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Switch on thump

Discussion in 'Electronic Basics' started by Chris, Sep 16, 2005.

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  1. Chris

    Chris Guest

    I have a guitar pre-amp and separate power amplifier .
    When the power amp is already on for a while , should I then power up the
    pre-amp , there is a loud ' thump ' .
    Can I modify the switch on the pre-amp to get round this problem .
    I'm in 230VAC UK .

  2. Kitchen Man

    Kitchen Man Guest

    Not stated but inferred is that when switching the two on together, or
    after the pre-amp has been on only a short time, there is no audible
    thump. This suggests to me that there is an abundance of energy built
    up on the pre-amp output during use that is discharging through the
    power-amp when you turn on the latter. It would appear that the
    solution is to power down the pre-amp prior to using the power-amp,
    and to power them up together when it is desired to use the power-amp.
    I don't know how modifying the switch on either the pre- or power-amp
    would alleviate the problem, unless it is paralleled with the
    power-amp switch to ensure they always power-up together. In that
    case, plugging them both into a switched power strip should do the
  3. Bob Masta

    Bob Masta Guest

    The thump probably comes because the preamp outputs are
    AC-coupled through capacitors to the power amp. (There
    are probably also caps in the power amp input.) Thumps
    are especially likely if the preamp runs off a single supply
    so its internal output is at half the supply, and relies on
    the cap to block this. When you first turn it on, that internal
    output point rises quickly to half of the supply, and that
    fast rise is essentially AC that passes right through the cap
    to appear as a thump. To prevent the thump, you'd need
    a special slow-turn-on circuit design.

    If the preamp has balanced + and - supplies, this is
    still a possible problem if the circuit doesn't come on
    in a controlled fashion.

    So no, there is no simple switching arrangement to
    cure this. A complicated arrangement would be to
    have a time-delay such that the preamp is allowed to
    power up and charge its output cap into a resistor
    to ground simulating the power amp, and then after
    the voltage across the cap is reduced to a very low
    value the switch connects the power amp.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  4. Bob Eldred

    Bob Eldred Guest

    Turn on the pre-amp first and let it settle for 10 seconds or so before
    applying power to the power-amp. Do the reverse when turning off, turn the
    power-amp off first then the pre-amp.

    The problem is caused by large swings in voltage in the circuitry as power
    comes up and amplification is established. Many amplifiers and
    pre-amplifiers suffer from this defect. Circuit designers usually go to
    great pains to insure their products don't do this by incorporating some
    form of muting circuitry in the design. Apparently your pre-amp does not
    have this.

    One way to solve it is to use a time delay relay to connect the speakers to
    the power-amp only after the power has been applied for some number of
    seconds. Then any power up thumps will simply not get to the speakers no
    matter where they originate. Some amplifiers may require a dummy load when
    not connected to the speakers which can be accomplished with a double throw
  5. John Fields

    John Fields Guest



    Bob might have it but, if the preamp is of any quality, it really
    shouldn't do this. If you have a Digital volt meter you might want to check
    and see if the is any DC at the output of your preamp, there should be none.
    When you power up the preamp the output might be jumping from 0VDC to 1 or
    2VDC, this would cause a pop. If this is the case, you might want to take
    it to you local tech and have it fixed, or at least compare it to one of the
    same model to see if it is a poor design.
  7. Jasen Betts

    Jasen Betts Guest

    possibly. more likely you'll need to add a de-thump unit inside the preamp.

    something that will disconnect the output until the pre-amp has stabilised
    (probably about 1/4 second after turn-on)...
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