I’ve started building this wonderful piezo amp using the schematics and instructions from this great site .
Unfortunately, I still use really bad cables (I thought using a USB cable with cut-off plugs would be a cheap way to get a still well shielded cable. It’s not :/.). Find below the recordings, denoised with a 50 Hz + harmonics notch filter.
Sand:
Sand2:
Table:
Water:
Keyboard: