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120 Hz noise

Discussion in 'Electronic Basics' started by Amanda Robin, Nov 4, 2003.

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  1. Amanda Robin

    Amanda Robin Guest

    Hi all--

    I am having an interesting noise problem. I have an acoustic setup
    whereby I output a signal from an MX31 DSP unit, take it through a
    custom made lowpass filter PCB, send it through a power amplifier, then
    out a speaker.

    Actually I have two speakers, so I have two setups like this in
    parallel, using the same DSP unit and different channels of the same
    power amp.

    I am doing acoustic work and normally send a 125 Hz tone to the
    speaker. I've been using this setup for about a year now with no noise
    problems to speak of. (I do actually get white noise now and then, but
    I can make it stop by off/oning the circuit boards.)

    Recently the system has gotten very sensitive to static electricity. If
    I scuff my feet on the carpet, I can hear an additional tone going
    through one of the speakers (not the other). It sounds like a
    vibration, but it is an electrically-based noise.

    I have taken the the SPL of the output of the speaker and the noise is
    120 Hz. I've looked at the signal from the PCB and performed an FFT on
    it, and the 120 Hz appears to be there.

    I have traced my wires and made sure they aren't close to power cables.
    I've checked my connections and grounds, etc. But here's the clincher:
    the noise is not present when I don't play a tone through the system.
    It stops as soon as I turn the tone off (even if I walk around and
    scuff my feet).

    Oh, and the noise is present when I send tones of different frequencies
    through, down to about 70 Hz. I haven't tried finding the upper limit,
    if there is one.

    I am going to do some comparison of the signals since my other channel
    is noise-free so far. I just wondered if anybody had any bright ideas
    about this (besides waiting for wet weather....).

  2. John Fields

    John Fields Guest

    Since it's 120Hz, I'd suspect that something has gone wrong in your (the
    amp's ;^) power supply and that what you're hearing is ripple which is
    occurring when the filter caps are being loaded and their voltage
    valleys are falling below where they should be. More than likely it's
    one or more rectifiers which have gone bad or one or more of the caps
  3. JeffM

    JeffM Guest

    the noise is 120 Hz.
    Lets get the obvious out of the way:
    How do your (full wave rectified?) supply rails look?
  4. Ross Mac

    Ross Mac Guest

    Same question here...Put a DMM on those rails and what does the VAC look
    like....should be almost nothing...
    Or better a scope?....whats the waveform....
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