a simple preamp for piezo disks based on the tl071 opamp. I only had a tl072, so i used this one.
https://www.eleccircuit.com/acoustic-guitar-pickup-circuit-using-tl071/ http://tagboardeffects.blogspot.com/2014/04/acoustic-piezo-preamp.html
http://www.richardmudhar.com/blog/using-piezo-contact-mics-right/ https://www.cbgitty.com/news/electronics-pickup-harness-disk-piezo-capacitor/ https://www.harmonycentral.com/forums/topic/1531094-best-diy-piezo-preampbuffer-school-me-on-impedance/
Next step is to build one with a fet and compare the quality.