had a nice page in oldwiki, here's a reboot. im placing it for now.
a lot of the info starts from collinshandmade_electronic_music.pdf, if there is a mention to chapter number below, it's from this ebook.
see: how to make a contact mike: using piezo disks to pick up tiny sounds, chapters 7-8
amplification and distortion: a simple circuit that goes from clean preamp to total distortion
A piezo element can be thought of as a sound-dependent voltage source in series with a large capacitance of about 15nF. Here the trouble starts. You need to put this into a load which is higher than the impedance of the series capacitor at the lowest frequency of interest. If that is 20Hz, since the impedance of the capacitor is 1/2pifreq*C then you want that to be > 530k. I’ve terminated mine in 3.9M because that’s what I had to hand.