INTRO: Ok, right up front you need to know this article is not one of my usual in-depth write ups. But here’s the deal. The battery powered O2 headphone amp has been more popular than I imagined with 2000+ PC boards already in circulation and more on the way. After thousands of O2 messages and comments I’ve learned many are interested in purely a desktop amp and many O2’s have gone into service sans batteries. Those considering an O2 for desktop use might want to read this “preview” article.
THE ODA: “ODA” simply stands for “Objective Desktop Amp”. The idea is to take the same objective principles behind the O2 and apply them to an amp optimized for desktop-only use (no battery power). Those principles include:
- Genuinely Transparent Performance – I wrote an article for InnerFidelity that describes what’s required of a headphone amp for transparent performance. I was even more stringent in defining the requirements for the O2. The ODA will meet or exceed the same requirements so you can listen to everything your music has to offer without your being forced to listen to your amplifier’s unwelcome contributions. This means no audible hiss, distortion well below audible levels, and a suitably low output impedance to maintain flat frequency response and proper damping with all headphones.
- Wide Headphone Compatibility – Music lovers shouldn’t have to worry about trying to figure out what headphones work well with what amp. The O2 has demonstrated it’s entirely possible to have one modestly priced amp that can accurately drive 98% of current headphones on the market.
- Focus On Design Rather Than Designer Components – The O2 demonstrates design and implementation is often far more important than using trendy expensive parts. The ODA places a similar emphasis on proper implementation to get the most from reasonably priced high quality components. As with the O2, I expect the ODA will outperform many other headphone amps using more exotic components and topologies.
- DIY Friendly Design – Several first time DIYers have already built an O2 and reported it worked the first time they powered it up. By avoiding surface mount parts, point-to-point wiring, and chassis mounted components, the ODA should be equally simple to build. Because DACs require surface mount components, and for other reasons, the optional DAC board will be available pre-assembled and tested (see below).
- Real Ground – The ODA will not have third channel masquerading as a virtual ground to mess up the performance. It uses a proper bipolar power supply referenced to true ground and direct coupled outputs.
- Maximum Value – Like the O2, the ODA is being carefully “value engineered” which takes into account everything from the size and complexity of the PC board to minimizing the number of different components on the parts list. Upgraded components are used where there’s a meaningful benefit or little price penalty. The ODA will use a low cost off-the-shelf enclosure.
ODA VS O2: So what makes the the ODA different and more desirable for desktop use? The main O2 thread on diyAudio is flirting with 1000 posts. And, collectively, there are another 1000+ posts and messages elsewhere. Some clear consensus stood out in what many wanted in a desktop headphone amp:
- Optional Internal 24/96 High Resolution USB DAC – The number one requested “upgrade” to the O2 is a DAC that’s designed and measured to similar standards of high performance. Just such a DAC is well under development for the ODA (see below).
- Higher Quality Inputs & Outputs – The O2, for size reasons, has 3.5mm input and output jacks. The ODA adds a 1/4 inch (6.5mm) Neutrik headphone jack for use with high-end full size headphones and RCA input jacks for higher quality connections and even lower crosstalk.
- Rear Panel Connections – The ODA has the power and input connections in back to keep the cables out of the way for a cleaner look and better ergonomics (the downside being another relatively expensive panel to buy).
- Wider Source Compatibility – The ODA should work with anything from an iPod Line Out Dock (LOD) to high output home DACs with a wide range of headphones. It has more flexible gain options compared to the O2.
- Higher Quality Power Supply – The half-wave power supply in the O2 is something of a compromise for size, cost, and battery charging reasons. The ODA power supply improves on the O2’s in several ways.
- Headphone Protection Relay – The O2, like many amps, produces a “click” in the headphones when you turn it on and a soft “thump” when you turn it off. The ODA uses a headphone relay to eliminate these noises and provide added protection for expensive headphones.
- Other Possible Upgrades – Depending on how the details sort out, the ODA may have some other upgrades as well such as a possible preamp/line output.
ODA + ODAC: What’s an “ODAC”? It’s a USB DAC designed the objective way—just like the O2 and ODA. It fits inside the ODA turning it into a desktop headphone DAC. Here’s some preliminary information:
- High Resolution USB – Relatively few reasonably priced commercial DACs, and almost no DIY DACs, support anything beyond 16 bits at 44 or 48 Khz over USB (despite most having 24/192 DAC chips). The problem is, until recently, there were no suitable options for high-resolution audio over USB without needing special drivers or spending lots of money. The ODAC will support up to 24/96 over USB.
