatives don’t have to be be expensive or made with dual phase aligned unobtanium.

SMALL CHANGES CAN EQUAL BIG IMPROVEMENTS: I’ve sometimes been amazed how even small changes have made fairly large differences in the ODAC’s performance. Several of the ODAC’s optimizations run counter to typical DIY audiophile beliefs. Here are a few examples of typical design myths:

  • Larger Value Capacitors Are Better – “Upgrading” certain power supply capacitors to larger values made the ODAC perform significantly worse compared to using the values specified in the chip manufacturer’s reference design. Bigger value caps often have higher ESR, more inductance, and much higher impedance at very high frequencies. They can also create other problems.
  • Top & Bottom Ground Planes Should Be “Stitched” Together – Stitching is the practice of applying a board-wide grid of small vias (plated through holes) that connect the top and bottom ground planes. Some argue this lowers the ground impedance and keeps ground paths shorter. But they’re not true ground planes on a 2 layer board. Instead you have a bunch of ground fill areas isolated by signal and power traces. When you have these “ground islands” rather than a true continuous ground plane, the stitching can easily create undesirable ground loops and send ground currents where you don’t want them. I’ve seen several DIY and commercial designs ignore this issue and other proper grounding practices.
  • Expensive Audiophile Dielectric Capacitors Work Best – I experimented with various types of poly film caps, including audiophile preferred SMT Polyphenylene Sulphide (PPS) types, and found they sometimes made things worse. I also discovered not all ceramic caps perform equally. There’s no simple rule of thumb that always works. You have to make the right measurements and sweat the details. It’s time consuming but proved worth it. Every capacitor associated with the DAC chip and analog circuitry of the ODAC has been carefully optimized using the dScope. Overall, my capacitor tweaking resulted in lowering the noise and distortion by more than 6 db. And, trust me, the final result is not what your average DIYer would intuitively think is best.
  • Fully independent Analog & Digital Power Supplies Improve Performance – While there are probably a few high-end DACs that measure slightly better with fully independent digital and analog power supplies, the reality is the DAC chip itself and related digital noise is more likely the dominant limiting factor. I’ve shown, with multiple measurements and tests, the power supply is not holding the ODAC back in terms of performance. And while there’s some carefully designed filtering and ground routing between the analog and digital sections, they’re both derived from the same source. A DAC will only perform as well as its weakest link allows. In an optimized design that weak link is usually the DAC chip and/or a certain amount of unavoidable noise from the USB and I2S digital buses. When that’s true, adding more esoteric power supplies won’t help much if at all.

PCB LAYOUT IS EVERYTHING: I can’t stress this enough. Just as the O2 board was designed for “function over form” so was the ODAC. Many DIYers and audiophile manufacturers want to show off their PC boards. So they often lay them out at least partly to look nice. That almost always means the performance suffers. In the case of a 24 bit DAC I’ve learned it might suffer quite a bit. Keeping the noisy digital signals out of the analog side of a DAC becomes challenging once you get past about 17 bit performance. And for those trying to judge if they got it right by using RMAA and a sound card, or an 8 bit oscilloscope with only 40 dB of dynamic range, good luck with that (see The Audio Analyzer Advantage above and my RMAA article for why). And good luck using typical sighted listening tests to verify a PCB layout.

BENCHMARK DAC1 REFERENCE TARGET: The Benchmark DAC1 models have been widely reviewed and praised—especially for their excellent measurements, sound quality, and hassle free 24 bit high resolution USB support. Hopefully many agree the DAC1 Pre is a worthy reference target to judge the ODAC against. While it’s a bit like putting a Mazda MX5 Miata up against a Porsche 911 it’s still a worthy goal. When I present the ODAC’s final measurements I’ll show many of the same results for the Benchmark and everyone can judge the end result for themselves. I’ll also be comparing the ODAC to the FiiO E10 as it’s probably the ODAC’s closest current 24 bit USB competitor in terms of cost and might even include a few pro audio interface results as well.

PRELIMINARY BLIND TESTING: In preliminary blind comparisons between the ODAC prototype and my DAC1 Pre, they sound the same. I’ll be doing more involved blind tests, but the initial results are very promising. If this trend continues I’ll be adding an “ODAC Public Blind Challenge” to my existing O2 and op amp blind challenges. If someone thinks they have a DAC that measures well and sounds better, let’s find out!

