Reducing NOISE in audio circuit

Discussion in 'General Electronics Chat' started by Rogare, Oct 30, 2012.

  1. Rogare

    Thread Starter Member

    Mar 9, 2012
    78
    0
    Hello,

    I've got a basic volume control circuit that works wonderfully—with the exception of noise. Specifically, when my hand gets close to the enclosure (plastic), the outputted audio really starts to buzz. (Note: this box goes through a 20 dB boost before reaching the earphones.)

    The schematic and a picture of the circuit itself are attached—as I said, super simple. If anyone has any ideas of how to make this more noise proof, that would be great!

    Thanks for reading.


    (P.S., Thanks for your previous reply MrChips: I've since updated the schematic and included an image of the circuit.)
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    The messy wires between the volume control and the circuit board are too long and pickup mains hum (rectified into a buzz).

    Why are you feeding fluctuating DC into the headphones? Headphones work best with AC.
     
  3. Rogare

    Thread Starter Member

    Mar 9, 2012
    78
    0
    OK, thanks for the tip re: shorter wires.

    The box (together with the 20 dB booster) is designed to amplify short, positive square waves.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    896
    Shielded audio cable between the volume control and the circuit board will eliminate mains hum pickup.

    Why does the headphones driver have a rectifier diode?
    Why does it have extremely high value (10M!) resistors?

    Why didn't you use an audio amplifier with a coupling capacitor?
     
  5. Rogare

    Thread Starter Member

    Mar 9, 2012
    78
    0
    Hmm, I've used LM386s and longer wires in other circuits without this issue. Though those could certainly contribute to the noise problem, could the main issue be something else?

    When I touch one the screws that holds the circuit board to the enclosure (from the outside), there's a big boost of noise. I tried grounding the screws for a moment and that had no effect. (Same with grounding the pot cases.)

    Could it be related to the plastic (not metal) case? Or, could it be related to the dual-gang potentiometer? It's my first circuit using one of these.

    Thanks again for reading.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Try the circuit without the diode then the pots will ground the input of the opamp much better than the 10M resistor.

    Why is the volume control a dual gang one and what does the 5k pot do?
     
  7. Austin Clark

    Member

    Dec 28, 2011
    409
    44
    Couldn't the issue be fixed via earth grounding of the pickup pre-amplifier?
     
  8. Rogare

    Thread Starter Member

    Mar 9, 2012
    78
    0
    Thanks for the suggestion. The diode was protection in case the input signal happened to go negative (the circuit doesn't react well to that), though I think I see how removing it could help with noise...

    The pots offer coarse and fine attenuation control.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Instead of using a dual pot and a fine pot, why not just use an ordinary audio-taper volume control? It has a logarithmic response so it adjusts levels equally when it is near maximum, near center and near zero. A linear pot has very fine adjustment near maximum and very coarse adjustment near zero.

    It is too bad that you opened too many threads about your single project because your thread about your preamp mentioned that your input pulses do go negative a little, then the diode should be replaced with a resistor, maybe 1k ohms.
    Then you are not amplifying the signal anymore?
     
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