Low-noise, high-fidelity preamplifier for piezo pickups, also known as contact microphones.  The design combines an OpAmp preamplifier (using TI’s TL072 or OPA2134), together with a JFET buffer that allows the microphone to be “recognized” by things like mobile phones and their headset input.  The design features TRRS output for connectivity with mobile phones, and a TRS connector for headphones (from the phone).  Note that the transistor Q4 in the buffer portion of the circuit requires ~2.8V from the phone to be powered.  Circuit runs on a single A23 12V battery.

Schematic and board designs in Eagle can be found this GitHub repo (look for versions 0-8 and 0-9b).

This preamp was created for Ticklers.

Piezo microphone amplifier
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