Piezo discs are popular as contact mics and instrument transducers. The major drawback is that they are extremely high-impedance devices. When connected to standard line or microphone inputs, the impedance mismatch creates a high-pass filter which emphasizes high frequencies and gives them a characteristic harsh and tinny sound. This preamp provides an ultra-high-impedance input to match the piezo, and an output matched to standard mixers/amps to improve the overall sound.
Based on the Jensen Transformer AS098 application note, this circuit presents a 10meg impedance input JFET buffer followed by a single 12db gain stage, and is ideal for basic impedance matching. One PCB includes two preamps, powered by a 9v battery source.
This circuit may be paired with one of Jensen's direct-box transformers to provide both impedance matching and isolation – but even without the transformer, piezo performance is greatly improved using the impedance-matching preamp by itself.
BOM quantities are based on populating both preamps included on one board; adjust if only building one preamp.
Example build #1 : Dual preamp in 125B pedal inclosure. The channel-1 input jack is a TRS type, with the ring used to connect the battery ground to the PCB when a (TS) plug is inserted.
Example build #2 : Single preamp (1/2 of the PCB left unpopulated), with a Jensen transformer (bottom left) and a ground lift switch, in 125B pedal inclosure.
Other configurations are of course possible; I have a quad build for my piezo-amped toy performance rig...