something to be aware of.

[Supermagnum]

The problem with piezo crystals is that they are not well matched to typical audio inputs. By their nature they can generate a lot of signal, but they cannot drive a 50 kilohm typical line input. The pickup needs to work into a much higher impedance, typically 1 megohm or so.

So what to people do? They go and plug a piezoelectric disks output directly into the line input of their recorder, typical impedance 50k, or the plug-in-power mic input of their recorder, typical impedance about 7k, and they start to bitch and moan that this damn thing sounds tinny. Which is does !

The reason why these devices often sound tinny is because the piezo sensor presents its signal through a series capacitance which is small, typically 15nF or less. When wired to a normal 50 kilohm line input this forms a high-pass filter, which eliminates the bass.

This circuit board solves that, and amplifies the signal around 30 dB. It is useable for hydrophones too.
https://github.com/Supermagnum/piezo-balanced