- 111+ dB Documented Dynamic Range – The most dynamic range you can get from a DAC operating in 16 bit mode is around 96 dB with most falling several dB short of that. While that can be enough if you control the volume in the analog domain somewhere after the DAC, it’s often not enough if you want to control the volume at your PC in the digital domain. That’s where those extra high resolution bits show their stuff. Even with the PC volume turned down, you can still get 16+ bits worth of resolution and dynamic range. That can mean the difference between hearing noise or pure silence in your headphones. And, unlike nearly every other DAC out there, the Dynamic Range of the ODA will be fully documented. Many DACs just quote the spec for the chip off the datasheet which is usually nowhere near the entire DAC’s actual performance. See the Tech Section for a preview.
- USB Audio Class 1 Compliant – Unlike most pro-audio interfaces that support 24/96, the ODAC requires no problematic proprietary drivers for XP, Vista, Windows 7, OS X or Linux. It’s true Plug-And-Play. It also does not require UAC2 drivers, like some DACs, which are not provided in any current version of Windows.
- ODAC vs DAC1 – I’ve done some preliminary blind testing against my $1600 Benchmark DAC1 Pre with a variety of music and my best headphones, and so far, a least two different people cannot tell them apart. The DAC1 Pre has won a lot of professional accolades for being one of the better DACs money can buy at any price. I can see a formal ODAC NwAvGuy blind listening challenge coming up.
- Redbook Compliant Output – For many reasons, it’s good to have a DAC that complies with the Redbook standard for digital audio output levels. Many USB DACs, like the AMB gamma, can’t produce the required 2 volts RMS of output and suffer lower dynamic range. And some go way over the specification which risks overloading the inputs of a lot of equipment. The ODAC is Redbook compliant. Among other benefits, this makes it much easier to do level matched A/B comparisons with other DACs and digital sources that are also Redbook compliant.
- Pre-Assembled Board – Because USB and DAC chips are typically fine pitch surface mount devices they’re not very DIY friendly. To make matters worse, if you want 24 bits over USB with native Windows drivers, I’m not aware of any suitable chips that are available to DIYers. All the current solutions require licensing and/or some sort of contractual agreement. They’re not stocked by DigiKey, Mouser, Farnell, etc. And to further complicate things, they also require custom programming before they will operate correctly. That’s 3 strikes against DIY for high resolution USB DACs. The obvious solution was finding someone to co-develop the board with and handle all the contractual, assembly, programming and financial details so they could offer a pre-assembled board.
- Line Input Retained – The ODAC won’t defeat the line input on the ODA. You can use both sources or even plug another Redbook compliant DAC in for direct A/B comparisons (even better if a friend does the switching out of the listener’s sight!).
- Standalone Capable – The ODAC is designed so it’s also usable by itself for other applications while operating entirely from USB power. No ODA required.
NO MONEY FOR ME – There’s been some speculation that an assembled DAC board is a chance for me to finally make some money. But that’s not the case. There’s no money in the ODAC for me. As with the O2, and ODA, I’m leaving that part for others to sort out. This blog, and hence my reviews and commentary, remain entirely non-commercial.
WHAT THEY’RE NOT: A few things the ODA and ODAC are not:
- No S/PDIF – There’s little need for an S/PDIF output as it seems unlikely anyone would want to use the ODAC as a USB-S/PDIF device for another DAC. And while there are a few applications for an S/PDIF input it’s not something most will ever use. For everyone else, an S/PDIF input would add complexity, size, and substantial cost with zero benefit and may degrade jitter performance. That’s not keeping with the “best performance for the lowest cost” O2/ODA approach.
- No Balanced Outputs – Balanced audio is great for pro use with long cables and electrically hostile environments. But it’s generally more of a liability than an asset in home audio gear. All else being equal, balanced stages usually have higher overall noise and distortion than their unbalanced counterparts. Headphone drivers don’t magically somehow work better when driven by a balanced source. Most of the stuff you hear and read about balanced audio for home headphone gear is myth, hype and even snake oil.
- No Arc Welding – If you’re looking for an ultra high output amp for those vintage AKG K1000s, or to weld that broken light fixture, the ODA isn’t it. Just like with S/PDIF, it doesn’t make sense everyone should pay for power they can’t use (and that could also more easily damage their headphones) just to support a tiny minority of potential users with unusual requirements.
- No Battery or USB Only Power – The ODA isn’t intended to be portable. That’s what the O2 is for. The ODA needs a proper power supply for the best performance and should outperform any USB powered headphone DAC. The ODAC board by itself, however, can be powered from USB for use in other applications.
