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AC noise in audio ckt

  1. Nov 30, 2008 #1
    I'm working on a 'Fuzz box' for a bass guitar, ckt diagram below. With fuzz on and gains up, I'm getting a strange (to me) AC artifact -- Oscope images below (AC waveform from isolation xfmr at 10V/div, signal out at 2V/div).

    With fuzz on, and the input shorted to ground, the artifact is timed to the AC peaks, and these peaks are flattened. With fuzz off, the AC peaks are 'normal'. The artifact is present (tho much smaller) at the input to the first amp stage, but I think it is somehow related to feedback from the clipping diodes (which make the fuzz).

    Any thoughts? Thanks in advance.

    Ckt diagram:

    Fuzz On:

    Fuzz Off:
  2. jcsd
  3. Dec 1, 2008 #2


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    Gold Member

    AC hum usually is introduced by 1) a poor power supply, 2) improper shielding or 3) improper grounding. Check the supply for your circuit, or use a battery, to address #1. (Using a grounded supply can introduce ground loops, see #3). Are there any long wires, and if so are they twisted? Is your device in a tight grounded metal box? These steps will largely take care of #2. #3, proper grounding and eliminating ground loops, can be more difficult. I can't offer specific advice since this involves cable routing, the other equipment you are using, earth grounds, and even things that are non-obvious. Try to learn about grounds and ground loops--there are many books on this topic (Ott's "Noise Reduction Techniques in Electronic Systems" is a good one), and it looks like a Google search has dozens of helpful online sites--then apply them to your system. Good luck!
  4. Dec 1, 2008 #3
    I am coming to the conclusion that it is #2 and/or #3 as you describe. Currently, its a raw pcb sitting on my bench. The gain controls are pots on long leads, not twisted. Flourescent lights overhead. So I guess I'll have to test it shielded.

    But one curious thing is that there is no noise when S1 (the fuzz on/off switch) is open. So I don't think its the power supply.

    Thanks for the input!
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