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Hum Loop - any ideas

Discussion in 'AV Pre-Amp/Processors & Power Amps' started by davehk, Nov 15, 2004.

  1. davehk

    davehk
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    I recently connected the 5.1 analogue outputs from my DVD (Pioneer 656) to my DSP800E processor (previously only had a fibre digital connection, but I want to play SACDs), and I now have a ground loop that appears to go as follows:

    DVD player (2 core mains) to DSP800E (2 core mains) to Preamp (2 core mains) to Power Amp (3 core mains) through earth conductor of ring main to Projector (3 core mains) to DVD player.

    Any ideas on how I can break this? Clearly both the power amp and the projector need to be earthed for safety, since they are not double insulated. I guess I could isolate the power amp Ov line from direct link to mains earth (I built it years ago) by putting a resistor and capacitor in the path - that's quite a common technique

    Thanks
     
  2. pcavelle

    pcavelle
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    I had exactly this same problem recently, with a similar configuration to yours, different brands though.

    I ended up fully curing it by connecting a single piece of speaker wire (as a ground cable) between pre amp chassis and processor. Most components have a little screw plug to enable this type of fix - then, its just a question of touching the non-processor (or preamp) end to different component chassis until the hum stops.

    Paul
     
  3. deckard

    deckard
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    Alernatively you could use a ground loop isolator to do the trick. I've got one going spare if you're at all interested.
     
  4. LV426

    LV426
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    Or....you could try simply pulling the RCA phono plugs (all of them) partly out of their sockets at one end only, so that only the centre connector is making contact.
     
  5. davehk

    davehk
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    Tried that - then I REALLY get mains hum.
     
  6. davehk

    davehk
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    That's the sort of thing I was thinking of - do you have any specs ? Impedance, maximum signal level (I assume it's a transformer, so there will be limits due to the magnetic characteristics of the core), Bandwidth, distortion?

    It would have to go in the preamp to power amp line, so would be used for music as well as HT - so it has to be very good.

    Found the Xitel website. It's 20-20K +/-3 dB, 47Kohms, Not good enough I'm afraid. I'd need the response to be +/- 1dB at least between those limits. Any no distortion or signal level info.

    I think I'm going to modiify the power amp to isolate the signal ground from the chassis for low level signals. There's a way to do this and still preserve the safety grounding in case of a primary to secondary short in the mains transformer, to ensure that the signal Ov can't go up to mains potential. NOt that that's very likely with a torroidal.
     
  7. LV426

    LV426
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    ho-hum! (sorry). Worth a try; it often does work and costs nothing.
     
  8. deckard

    deckard
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    No prob re the isolator, I didn't want to use it either.

    I found a way around the problem in my case. My situation is not dissimilar to yours (I used to use a DSPE800 in fact) in that I take the front pre-outs from my AX5i and feed them into a tape loop on my Linn Majik for the front channels. Now if I don't also connect the analogue 2 channel output of my 757Ai into the Majik I get hum (AX5i and 757Ai connect via i-link). It might be worthwhile therefore trying an additional electrical connection, ie using a coaxial digital interconnect rather than an optical one (a bit like pcavelle suggested now I think about it except using a signal cable).

    Might be worth a try.
     
  9. davehk

    davehk
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    I'm going to give this a try - it's one of those things that just might work. I'll also try power amp to preamp chassis too.
     

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