Francis Deck Phantom Powered Piezo Preamp modification

[idranoels]

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: http://personalpages.tds.net/~fdeck/bass/quickand.pdf

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

 

[AnalogKid]

What is the gain of this preamp?

It is more of an impedance buffer than a preamp. The gain is slightly less than unity.

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);

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.

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?

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.

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)?

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.

ak

 

[idranoels]

Thank you very much