my DAC design - pup1 DAC

Discussion in 'DIY (Do-It-Yourself) Discussions' started by cobaltmute, Nov 26, 2008.

  1. tomb
    0.13V offset is perfectly acceptable for the pupDAC ...  it may be that the OPA2835 got damaged somehow.  Sorry you had to go this route, but you'll be happier with the OPA2836 anyway - it's a far superior opamp.  JMHO, but the 2835 was just a silicon prototype for the 2836.  When TI/Burr-Brown felt the design was vetted, then they really turned up the performance on it for the 2836.
    The ringing pulse could be caused by anything, but if it's in the MHz range, it's not audible.  The only other thing I would suggest checking is the lead length on the mica capacitors.  Once again, it's the parasitic effects of the lead lengths, but it takes some effort to really get those micas down low to the PCB (like pliers pulling the leads through - carefully).  As for the 50Hz, that's probably just a USB-power artifact.
  2. geofftnz
    I stood at my workbench last night for two hours going through all the tracks that I gravitate towards when testing audio - it was hard to turn everything off and go to bed... I'm not unhappy at all for having to swap out the opamp - thank you for including both in the kit! As for power & noise, I'm throwing some pretty nasty stuff at it: My laptop on AC has rubbish power (visible on USB power and very audible on the on-board sound) and I have CFL lighting very close to where I'm working, which likes to inject noise into anything and everything. Most of the power for my house flows through the wall behind my workbench, so that's another 50Hz source.
    I had to put a couple of 90° bends in the mica leads to get them to fit - you were right about the quality control on these. They're as flush as I can get them though - probably added 0.5-1.0mm of lead length. Would SMT mica caps be an option here?
  3. tomb
    Hmm ... that's a cobaltmute question.  Seems like at one point he said we could replace them with film caps, but I'm not sure.  During prototyping, he seemed to think offset issues were focused on the opamp resistors.  That's why he made the change to put four of them on the bottom of the PCB.  However, if I'm interpreting things correctly, the mica capacitors are essentially the decouplers for the power supply to the opamp.  That's why I suspect that perhaps long lead lengths with the micas could cause an issue.
    Hopefully, he will see this and reply.
  4. geofftnz
    My interpretation was that the micas were part of a low-pass filter on the differential pair coming from the DAC, since the opamp has bandwidth well above audio frequencies... I'll defer to the actual designer though :)
  5. tomb
    You're right.  Looking again at the schematic, they're applied to the signal outputs of the DAC.  Further, "The analog filtering is minimal, only using one high quality silver mica capacitor in the signal path."  That's a quote from the EZDac technical info, which was some of the basis for the differential-to-single-ended-gain-stage on the pupDAC.
    So as far as what I had been saying ... never mind.[​IMG]
  6. scootsit
    Hey gents, I've been away for a while, it's been a crazy couple of months, two moves, new jobs, etc, etc. Anyway, a year or so ago, I was working on building an effects pedal for a friend, then well, things got busy and it didn't happen. Now that I've settled, I want to get back to work on it. I recall someone posted somewhere on head-fi (I'm pretty sure it was this thread) a link to a company that custom engraved and silk-screened electronics boxes. I think it's the company used for the Pup. Anyone have any ideas?
  7. geofftnz
    Might be wrong, but I think it's Hammond.
  8. scootsit
    Cool, thanks. I checked with them, I remembered seeing a service with an online interface that also used CAD, so you could edit it and upload it and get an immediate quote. I'll check with Hammond tomorrow and see if they're not too outrageous for just one or two.
  9. Avro_Arrow

    Front Panel Express
  10. scootsit
    That's the one, thanks a ton!
  11. tomb
    Hammond certainly makes the pupDAC cases, but the anodizing and laser-etching was done locally in Atlanta:
    Hammond is now making those cases in the same blue, so I'd order them directly from them if you are interested.  Their quality, quite frankly, is not that good if you order them in RAW aluminum.  I get the feeling they dump them all in a giant dumpster to get scratched up, first. [​IMG]
  12. geofftnz
    It's called "Vintage", so your DAC goes with your threadbare tshirt and $250 ripped jeans.
  13. scootsit
    I'm not building the same thing. If possible, the effects pedal would actually work better with the Bud cases, anyway. I appreciate the tip, though. I'm going to try Front Panel Express.
  14. tomb
  15. funch
    I thought I'd pull this thread up out of the dead pool. I finished mine a couple of days ago, and am quite impressed. For no more than this DAC costs to build, it's a great performer. I was looking for a
    small DAC to go with my 'Wintergreen Handwarmer' tin amp, which itself is a great performer as well. Fabulous match. Of course, I'm feeding mine with a Beezar Doodlebug USB de-crappifier.

    Even through my Eddie Current BA, it sounds excellent. Bang-for-the-buck ratio is high. Definitely recommended if you're looking for a cheap, small DAC.

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