Mr. NWAVGuy has published one of his most interesting and technically revealing posts: The ODAC
LESSONS LEARNED: If this project has taught me anything, it’s that getting much better than 16 bit (96 dB) performance can be challenging. The first version of the ODAC, despite following the reference design, only had about 98 dB DNR. That’s about the same as the FiiO E10. The photo to the right shows a few dozen assorted surface mount parts that were laboriously swapped out one at a time and measurements repeated dozens of times using the dScope. Some improvements were far from intuitive. Audiophile preferred polyphenylene capacitors performed worse than less expensive types. Additional filtering on the digital power supply dramatically increased jitter. Chasing down the last few dB of dynamic range the chip is capable of proved to be especially challenging. When it was said and done, the DNR went from 98 dB to over 111 dB. That’s a huge difference and something the design-by-ear crowd would have never achieved.
The ODAC is based on very familiar components (Tenor 7022l and ESS 9023 DAC -both not having a freely available datasheet), but according to NWAVGuy, only after extensive component experimentation and measurement, was he able to achieve the numbers published in the DAC’s datasheet. This indeed is a very remarkable achievement given the fact that the entire board is USB powered, including the ESS DAC.
The final product is a collaborative effort with Yoyodyne Consulting and very affordable priced: $99 starting price.
(Another photo of a prototype board from diyaudio)
(Update 5/10/12): Production (with minor tweaks to the layout, resulting in improved jitter (-106 db)
I’m pleased to report the production version has even lower jitter. In the last ODAC article I reported the jitter components adding up to –103.3 dB. In the production version, as you can see below, it’s over 3 dB better at –106.5 dB with far fewer components.
I believe it’s related to some minor changes in the USB ground scheme. When you’re dealing with jitter components below -110 dB, even small changes in the digital grounds can impact jitter. You’re correct such changes carry a risk of making things worse instead of better. So, especially where the PC board layout is involved, we tried to play it safe with the final revisions based on everything I had learned along the way.
(Rev 2 (left) and Rev 1)
Notice the following:
- Replaced can-type of bypass capacitors with SMT types
- Replaced radial through-hole bypass caps with SMT types
- Use of large inductor in front of DAC supply regulator
Perhaps these larger-feature capacitors act like antennas and pick up more air-born EMI causing jitter problems? I notice a similar “trend” on the Musiland USB devices: newer models have done away with through-hole components (maybe it is just cost-savings). Compare the new 01 USD with the old 01 USD:
Some wisdom from the comments section
(Whether you agree or disagree with NWAVGuy’s approach, there is always something to learn in his posts and comments)
Cable ferrite for noise suppression
Use of ferrite bead on USB cable and choosing a “free” USB port [link]. I have been doing this for a while in my own setup
The ferrites do suppress common mode noise and also radiated noise. That lowers noise on the ODAC’s power supplies, which in turn, slightly improves the overall performance. The ferrites are also a good idea for RFI suppression from cell phones, etc. The ODAC has ferrite filtering on board, but it helps to keep as much noise as possible from even reaching the ODAC.
Audibility of jitter [link]
The ODAC and DAC1 both measure sufficiently well to be transparent. But they do measure differently and the DAC1 has a performance advantage in several areas, especially jitter. Yet they sound the same. This is further evidence you’re not going to get better sound by trying to further improve the ODAC, use the XMOS interface, etc.
Put another way, I doubt the XMOS interface beats the DAC1 for jitter. Yet the ODAC and DAC1 sound the same. And it’s not surprising. All the audible ODAC jitter components, using a worst case signal designed to expose the maximum amount of jitter, are below -110 dB and the total is below -103 dB. Consider Ethan Winer’s distortion audibility test. The distortion is entirely masked by the music around -75 dB. A level of -103 dB is massively lower. Ethan very conservatively sets the bar at -100 dB for assured total transparency.
Stay tuned to the next article which will be more technical in nature. Mr. NWAVGuy will go into more detail how he went about the improvements. This is a very valuable source for us diy types with no measuring equipment (and with bad ears :-)).