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I experimented with different types of piezoelectric disks as double bass pickups and found two that picked up different parts of the frequency spectrum decently. The specification of the two piezos are:

  • "High frequency" piezo
    • 1000 Ω
    • 2.9 ± 0.5 kHz
    • 30000 pF ± 30%
  • "Low frequency" piezo
    • 800 Ω
    • 1 ± 0.5 kHz
    • 70000 pF ± 30%

I combined them in parallel, which seemed to work. Now I'd like to be able to do more fine-tuning of the sound by balancing them out. I guess this could be done like so:

schematic

simulate this circuit – Schematic created using CircuitLab

If this schematic is sensible, my question is: How can I determine what resistance I should pick for the potentiometers? The combined pickups will be hooked to 1 MΩ impedance inputs.

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  • \$\begingroup\$ A little off-topic for electronics.stackexchange: If anybody knows a good strain relief technique for the solder joints of piezos without housing (I clamp them right onto the top of the instrument for direct contact), I'd be interested. Hot glue works mechanically but totally destroys the sound. \$\endgroup\$ – Thomas W. Jun 30 '14 at 5:17
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As the piezo's have very high output impedance, I would use a amplifier/buffer-stage as close to the pick up as possible. That way you reduce the pickup of (electrostatic) noise and it is much easier to mix the signals without one interfering the other. Once you have amplified the piezo signals to somewhere near regular line levels (1Vtt), you can use potmeters in the order of 10k for a passive resistive mixer and use a regular line input for further amplification.

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  • \$\begingroup\$ I'm currently less worried about noise and signal quality. The piezo elements are just very low cost compared to a pickup you can buy and I'm not targeting a recording studio sound quality, just something that works for an absolute amateur. So, I want to keep it as simple as possible, especially without any amplification. The pickup is meant to work with "guitar level" (1 MΩ impedance), not "line level" inputs - most of the commercial piezo pickups have no built-in amplification. \$\endgroup\$ – Thomas W. Jun 30 '14 at 6:15

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