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possible ground loop

Discussion in 'Misc Electronics' started by Ralph E. Dodd, Apr 5, 2012.

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  1. Hello all,

    I am trying to power an Atari 2600 video computer system and an audio
    amplifier from the same 9 volt wall wart. I've applied a stereo
    modification to the 2600 and mounted the amplifier and speakers from a
    Sony SRS-48 powered speaker system in the Atari case. The Atari came
    with a 300ma supply and the Sony had a 600ma supply, both 9 volt. I'm
    using a 9V 1 amp wall wart but if I power both from the same supply, I
    get a loud hum. I'm assuming that this is a ground loop. If I power
    them from separate supplies, I don't get any hum. Is there a simple
    cure for this or should I just mount another power connector in the case
    and use 2 wall warts? Thanks a lot.

    Ralph
     
  2. Jasen Betts

    Jasen Betts Guest

    you could try an audio isolating transformer between the computer and
    the amplifier, These are often found in the car audio section of the
    store.

    If the hum is flowing over the 9V power bus that won't help, check for
    this first by disconnecting the amplifier inputs from the computer and
    shorting them to ground at the amp end, if the hum persists you have a
    powersupply decoupling or conducted RFI problem, or it could just be
    that your powersupply is poorly regulated.
     
  3. Thanks for the response. I'm going to take the easy way out and use 2
    wall warts. I'll be at VCF East 8.0 with a bunch of modifed Atari stuff
    and there are other issues that I have to straighten out before the show
    (May 5-6).
    http://www.vintage.org/2012/east/

    Ralph
     
  4. I'll bet regulation - or more accurately, filtering, or the lack thereof.

    Maybe add a filter inside the case, a few larger electrolytics of 16V to
    25V (say a 4,700/25V electrolytic) across it? I bet that 9V supply is
    barely filtered, or not filtered at all, and the smaller one for the amp
    IS filtered.

    Just a suggestion if you wish to keep it to one wall wart.

    RwP
     
  5. RwP,

    I'll try this right away. Nice easy fix if it works. Thanks.

    Ralph
     
  6. 25+ year old caps probably don't help at all either.
     
  7. Hello again,

    I got distracted from this for a while but now I've finished it up. I
    put an additional 2200mfd 16v. cap across the wall wart input and it had
    no change at all. I tried using the Sony supply that is quiet with just
    the amp connected, but it made the same noise. I mounted another 1/8"
    phone jack on the Atari and put an 1/8" plug on the Sony supply so I
    using 2 warts and I have crystal clear audio. Thanks for all you help.

    Ralph
     
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