I have some theoretical electronics knowledge from uni, but when it comes to applying it in practice, things can get a little bit complicated.

I've built simple stomp box, basically a piezo transducer glued underneath a top of a box. When you stomp on it, it is supposed to capture a sound. It was way too quiet when plugged in to my guitar am. As it is supposed to imitate kick drum, treble and some mid frequencies should be filtered out. I decided to assemble a simple active low pass filter to work as a preamplifier - amplify the signal a little bit and filter out high frequencies.

Low pass active

(source: https://www.electronics-tutorials.ws/filter/filter_5.html)

How do I build preamp like this around an LM386 without frying my guitar amp? I know that the LM386 has something more "on board" than a standard op-amp, so it might be a little bit harder than treating 2nd pin as op-amp minus, 3rd as plus and so on, leaving 1, 7 and 8 (gain and bypass) unconnected. Especially that somehow its default amplification seems to be set to 20. I've seen people building full-time amps around this chip, but I've been wondering whether pre-amps are also doable.

I want to use an LM386 specifically because I have handful of them lying around my house and I don't feel like waiting for a different op-amp.

  • 3
    \$\begingroup\$ First up, have a good read of the datasheet to understand how it works and look at suggested circuits. If it gives too much amplification, just use a pair of resistors to reduce the output. And probably add a capacitor to block the DC component of the output from reaching your amp. \$\endgroup\$
    – Finbarr
    Jul 5, 2021 at 22:38
  • 3
    \$\begingroup\$ I'd strongly recommend, even for a distortion effect pedal, to not use an LM386, but anything that is not "the worst audio amplifier still sold commercially". \$\endgroup\$ Jul 5, 2021 at 22:38
  • 1
    \$\begingroup\$ Why would you want to use a LM386 for this? That's not to say it can't be done, but it's an audio amplifier chip, it has moderate input impedance and can drive a speaker directly. A real op-amp would have extremely high input impedance that is needed for piezos and they can drive an amplifier input easily. \$\endgroup\$
    – Justme
    Jul 5, 2021 at 22:50
  • \$\begingroup\$ You didn’t specify any values. Why? If it’s a single supply, you have to Bias it midscale. \$\endgroup\$ Jul 5, 2021 at 23:06
  • 2
    \$\begingroup\$ Jakub, as others have suggested already, the LM386 isn't an opamp. It's got internal NFB that sets the voltage gain and an output stage that is designed to directly drive low impedance speaker loads. It's input is designed more for the standard, sub-standard audio found on those red and white connectors coming out of a TV set or the back of an amplifier/tuner deck (olden days, style.) It's not going to work well with a piezo transducer as its input. You'll need a pre-amplifier. And that's assuming you still have a reason for the LM386. Instead, you want a pre-amp for your existing amp. \$\endgroup\$
    – jonk
    Jul 5, 2021 at 23:28


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