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Francis Deck Phantom Powered Piezo Preamp modification

Discussion in 'General Electronics Discussion' started by idranoels, Apr 8, 2016.

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  1. idranoels


    Apr 8, 2016
    Hello everybody
    I'd like to modify Francis Deck circuit Phantom Power Piezo Preamp in order to control the output volume of the preamp itself rather than be fixed-gain and have one DIY preamp for my banjo.
    The original circuit is the last one in this PDF labelled PPPP:

    My questions are:
    1. What is the gain of this preamp?
    2. Is it possible to change some resistor with a logarithmic pot in order to control the gain from -inf (0 volume) to max gain (top of the volume); I guess I could use a dual-gang 15k pot to "replace" R3/R4 connecting the middle contact of each resistor to XLR contact (respectively XLR+, XLR-), or is there a better way?
    3. should I tune some condenser value to adjust the bandwidth of a banjo instead of a bass (as far as I understood Deck designed it to be the preamp of his bass)?

    Thank you in advance

    Sergio PhantomPoweredPiezoPreamp.png
  2. AnalogKid


    Jun 10, 2015
    It is more of an impedance buffer than a preamp. The gain is slightly less than unity.
    If the two outputs are perfectly balanced, a single pot between them (after the coupling capacitors), wired as a rheostat, could reduce the volume to zero.
    That would inject more and more power supply noise into the audio as you turned down the pots. A better way is to place the pots at the points marked xlr+ and xlr- to GND, and take the signal from the wipers.
    The circuit has an asymmetrical output impedance. R2 and C1 form a 16 Hz highpass filter. I would leave it alone and add a real eq stage later if needed.

  3. idranoels


    Apr 8, 2016
    Thank you very much
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