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Altec 1567A mixer hum, help needed

Discussion in 'Pro Audio Equipment' started by pdsepple, Jul 21, 2008.

  1. pdsepple

    pdsepple Guest


    I got an Altec 1567A mixer and hooked up a line level signal (like a CD player) to input 1 octal socket pin 7 hot and 8 ground. Pins 5 & 7 are linked. The output is connected to output octal socket pin 7 hot, pin 8 ground. It has a bad hum/low frequency buzz, I think its the way its hooked up. I don't think its the unit itself and I tried lifting grounds on everything except the mixer. The source is battery powered and the amp is a quiet soundcraftsmen amp. Anyone who can help I'd be happy to trade studio parts, tubes, caps, octal plugs. Incidentally, I shorted pins 7 & 8 of input and the hum/buzz is still there.

    The schematic and manual are at http://www.triodeel.com/al1567a.gif
  2. pdsepple

    pdsepple Guest

    Also, the hum gets louder when I raise the master volume or ch. volume. The hum is the same on all ch.
  3. Boswell

    Boswell Distinguished Moderator Resource Member

    Apr 19, 2006
    Home Page:
    Yes, those units suffered badly from ground-induced hum. DC heating of the input valves (tubes) was a nice idea, but it causes heavy pulse currents through the rectifiers that permeate the rest of the circuit. The optional input and output transformers were not there for fun - they were needed to isolate the mixer from what was driving it and what it was driving.
  4. Greener

    Greener Guest

    Is the ground clean?
  5. Codemonkey

    Codemonkey Distinguished Member

    Dec 11, 2007
    Scotland, UK
    When was it last swept?

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