Hi, I know this problem is quite old, but I finally found out how to solve this, and did not know where else to share it. The piezo signal is very sensitive to electromagnetic interferences. The solution is to shorten this wire as much as possible and as far as possible from the power wires (heater, stepper motors...). A shielded wire may help maybe. So as a conclusion, place the PCB amplifier as close as possible to the piezos and protect the link between the piezos and the amplifier from electromagnetic fields as much as possible.
