◊ PCB Documentation

Piezo Preamp

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.

PCBs are $15.00 @ — ORDER HERE

Bill of Materials/Build Notes

BOM quantities are based on populating both preamps included on one board; adjust if only building one preamp.

  • Resistors
    • 100k x 4
    • 1k x 2
    • 3k1 x 2
    • 10k x 7
    • 10meg x 2
    • 2k trimmer x 2
  • Capacitors
    • 100 pf x 2
    • .01uf (10nf) x 2
    • 100uf 16V electrolytic x 2
    • 220uf 16V electrolytic x 2
  • Semiconductors
    • PN4393 (JFET) x 2
    • 2n4403 (PNP) x 2
  • Misc
    • OPTIONAL: JENSEN JT-DB-E Direct Box Transformer (1 per preamp)
    • SPDT switch (optional, for ground lift with transformer option)
    • Input/output jacks, enclosure

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...