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My new DAC (long, but with pictures)
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Posted on October 24, 2010 at 16:45:50 | ||
Posts: 10297
Location: Seattle Joined: December 29, 2000 |
Howdy all It's been a while since I posted at the Asylum so I thought I might update you all on what I've been doing: I built a DAC: My goal wasn't to build the most cost effective solution: I wanted to verify some ideas I had. If I built a no holds barred board and it didn't work I would know that I had failed fundamentally and not just because I chose cheap solutions. The board is a monster: Number of copper layers: 6 It's features are outlined below. But first some history: A few years ago I thought I'd make a DSD based DAC and since I'm a software guy I started out by prototyping a quick and dirty output stage, just some passives filtering the raw DSD in an Sony DVP-S9000ES: Then since I'd never built hardware before I cobbed a quick dual output power supply: I simulated some of my ideas with the demo version of MicroCap 9 (http://www.spectrum-soft.com/index.shtm) and tho things looked good I didn't know what tools were available so I contracted a quick layout of my schematics: The boards were daisy chainable and I had three built with differing output stage component selections. I could just use my preamp input selections to A/B/C the boards. The boards sounded pretty good, but there was some "breathing" in the gaps between notes and the output level was much to low for most systems. I found FreePCB (http://www.freepcb.com/) and figured out how to export my schematics from MicroCap and built my next board with better power supplies and a different DSD amplifier and a custom VCO from Vectron with 200ppm pull and DSD x 8 frequency: It sounded like crap :) Too much aliasing, worse "breathing", more crunchy. Yuck! Also the clock circuitry drew so little current that the voltage in the clock power supply floated too high: After thinking for a while I surmised that I needed double rate DSD and more solid power supplies. After running some experiments to verify various hypotheses I laid out my current board using the following guidelines: 1) Clean power I built better power supplies with lots of filtering. (There are 9 of them including 3 for the FPGA, left analog, right analog, 2 ECL and two for the clock.) I used ECL for clean distribution of the DSD. I kept the custom Vectron oscillator modules, but I added more filtering of the control signals and their own power supply. The board is dual mono, balanced differential so there is essentially no chance for even order harmonic distortion and I selected my bypassing, etc. to keep the THD below the 120dB noise floor, at least in the simulations. I haven't had the chance to use something better than my PicoScope to look at distortion, but it shows no distortion withing it's noise floor from about 7Hz up to 20kHz (where I stopped looking.) To assist in noise control I made sure everything was electrically isolated: output transformers, power supply transformers, AES/EBU and SPDIF transformers, optical connectors for TOSLink or Meitner ST glass DSD. To save on popcorn logic and to allow easy processing of PCM to DSD I put on a Xilinx FPGA along with the MIPS MCU controlling processor. The FPGA also allows me to avoid other LSI chips like AES/EBU and S/PDIF receivers... No PLLs here :) The board can process: The PCM can be 44.1, 48, 88.2, 96, 176.4 or 192. I synchronously upsample all PCM to 28,224,000 Hz then to double rate DSD (5,644,800 Hz) and also I upsample DSD to double rate DSD. It's dynamic, "fast", easy to listen to, has a flat freq response and has no grit or jitter edginess. The board with no case, just sitting on a cardboard box, and with no magic power cords or interconnects, etc. and using a USB to S/PDIF converter then a S/PDIF to TOSLink converter sounds better than my Meitner or anything else I've heard. When I had my hypersensitive pregnant daughter A/Bing it vs. the Meitner DAC6e her body jerked each time I selected the Meitner and on about the fourth switch she said "Stop that" :) A few more pictures: The raw board: The processor and FPGA (and lots of bypass) Bring up of 2nd board: Early FPGA dev with a Xilinx eval board: -Ted P.S. I still don't have enough time to read everything here, but I'll at least watch this post for a while and try not to be too cagey :) |
RE: My new DAC (long, but with pictures), posted on October 24, 2010 at 17:06:24 | |
Posts: 13158
Location: Kent Joined: June 1, 2002 |
Very well done! |
Thanks. [nt], posted on October 24, 2010 at 18:02:14 | |
Posts: 10297
Location: Seattle Joined: December 29, 2000 |
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Thanks., posted on October 24, 2010 at 20:22:33 | |
Posts: 10297
Location: Seattle Joined: December 29, 2000 |
Howdy It's always fun learning new things :) -Ted |
RE: Two questions..., posted on October 25, 2010 at 15:37:03 | |
Posts: 10297
Location: Seattle Joined: December 29, 2000 |
Howdy Unfortunately the only relevant devices I have are a PicoScope 3206 (http://www.picotech.com/computer-oscilloscope.html) and various (relatively cheap) 24/96 or 24/192 A/Ds. The scope only has 8 bits precision @ 200MHz bandwidth (more precision if you do averaging) and the audio A/Ds only cover the audio range of frequencies. Neither really reach their theoretical accuracies, e.g. I still see noise with the ground wire directly clipped to the probe tip. I do most of my debugging with simulations, ears, AM radios, A/D audio input recording, whatever I need that doesn't cost a fortune :) On my previous boards even tho I had good isolation and noise rejection thru the DSD inputs I found noise in the 100's of MHz coming in via the power cord and also thru direct radiation from the open S9000ES, so I made sure that my new supplies have high input noise rejection and low output impedance over a wide frequency range. I can no longer hear differences when I use the S9000ES transport controls so I infer that I've moved in the right direction :) When I run out of improvements I can make in software, etc. I'll see if I can borrow (or rent) some real test equipment. I'm sure that many of the things I've done are way overkill and that I could get most all of the same quality of sound with a lot less hardware, but as I mentioned at the outset I didn't want to wonder what would happen if only I did this, that or the other thing. Here is the data from the previous board. It will illustrate what the PicoScope can and can't do. I apologize for the fuzz, but I didn't want to post the 6Meg versions of the .jpgs. Also I apologize that the vertical scales can't be taken literally and that they have different horizontal scales: I captured these for my own debugging not for public consumption and didn't take care to normalize the results. Here's the noise floor with the S9000ES stopped: Here's the noise floor with the S9000ES spinning, but paused: Here's the a similar plot but zoomed in: -Ted |
RE: My new DAC (long, but with pictures), posted on October 26, 2010 at 01:17:10 | |
Posts: 213
Joined: December 23, 2006 |
Hi Ted, Forgive my ignorance but which DAC chip is used? |
RE: Oh, well, posted on October 26, 2010 at 07:57:49 | |
Posts: 10297
Location: Seattle Joined: December 29, 2000 |
Howdy Thanks. The company that did the physical board assembly seemed surprised that I was one guy in a basement and I was a little surprised that they didn't have many other "hobbyist" customers: they mostly support Boeing, Microsoft, various local medical companies and obviously a lot of small and medium sized companies. An ALW Super Regulator supplier: http://www.at-view.co.uk/alwsr.htm The super regulator has a long history and rather than confusing the issue or taking forever I'll just give you a link to the beginning :) http://waltjung.org/Regs.html -Ted |
That's why you write English and I write code :) [nt], posted on October 26, 2010 at 16:21:48 | |
Posts: 10297
Location: Seattle Joined: December 29, 2000 |
RE: Thanks, posted on February 21, 2013 at 18:25:23 | |
Posts: 5
Location: Brisbane Joined: August 9, 2012 |
Not sure if Im a member here |