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Allowable drop in gain of a band amp over an hour

Discussion in 'Electronic Repair' started by N Cook, Jun 13, 2007.

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  1. N Cook

    N Cook Guest

    What would be normal and what acceptable for a solid state or valve/tube
    stage amp. ?
    Well you would not want the gain/ output audio to go up over an hour of use
    with a constant input.
    Testing a solid state amp that had a problem that certainly affected the FET
    gain killer protection circuit so (falsely) cutting the output over an hour
    to close to zero. But checking to see if there was any other gain drop
    problem but then realised I don't know what the norm is.
    Amp rated at 200W (music power) which I've taken as 100W (continuous RMS if
    you could). Running a constant source of 1Kz through amp to give a
    continuous 1/5, 20W in a dummy load , which with heating itself is not
    necessarily constant. Result of testing over 40 minutes was a 9 percent drop
    in V rms into the load so about 20 percent drop in equivalent audio watts.
    No fans on this amp and the heatsink settled at 68 degrees C after about 30
    minutes. The output was still dropping after 30 minutes but very much slower
    and decided to cancel after 40 minutes
  2. Ron(UK)

    Ron(UK) Guest

    In an ideal world I wouldn`t want a pro amp to drop any level over any
    amount of time. You should easily be able to locate just where the
    signal is dropping with a scope. There may be a temperature sensitive
    device in contact with the heatsink.

    A constant sine wave isn't a very good way to test an amplifiers long
    term performance, I`d use a signal more indicative of real life conditions.


  3. Eeyore

    Eeyore Guest

    Are you sure the drop in power isn't simply due to the transformer heating up
    and the consequent rise in winding resistance ?

  4. Meat Plow

    Meat Plow Guest

    I made a audio track on CD of a guitar track from a multi track studio
    recording on CD to test amps with and just repeated the track over and
    over. I would take a 12" 150 watt speaker and lay it on it's face on
    carpet then wrap a blanket around the outside of the basket to get an
    inductive load rather than run the amp into a 300 watt resistor. I don't
    expect any power drop in a guitar amp over a period of time but pumping a
    1KHZ sine wave into a non-inductive load may produce such results where
    real life playing wouldn't.
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