Piezo Element Amplifier for Guitar and Violin - Electronics-Lab.com
Piezo Preamps
Building (or Buying) Contact Mics – Zach Poff
operational amplifier - What is a good circuit for recording a piezo contact microphone or an electric guitar pickup? - Electrical Engineering Stack Exchange
Building (or Buying) Contact Mics – Zach Poff
Building your own Piezo-Preamp – The Center for Haptic Audio Interaction Research