Weird problem. My passive pre-amp is introducing hum & noise into my amp! HELP! ;)

Discussion in 'Audio Hardware' started by fjhuerta, Aug 16, 2003.

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  1. fjhuerta

    fjhuerta New Member Thread Starter

    Location:
    México City
    Hi all,

    I just picked up my new amp.

    It's a MONSTER. :)

    I plugged it in, using my new CAT5 cable and radio shack interconnects. I used a pair of Definitive Technology speakers and a cheap CD / MP3 player as a source. My preamp is a small Creek OBH-12.

    The amp sounds heavenly. But I noticed TONS of noise and hum.

    As to troubleshoot, I first unplugged the preamp from the amp. Noise and hum were gone (I'm thinking it's a 60Hz signal, but I could be dead wrong). I then switched the amp from unbalanced to balanced mode. Noise is at the same level. Good. The amp is not to blame.

    Next thing - I connected the whole thing together again. Noise and hum are back. I disconnected the CDP. Still lots of hum and noise. I now disconnect the cable connecting the CDP (leaving just the pre and power amp connected using RCAs). Hum and noise still at the same level.

    I then disconnect the passive pre-amp, and leave the RCA's connected. The noise is gone.

    Next thing I try is to play with the volume level. Yep. Hum lowered its level as soon as I turned the knob, and then went from the left speaker to the right one as soon as I put the pre-amp at full power.

    I'm completely perplexed. Shouldn't a passive pre not add any hum or noise? It's passive!

    OTOH, am I experiencing an impedance mismatch? If so, is there any way to take care of it?

    Thanks a lot for any and all help! It'll surely be appreciated.
     
  2. sgraham

    sgraham New Member

    Location:
    Michigan
    To be sure it's really the preamp, unplug *everything* from the preamp, just have the preamp connecte to the amp. Is there still hum? If not you could have some sort of ground loop, though that seems unlikely if it's there with balanced inputs as well.

    More likely the preamp, or one of the cables, is picking up hum induced by a transformer, perhaps in the power amp or some other component. Have you tried reorienting the preamp to see if that's the case? If so, physically separating them will help.
     
  3. Clay

    Clay Forum Resident

    Location:
    Saratoga, CA
    I had major hum and I added a cheater plug so all components are only using 2 plugs into the outlets. It did help.
    If one is a 3 prong and the other is a 2 prong, lift the ground on the 3 prong and presto hum gone (hopefully).
    Another idea is to attach all the components together with a ground wire.
    Another idea is to switch the direction of the 2 prong plugs. (pull it out turn it 180 degrees and reinsert it) Also can try with the 3 prong unit after the ground is lifted.
    Try plugging the preamp into a different outlet.
    Be sure the speaker cables are not near electrical cables.
    Try the old speaker wires (shot in the dark - no idea if it will do anything)
    Search the archives for hum, ground loop, etc.
    Report back -
    I had a similar problem so I have just been through these tasks.
     
  4. Dave

    Dave Esoteric Audio Research Specialist™

    Location:
    B.C.
    Being that you just finished making DIY IC's I'm willing to bet that they are your problem Javier. Try a common type IC to check this out, even a cheap set will do.

    Just a hunch.
     
  5. Metralla

    Metralla Joined Jan 13, 2002

    Location:
    San Jose, CA
    Dave, I think he just finished Cat 5 speaker cables, and he indicated in his post that he used "radio shack interconnects".

    I would remove the volume control device and connect the CDP directly to the amp. Find the quietest piece of music you own (the opening to "Bolero" is particularly quiet) and have a listen. Be ready on the STOP button if it gets too loud. If the sound is clear and hum-free, you know that the Creek is to blame.

    Regards,
    Geoff
     
  6. Ted Bell

    Ted Bell Forum Dentist

  7. fjhuerta

    fjhuerta New Member Thread Starter

    Location:
    México City
    Steve: the first test I did (outlined in the original post) was to unplug everything from the pre-amp, and just leave pre-amp and amp connected. That's when I discovered where the hum was coming from :(

    Clay: The preamp is passive, so there's no way for me to plug it into something. That's part of my problem - how can a passive device be generating hum??? Sounds next to impossible to me.

    Dave: Geoff is right, I didn't do any IC DIY work; I did the Cat5 speaker cable. Still, since I have tried the test with the preamp unplugged, but the speaker cables connected, I'd assume the speaker cable is not to blame.

    Geoff: I was warned by the amp designer never to use the amp while having a source directly plugged into it, since the amplifier gain is very high (I wonder if this is part of the problem...). The designer told me he had an exceedingly high ratio of clients who had blown their speaker system with his amp, so he asked me to be careful.

    Ted - I'm looking into the web site you mentioned.
     
  8. Clay

    Clay Forum Resident

    Location:
    Saratoga, CA
    If you have a old pre-amp or receiver with pre outs, you could try that or borrow one from the store. Hope you getting it working soon.

    I have a few extra of everything so when something is suspect, I switch it out. Wish I has an AA or EE in electronics.
     
  9. fjhuerta

    fjhuerta New Member Thread Starter

    Location:
    México City
    Clay - I have an EE degree. It's not much help right now :)

    I measured a 36V AC difference between both terminals (pre and power amp). Wow. I tried grounding both chassis to a common ground, then "floating" both pieces of equipment, in order to break the ground loop. It halved the voltage difference, so I'm now assuming the problem is with the amp itself (NO way a passive amp can introduce any voltage difference, so I'm assuming the amp circuitry has a faulty ground connection).

    I wrote the amp's designer today. Let's see if he can help...
     
  10. fjhuerta

    fjhuerta New Member Thread Starter

    Location:
    México City
    Update.

    Called the amp's designer. Told him what I found out.

    His answer? "Uhm... there's a cap in there... it might not be discharging correctly. Tell you what. Bring the amp back, with your preamp. I'll modify the circuit so that it works correctly with your pre". :eek:

    :bigeek:

    :righton:

    I'm not buying ANY other piece of mass manufactured items, as long as his company is around.
     
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