DESIGNED AND (for now) ASSEMBLED IN THE USA: This may not matter to everyone, but living in the USA myself, it’s something I take at least some pride in. Steve Jobs famously told President Obama “those jobs are not coming back” referring to Apple making nearly everything in China. And while Apple products are generally made to a high standard in China they’re still designed in the USA. The same cannot be said for a lot of reasonably priced “boutique” audiophile gear. NuForce, Audio-GD and FiiO are a few examples with well documented, and sometimes embarrassing, problems. And, based on what I’ve seen, a lot of the gear being sold mainly on eBay and direct out of Asia can be even worse.

USB POWER ADVANTAGES: A self powered DAC that operates entirely from USB power has a number of practical advantages:

  • One Size Fits More – Because the ODAC needs to be commercially assembled in volume using automated equipment it’s much more cost effective to have a single version that works in the O2, ODA, standalone, and can be added to other DIY or commercial designs. USB power is virtually required to make this possible. Higher volume manufacturing of a single board brings the price down for everyone.
  • O2 Compatibility - The O2 has a dual +/- 12 volt power supply delivering around 200 mA peak to each channel of the amplifier. The ODAC requires around 125 mA at a far lower voltage. The O2 cannot power a high quality DAC without adding a switching regulator or DC-DC converter which would add noise and still tax the O2’s power supply. There’s also no physical room for the added hardware. So USB power is the only viable option for the ODAC in an O2.
  • Stand Alone Operation – For those wanting to use the ODAC by itself, USB power has a clear advantage. It eliminates the need for another power supply and is especially desirable for portable use.
  • No Enumeration Problems – Some USB interface ICs are designed to be USB powered. They will not enumerate correctly with the host device if they’re already powered when the USB connection is made. This can necessitate using USB power for the interface chip while using a different power source for the DAC chip. This creates various other challenges including the next point.
  • No Unpowered Inputs - USB power also simplifies problems associated with having unpowered IC’s connected to powered ICs. Digital ICs that are not powered don’t generally like being connected to ICs that are. It can result in a potentially destructive condition where the normally high impedance inputs of a powered IC represent a low impedance when the IC has no power. The powered IC pumps abnormal levels of current into the unpowered inputs. When power is applied with the IC in this invalid state it can “latchup” and draw large amounts of power supply current destroying itself. Some of the solutions to this problem can degrade jitter performance because the I2S bus is subject to this issue on a USB DAC with a split power scheme. Powering the entire DAC from USB power neatly solves these problems.

USB POWER CHALLENGES:  USB power also presents some challenges:

  • Potentially Greater Noise -  USB power is more of an unknown compared to a dedicated power source. So it requires extra filtering and careful design to avoid degrading the performance of the DAC. This is especially true when you’re aiming for much better than 16 bit performance. The USB powered FiiO E10, Creative X-Fi Go, and NuForce uDAC-2 all promise 24 bit performance but only deliver around 16 bit performance. Likewise, when running from AC power, the latest MacBook Air also fails to deliver better than 16 bit performance from its 24 bit DAC due to extraneous power-related noise.
  • Audio Output Voltage - USB power can be as low as 4.5 volts, and when you add in losses from power filtering and the DAC/op amp circuitry, you’re lucky to get 4 volts peak-to-peak of output swing without clipping. That works out to 1.4 Vrms which is a significant 3 dB shy of the 2.0 volt Redbook standard for digital devices. That’s 3 dB of potential dynamic range lost and a 3 db drop in level compared to normal home sources (like a CD player, network media player, etc). Indeed most of the USB DACs I’ve tested, and even popular DIY DACs like the AMB gamma, have this shortcoming.
  • Capacitor Coupled Outputs - A single-ended DAC power supply usually requires an output coupling capacitor to block the 1/2 Vcc voltage at the DAC output. Such a capacitor, to drive a 10K load, needs to be a fairly large value to avoid low frequency roll off and excessive phase shift. It also should be a high quality film, rather than electrolytic, type. But many USB powered DACs, in the interest of saving money and space, use electrolytic or otherwise compromised output capacitors. This is even true of some audiophile DIY designs like the AMB gamma.
  • Power Related Jitter - Power and ground “pollution” at various frequencies can have a significant impact on jitter performance. Noise from the power supply can, in effect, modulate the digital bit stream creating jitter.
  • USB Maximum Current Limit - While the USB ports on any PC or laptop made in the last 6+ years can nearly always supply 500 mA of current there are some exceptions. There are a few ultra low power netbooks that have 100 mA USB ports and unpowered USB hubs are also, at least in theory, limited to 100 mA per port. I’m not sure about iPads and Android tablets but I suspect they may be rated for only 100 mA as well.

ODAC POWER DESIGN – The above challenges can be largely or entirely overcome with careful design and by using the right components. Here’s how the ODAC addresses them:

  • Power Noise Below the DAC’s Noise Floor – It turns out ground and conducted (electromagnetic) noise are typically as significant as noise on the USB power line. Because a connection to the PC’s ground is required regardless, simply using an external power supply doesn’t automatically mean freedom from USB bus noise. With the ODAC the solution involved careful routing of ground currents, different power supply conditioning for the digital and analog sides, careful capacitor selection, and inductive filtering. The result is the latest prototype’s noise floor is mainly determined by the DAC IC itself not the USB power bus. Put another way, an independent power supply wouldn’t make much difference.
  • Redbook Standard Audio Output Voltage – The ODAC delivers delivers the Redbook standard output of 2 Vrms without clipping by using a bipolar power supply. This alone adds roughly 3 dB of dynamic range to the ODAC’s performance compared to many USB powered DACs.
  • Direct Coupled Output – The ODAC uses a ground referenced split supply so no virtual ground or output capacitors are required. This assures accurate low frequency amplitude and phase response with no bass roll off and avoids potential capacitor-induced distortion.
  • Power Related Jitter – The ODAC’s power filtering was optimized not just for the best noise and distortion performance but also for the lowest jitter. Interestingly some of my attempts at filtering did help lower the power supply noise but increased the jitter. The dScope’s J-test proved invaluable in optimizing the PC board layout and power supply design for simultaneously low jitter and low noise.
  • USB Current Limit – I’m not aware of any 24 bit USB interface and high performance DAC ICs that, combined, come in safely under 100 mA total. The bipolar power supply that allows a 2 volt output does require a bit more power. And the choices in 24 bit interface chips are very limited. The only way to keep the total budget under 100 mA would involve a much lower performance DAC and that’s not an acceptable trade off. The good news is I’ve tried the ODAC on many PCs, including two netbooks, and it works fine. It even worked on an unpowered USB hub despite being slightly over the 100 mA limit.
  • Worst Case – If someone does encounter a USB port that is either extremely noisy or can’t provide 125 mA of current, the solution is simply to use a powered USB hub. They can be purchased for as little as $20 or so.

SO HOW QUIET IS IT? Using the industry standard A-weighted dynamic range test with a –60 dBFS signal, the current ODAC prototype has an impressive 112 dB of dynamic range. How good is that? My $1600 Benchmark DAC1 Pre, on the exact same test referenced to the same 2 volts, is slightly worse at 111 dB. CD quality audio, in comparison, has only 96 dB of dynamic range. I should note if you have an application where you can use the DAC1’s full 7+ volts of output, it can manage 116 dB of dynamic range referenced to it’s maximum output. So you do get something for your extra $1500.

ENOB: ENOB stands for Effective Number of Bits and is another measure of a DAC’s performance. No 24 (or 32) bit audio DAC can achieve true 24 bit performance, In fact, 20 ENOB is generally considered the “Holy Grail” of real world DAC performance. The ODAC is just under 19 ENOB and the Benchmark, even referenced to its full 7+ volt maximum output, is 19.3 ENOB. The FiiO E10, even in 24 bit mode, is only 16.2 ENOB.

DISTORTION: The ODAC prototype on a standard –1 dBFS 1 Khz signal has about 0.003% THD+N while the Benchmark DAC1 is only slightly lower at 0.0025%. The ODAC, on every distortion test I’ve run, is well below my ideal 0.01% maximum THD+N. That’s true even at 0 dBFS which wrecks havoc with some DACs (like the NuForce uDAC-2). The ODAC’s performance does not noticeably degrade in any way at 0 dBFS so I can officially certify it Lady Gaga compatible (scroll down to Lady Gaga In Audacity here).

JITTER: You can’t put a single number on jitter performance but I’m quite happy with the ODAC’s results so far. It can’t match the expensive ASRC jitter reduction used in the Benchmark DAC1 but it’s significantly better than most DACs, including the FiiO E10, and several of my pro sound audio interfaces. The ultimate proof will be in the blind listening tests.

