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I have found several projects on piezo preamps but I cant find anything that can run 2 piezo inputs to make it stereo and uses potentiometer to control volume gain etc.

What I found is "Collin’s Lab: DIY Contact Mic" from make magazine website that uses an MPF102.

a busy cat
1. Can I just make 2 of them and make them share 1 ground?
2. Which component do I add/change to have a volume gain with a potentiometer?
3. Is it possible to have a "balance" from left and right inputs using sliding pot?
4. Can I use a 3.7v rechargeable battery instead of a 9 volt? I was thinking of salvaging parts from an old powerbank.

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  1. Common GND , +9V and 10uF decoupling capacitor do well in stereo version

  2. It would be useful not to add the output impedance of the preamp, so replace the 1.5kOhm resistor with the volume pot. Use logarithmic pot. 2200Ohm should be ok. Connect 4.7uF cap to the slider and the low volume end connector of the pot to +9V

  3. Balance pot can be a 4700 ohm lin pot between the outputs, connect the slider to GND. The higher is the pot resistance, the less you lose the total volume, but the wider is the dead "no effect" area in the middle. Experiment. NOTE: It is good to change the 4,7uF capacitor to bigger value, say 47uF because this simple balance control increases the loading and causes attenuation at bass frequencies. This is negative loudness: when the balance control decreases the volume, low frequencies get more attenuation. This can be good - lower volume makes the sound less bassy and preserves some hearability in the total mix.

  4. The preamp should be totally redesigned for low supply voltage to be sure. Simulate or do a test circuit. I have done no calculations, but I'm afraid of increased distortion. Piezo mics in guitars or percussion instruments can give hefty peak voltages and low supply voltage possibly does not provide enough room for the signal voltages to swing.

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Sure, you can make two circuits with a shared power source.

To change your volume, I wouldn't recommend modifying the existing circuit, I suggest connecting an op-amp amplifier with volume control after the preamp.

To control the balance, you can then modify the volume of individual channels.

To power the circuit with a 3.7v battery, you can use a step-up converter together with a charging circuit.

Hope that helps!

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  • \$\begingroup\$ so to clarify.. shared power supply, both piezo would also share 1 common ground and would also share output as stereo? \$\endgroup\$
    – kit
    Jan 1 '18 at 11:53

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