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Semi-Customized DAC Part II (in which we hit a roadblock and ask for help)

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Disappointing news from the trial run (no chassis or ribbon conductors, just everything laid out and hooked up with regular wire for testing):

Everything is soldered and connected, with a 1A fast blow fuse in the IEC inlet. When I plugged it in this morning, the transformer (an R-core transformer I purchased separately from the boards) got piping hot to the extent that I smelled plastic and was afraid to leave it plugged in long enough to listen to anything. One thing I did, though I am guessing it wouldn't cause this, is that I twisted the secondaries of each color together before soldering them individually to the appropriate contacts on the PSU PCB.

The fuse is intact and all the lights on the PCBs still went on the second time I plugged it in (though the solder contacts under at least one of the PSU capacitors also got very warm, and a capacitor or two was also warm). But the transformer again got way more hot than I think it should be, in half a minute or so getting so hot that after I unplugged the power it was still too hot to comfortably touch.

I'm supposing this is (very) wrong. Any notion of what I might have screwed up?


  1. Jud's Avatar
    Another data point: Powered up briefly again, long enough to see Audirvana Plus recognized the DAC in its preferences, and to find out I get no volume. :(
  2. Bones13's Avatar
    Power to the output stage may be shorted, causing no output, and hot power transformer, just a thought. Short could be anywhere from the power transformer to the end of the output stage. Assuming of course there is separate power to a separate output stage. The SDPIF was on enough to be seen, so the receiver board/DAC portion is getting power clearly.

    I doubt an appropriately sized, and connected power transformer will be getting that hot. DAC loads are not that high usually, so I am leaning to some sort of short, or a component that has gone bad, causing an internal short.

    This type of situation requires the schematic, and checking the power input to output, and the signal input to output.
  3. Jud's Avatar
    Thanks Bones, appreciate it. Yep, I think a short has a very good chance of being the explanation. Can't provide schematics publicly (see first post in this series re my agreement to keep PCB supplier's identity confidential) but have sent photos of the relevant stuff (tranny, IEC inlet, all the solder connections I made including outputs) to the fellow helping me with the chassis and the PCB supplier.

    The former says everything looks good so tranny may have a defect. I'll wait for my PCB supplier to get back to me and confirm whether or not I made a wiring mistake. If not, I'll test the tranny (stupid me, obviously should have done this before). That would be my guess, as the PCBs were tested before shipping, and if I haven't wired something wrong, there are no obvious solder bridges or anything like that.
  4. Jud's Avatar
    Wiring error at the outputs (soldered to metal-rimmed mounting holes, which looked like contacts, but weren't) - explains no sound. Wiring from IEC inlet to tranny, and tranny to PSU is OK. Unconnected outputs I'm thinking are unlikely to explain the hot tranny, so I'm ordering a new tranny from someone I trust.

    Couple of "shouldas" - Shoulda ordered the tranny from the trusted source in the first place rather than thinking all R-core trannies are the same and saving a couple of bucks; shoulda very definitely tested it before running power through it, lucky things didn't turn out worse. Hoping to avoid too many more "shouldas" along the line.