IT MAY GET EVEN BETTER: It’s a bit like peeling an onion. As you improve one area of the design, the lowered noise and distortion reveals other areas that can benefit from refinement.While there was some hope the version on my bench now would be close to the final production ODAC, we’ve decided to go one more prototype iteration and incorporate several more incremental improvements. Some of those changes may improve the performance still further but it’s hard to know by how much. When it’s all said and done there will be at least 4 generations of ODAC PC boards. Despite the more than acceptable performance of the current prototype we’re not done chasing the Benchmark DAC1 yet.

FAQ (please have a look here before posting questions!)

WHAT WILL THE ODAC COST?: The ODAC is still on track to hopefully come in under $100 for a completely assembled, programmed and operational board ready to slip into an O2 or the upcoming ODA. If you want a standalone DAC you have to add at least a few bucks for an enclosure, panel and output jack. We won’t know the final pricing until the production design is bid out for assembly.

WHEN WILL THE ODAC BE AVAILABLE? Given we have another prototype cycle ahead of us the best case is likely late April if all goes reasonably smoothly. But that could slip into May or even later if there are unforeseen problems.

IS THERE AN OFFICIAL ODAC FORUM THREAD? Not yet but I’ll post an update as soon as there is.

CAN I BUILD MY OWN ODAC? Unfortunately no. The ODAC will be offered only as an assembled board. Please see the next question. (PS – You probably wouldn’t want to anyway. It has a lot of 0603 components which are really tiny and a fine pitch quad IC package which are no fun to work with).

WHERE IS THE SCHEMATIC & PARTS LIST? As explained in the first ODAC article, there are no DIY-friendly 24 bit USB audio chips that meet the design criteria. So we’re forced to use components licensed for OEM use that are not available through normal distribution. Even the datasheets are marked confidential and access to the interface chip requires signing an agreement. Given the ICs have to be purchased direct from the manufacturer in substantial minimum quantities it’s not reasonable to expect someone to make a substantial initial investment only to undermine them by giving the same design to other commercial interests. It would be a bit like designing a new car for Ford and then saying “oh by the way we’re also giving this to the Volkswagen Group, General Motors, and anyone else who might want it. You better hope there’s enough sales volume to divide between all of you.”  Having the ODAC be an open source design would only increase the financial risk and the end result could easily be a “chicken and egg” situation. Everyone might choose to wait and see who else might choose to produce the DAC. And, regardless, individual DIYers can’t get the required parts anyway. Hopefully everyone can understand it’s only fair the person taking all the initial risk, and spending the money up front, should not have the rug potentially pulled out from underneath them by sharing the same design to anyone with the commercial means to produce it.

WHAT DAC AND INTERFACE CHIPS DOES THE ODAC USE? For the same reasons we’re not releasing the schematic, we’re not yet releasing certain other details yet either. Please see the question above.

IS THE ODAC BASED ON THE XMOS USB INTERFACE? No. The XMOS solution requires a proprietary Windows driver that must be licensed for a fee. We also don’t believe the XMOS chip offers any real world benefits in this application. See the next two questions.

IS THE ODAC USB AUDIO CLASS 1 OR CLASS 2 COMPLIANT? The ODAC is USB Audio Class 1 (UAC1) compliant because it’s the only standard that allows native 24/96 support in Windows, OS X and Linux without any special drivers. USB Audio Class 2 (UAC2), while newer, offers no meaningful advantages for 2 channel audio playback and is not yet natively supported in Windows. Its main benefits are for multi-channel audio and recording at 24/192. Some of the best performing DACs available at any price are UAC1 devices (including the Benchmark DAC1 series).

IS THE ODAC ASYNCHRONOUS There’s a lot of marketing and other hype lately surrounding asynchronous DACs. Much of it is myth. Just like op amps got a bad name from the old 741 released many decades ago, so did USB audio based on early synchronous designs. But most modern USB audio devices use an adaptive interface where a local clock controls the DAC and is only loosely coupled to the PC’s timing. Contrary to popular belief, with an adaptive interface the data is not directly clocked by the USB port. This method has been refined over the years and can work very well. It’s also natively supported by all major operating systems. Most methods of true asynchronous USB audio require proprietary drivers under Windows and proprietary drivers are rarely a good thing. Judging a USB DAC by whether it’s asynch or adaptive is a bit like judging a car by the engine configuration—i.e. an inline six in a BMW vs a V6 in a Nissan GT-R. Other aspects of the design are more likely to limit the performance and there are plenty of examples of outstanding USB DACs using adaptive interfaces.

