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
Board outline(s) extent: X = 455 MM; Y = 330 MM
Number of parts: 1471
Number of pins: 5284 (258 through-hole, 5026 SMT)
Number of vias: 3205
Number of holes: 3463
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
2) Clean DSD switching
3) Clean clocking
4) Good isolation
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 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:
1) DSD hard wired (well thru a digital isolator)
2) DSD via Meitner orange ST glass
3) TOSLink
4) RCA S/PDIF
5) XLR AES/EBU
The PCM can be 44.1, 48, 88.2, 96, 176.4 or 192.
I synchronously upsample all PCM to 56,448,000 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 :)