- No Recording – The ODAC is playback only. If you want a microphone input for Skype calls, check out the $12 Syba CM-119 I reviewed. If you want to record a string quartet, there are lots of great reasonably priced professional USB audio interfaces optimized for recording rather than playback.
DAC INTERPLAY: I’ve held back on some ODA design details waiting to see if the ODAC was going to be a reality. So in some ways, the ODAC has come first. The ODA PC board layout and other details are subject to change greatly depending on what form a DAC option might take. Because I want to remain non-commercial and not profit from any of this, I had to find someone interested in offering the ODAC for sale in pre-assembled form. It might be a bit premature, but so far the ODAC is looking viable and I’m very pleased with the performance and testing so far. It’s been a joint effort.
HOW MUCH? Many have asked about the cost. I’m guessing the ODA itself will be around $30 more than a complete O2. That would put the total DIY price somewhere around $130 without the DAC option. Just the board would be under $70 complete and fully functional if you build it yourself. And I’m told the optional assembled, programmed and tested ODAC board should be under $100. These are all just estimates and subject to change. The more popular the ODA and ODAC become, the lower the price will probably go.
WHAT’S NEXT & TIMING: My current plan is to publish another ODA/ODAC article sometime in December with more details, more test results, etc. Depending on how things go, the ODA documentation package (including PCB artwork), and even the ODAC board, could be available as early as late January. Being realistic, February is more likely and it might slip into March. It all depends on how many board revision cycles are required, what changes/problems come up, etc. The O2, for example, had a last minute re-design and delay because the main distributors ran out of volume controls and a few other critical parts. For now I just wanted to let everyone know there’s progress, and if you’re thinking about an O2 for desktop use, or perhaps another desktop headphone amp or headphone DAC, you might want to wait for the ODA.
TECH SECTION
DYNAMIC RANGE BACKGROUND : Probably the biggest spec thrown around by DAC chip makers for bragging rights is the dynamic range (DNR) performance of their chips. Some sleazy audio DAC marketing types just publish the number from the datasheet as the spec for their completed DAC. But that’s usually cheating in a big way and a bit like saying the tires on your car are rated for 155 MPH so that must be how fast the car can go. Uh, no. But then again some of those designing and selling USB DACs probably have no way to measure the actual DNR so perhaps that’s why they cheat. Getting even close to the chip’s ultimate performance requires great care with the power supply, PC board layout, grounding, and more. For 112 dB of dynamic range with the Redbook standard output of 2 V rms at 0 dBFS, there can be only 5 microvolts of total noise. That means all the noise in the audio band added together has to be less than 5 microvolts! That’s hard to do even if the DAC chip itself were somehow noiseless and that’s hardly the case with lots of noisy digital signals just a few millimeters from the analog pins and noisy USB signals not much farther away.
ODAC DYNAMIC RANGE: Shown below is the performance of an early prototype of the ODAC. The final design could be worse, but hopefully might be even better. Regardless, the results show nearly 112 dB of genuine dynamic range (A-weighted or “dBA” as is the industry standard). Most USB DACs are limited to 16 bits and around 90 dB plus or minus a few dB of dynamic range. I used a –60 dBFS signal instead of my usual –90 dBFS as the former seems to be more of an industry standard for this measurement:
WHY 24 BITS CAN MATTER: Many find it convenient to leave their headphone amp turned up and use the volume control in your PC’s operating system or player software. For one thing it allows locating the headphone DAC out of reach and/or out of sight. But when you do that with a typical DAC capable of only 16 bits over USB you get less than 16 bit audio delivered to the DAC at anything less than maximum volume on the PC. Turn it down only 6 dB and you have 15 bit audio. Another 6 dB and you’re getting only 14 bits. And so on. Here’s a graph of the ODAC running in 24 bit mode in blue. It’s playing a 6 Khz test track just below maximum at –1 dBFS but the volume control in Foobar was turned down about –45 dB. So the end result is a signal that’s about –46 dBFS being sent to the DAC. In 24 bit mode the THD+N was impressively low at only 0.01% relative to the –46 dB signal. Shown in yellow is the much higher noise floor in 16 bit mode without changing anything else. The dScope can’t show FFT readings from previous sweeps easily, but the 16 bit THD+N was nearly 20 dB worse at about 0.08%—or eight times as much noise and distortion in 16 mode (As has been pointed out in the comments, the readings in this graph may not be correct but the spectrum is correct. I plan to re-run the test):
MORE TO COME: Check back in December for more on the ODA and ODAC!