WHAT DIGITAL FILTERING DOES THE ODAC USE? Another trendy DAC topic is the type of output filter. It’s mostly about chip and DAC marketing types dreaming up added ways to differentiate their products. But, ultimately, what matters most is the measured performance at audible frequencies and listening tests using proper blind techniques. Judged by those criteria, I’m not sure either filter has an overall advantage. I have not seen any credible blind listening tests that support one filter type sounding better than the other. The ODAC, like many great DACs, uses a linear phase filter. Just like with asynch vs adaptive, there are many other variables that determine the overall performance. You have to consider the entire product as a whole. It’s foolish to dismiss a BMW or Porsche because the engine cylinders are not arranged in a “V” because, ultimately, that’s only a small part of a much bigger picture.

WHAT AUDIO FORMATS DOES THE ODAC NATIVELY SUPPORT? The ODAC supports 44, 48, and 96 Khz each at 16 or 24 bits. 24/192 is not natively supported by Windows over USB, nor does it offer any audible advantages for audio playback (in fact most DACs that support 24/192 perform worse at 192 Khz than at 96 Khz).

ARE YOU PLANNING TO MAKE MONEY OFF THE ODAC? No. I will not get any money from ODAC sales. Someone else is taking all the financial risk and coordinating all the manufacturing and sales logistics. If the ODAC proves popular, any profit is rightfully all theirs. Frankly, given the 4+ design iterations, steep minimum order quantities, and all that’s gone into it so far, they might be lucky to break even. But it’s ultimately no different than JDS Labs and Epiphany making a profit selling the O2. For me this remains an entirely not-for-profit blog and endeavor.

WHERE ARE THE MEASUREMENTS? Given the performance may change (hopefully for the better) with the next iteration it would be a lot of wasted work to publish detailed measurements and graphs only to do it all over again once we have the final design done. I made that mistake with the O2.

WILL THE ODA BE OPEN SOURCE? Yes it will. The ODA will be just like the O2, and anyone who wants to sell the ODA + ODAC can likely negotiate buying assembled ODAC boards with some sort of quantity discount. They might even want to weigh in before the initial production quantity is determined.

WHY NO S/PDIF INPUT? I know there have been several requests for S/PDIF, but as was explained previously, it’s not a trivial or even logical addition. To summarize, here’s why:

  • O2 Compatibility – Having the ODAC fit inside the standard O2 is a pretty cool thing. O2 compatibility seems to have more interest than S/PDIF support. It would be very difficult to add an option for S/PDIF and still have the ODAC fit in the O2. If it’s even possible with a 2 layer board it would require components on both sides which would substantially raise the price for everyone. And it would likely have inferior performance due to a less than optimal PCB layout. Likewise splitting the ODAC into two different boards would also raise the cost in multiple ways.
  • S/PDIF Doesn’t Fill a Niche – There are lots of 24 bit capable S/PDIF DACs as it’s much easier and cheaper to implement compared to 24 bit over USB. Even the $25 FiiO D3 does 24 bit over S/PDIF. But there is a shortage of reasonably priced 24 bit USB DACs. See Filling A Void near the beginning of this article.
  • No Native Support - There are very few suitable 24 bit USB audio compliant (i.e. driverless) interface ICs available. Likewise there are not many relatively high performance DAC chips that work well in a simple 24 bit USB DAC design without a microcontroller. The chips used in the ODAC lack native support for an alternate S/PDIF or second I2S input and the lack of a microcontroller complicates things further.
  • More Jitter – Adding an S/PDIF input option would mean “breaking” the I2S bus between the USB interface and the DAC to allow the insertion of an S/PDIF interface and/or switch. Ideally, for lowest jitter, you want to keep the I2S routing as short, clean, and direct as possible. With the current PCB size adding S/PDIF could easily compromise the jitter performance.
  • Possible Future Version – There is more room in the ODA and it’s possible there may be a more expensive version of the ODAC someday as a “plug and play” replacement for the current ODAC. Such a DAC would likely use more expensive components and possibly a 4 layer PCB raising the performance bar even higher and could also more easily include an S/PDIF input. That way the cost is kept as low as possible for the mainstream version and those who want more can pay more. I also expect some better 24 bit USB audio interface ICs to come on the market compared to what’s available now. Some might even more DIY-friendly and suitable for an open source design.