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Hum from passive-pre

Discussion in 'audio' started by lordsummit, Nov 10, 2017.

  1. lordsummit

    lordsummit Moderator

    I’m confused, very confused. Andrew yesterday very kindly has loaned me a Moodlab DAC to play with. This afternoon I plugged it in with matching interconnects, I’m a bit geeky and didn’t like the fact I had different ones on the DAC to pre and the pre to power. They’re Chord ones, and brand new.

    Anyway, I noticed a hum. At first it was linked to where the wires were routed. Some messing around and moving a power supply has eliminated this. But I can still hear a small hum when I put my head near the speaker.

    I know I have damaged some internal wires on the passive pre, and only one input works. I did this wrestling off the interconnects (Why do they make them so tight?)

    So I’ve tried to isolate the noise. I’ve swapped interconnects. I’ve switched off the DAC at the wall. I’ve connected the DAC direct to the amp (This got rid of the noise). I can get rid of the noise by opening the volume pot fully. I’ve waved the pre around everywhere to try and see if it is picking up something. No avail. It sounds great, but I know there’s a bloody hum/buzz there. Do I need to replace the volume pot, or am I missing something?
  2. Vinny

    Vinny pfm Member

    Earth loop somewhere? Although opening the pot' fully rather confuses that...............

    I changed to screened interconnects (cable - Klotz MC5000) a few months back and the difference in background noise (hum) was really, genuinely, immense. Worth a try? Deadly cheap cable!!
  3. lordsummit

    lordsummit Moderator

    Indeed, the only item grounded is the power amp, a Quad 306. The DAC is on a wall-wart, the pre is just a pre. The DAC is fed from an Airport Express via an optical lead. That’s not the culprit...
  4. Rick_F

    Rick_F pfm Member

    If you've damaged the RCA inputs on your amp you may have disconnected, or shorted one of the wires. Check to see if any positive wire is touching the case or the other wire connected to the outer part of the RCA socket. If it's the positive that has come away simply wrap some insulation tape around the end if you decide not to re-solder it.
  5. lordsummit

    lordsummit Moderator

    It's a plastic case, I'm thinking though of converting it to single input and bypassing the input selector as I don't need it.
  6. Rick_F

    Rick_F pfm Member

    It could still be a disconnected wire touching another one, worth taking a peak if you can.
  7. lordsummit

    lordsummit Moderator

    I'll tape them all up tomorrow. It really is very quiet, maybe I'm over imagining!!
  8. Rick_F

    Rick_F pfm Member

    Some systems do let a bit of mains hum through. I currently have an issue with mine!
  9. lordsummit

    lordsummit Moderator

    In fact, I'll just remove any unconnected wires
  10. flatpopely

    flatpopely Prog Rock/Moderator

    I did notice hum with this DAC.

    BUT it proved to be an inadequate PSU.

    Is the hum there with the DAC removed?
  11. flatpopely

    flatpopely Prog Rock/Moderator

    And, did it only happen when you matched the interconnects?
  12. Cav

    Cav pfm Member

    Don't all speakers hum when the amp is on? You can usually hear it only when you put your ear very close. If you can hear it from where you normally listen, then something is probably wrong.
  13. Tim Jones

    Tim Jones pfm Member

    FWIW, my experiments with passive pres a few years ago absolutely required a replacement of my (then) preferred and unshielded DNM ICs with a fully shielded, and expensive (though utterly excellent) Mark Grant IC.

    Careful about how you 'wrestle off' those plugs btw. If you break a solder you tweeters could be toast.
  14. talkinghorse

    talkinghorse pfm Member

  15. lordsummit

    lordsummit Moderator

    I think I may be slightly over-obsessing... It’s just audible when Input my ear next to the speaker. I’m going to remove excess wire, and look to wire the input direct to the volume control.

    I tried a different power supply, one of the eleventy trillion Yamaha keyboard ones I have, that made no difference, so I tried turning it off at the wall. That made no difference either. I suspect I may have just noticed something that has long been there. Bugger!!
  16. hifi-dog

    hifi-dog pfm Member

    If it’s only just audible with ear against the speaker then nowt wrong really. Just normal amp noise and you’ll never notice it once you’ve forgotten about this thread!
  17. Tony L

    Tony L Administrator

    It isn’t normal with a Quad 34. Richard’s is my old one that was recapped/serviced by Rob. It is exceptionally quiet, in fact one needs Klipsch La Scalas to hear any hiss or hum from it at all. I couldn’t hear anything via my Tannoys and they have to be 10db more efficient than Richard’s AVI stand-mounts.
  18. flatpopely

    flatpopely Prog Rock/Moderator

    If you turned of the DAC it's not the PSU! Have you disconnected it from the amp - does the hum go?
    If you unplug everything from the pre-amp does the hum remain?
  19. Jim Audiomisc

    Jim Audiomisc pfm Member

    As has been said, it's common to find you can hear low-level hum/buzz if you put your ear right up to the speaker with no music playing. Ideally, this doesn't happen, but in the real world it isn't surprising. It should be totally inauble from your listening seat, though.

    Also, no surprise if the amount varies with a passive pot volume control adjustment. Having no hum when this is wound up to max, but getting it at a lower setting is a clue that changing the resistances at this point is allowing a small hum current to now produce a voltage the following amp treats as input. Classic case may be where there is no hum with the volume pot wound fully up *or* fully down, but peaks loudest at some intermediate setting.

    FWIW I tend to use the preamp as the point for ground reference return and connection to mains earth. But in general, experiment as the best arrangement will vary with what you have connected together, etc.
  20. Paul R

    Paul R pfm Member

    I would fix the damaged wires. But I'm slightly confused by the thread title with the 'passive pre' and references to a plastic case and Tony's comments about the Quad 34. Perhaps I've not read it thoroughly.


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