The imP is an impedance matching circuit for piezo discs (original circuit from here and here), that buffer a piezo disc so it can be fed directly into an audio interface. They dramatically improve the sound of piezo discs, from unusable to decent. For two dollar microphones, they just shine with this circuit.
As you can see, it has a low parts count, it’s mounted to a faceplate or enclosure by the in/out jacks, it’s low noise, it’s wonderful! On the faceplate, just mount the two jacks, and the 100KA potentiometer, and maybe a power switch. You can see that really nothing else off board is necessary, as the power is switched on when a Piezo disc (wired to a tip/sleeve 1/4″ jack) is inserted to the input jack.
If you’re looking to mill the board, I have mill files with copper and holes in separate PNGs. If you’re etching, here’s the PNG of holes and traces combined. If you’re really curious, here are the Photoshop and Eagle files. Beware, I added a trace later in Photoshop, and an extra pair of vias for power to other boards, so the Eagle file is not exactly the same as my end copper. Just compare the two to see where those are.
I’ve made several of these, in two different configurations. While they both look more or less like guitar pedals, the first, smaller red boxes are geared towards use in a live sound/installation setting where external power (provided by battery or wall wart) is available. They are also controlled by side mounted controls, and stack nicely. Each is two channel. The second configuration is more like a traditional guitar pedal, the controls are mounted on top, it has an internal battery, and a stomp switch. I was thinking this would be a fun unit in a pedal board setup, both for guitarists looking for new sound sources (thinking of a piezo disc mounted to the headstock, or on a box they’re standing on, or a bell they can strike) to feed through their pedal setups or in a noise setup, where pedals are often use to process tape, electronics, microphones, etc. The circuits are the same in both, just a slightly different control/housing/power configuration. Here they are:
I’ll have sound samples up here once I get a chance to make some! Enjoy!