Amplifying sound from a microphone
Piezos have a high output impedance (capacitive, really), so you need an even higher input impedance on your mic pre or you'll get a "tinny" sound from the low end being rolled off.  

A piezo with 1000 pF capacitance would have a reactance of 3 M&Omega; at 50 Hz, for instance, so if you wanted to pick up 50 Hz, you'd need an amp with several M&Omega; input impedance.  

Signal Conditioning Piezoelectric Sensors from TI shows a circuit with 10 M&Omega; input impedance, for instance.   Interfacing Piezo Film to
Electronics suggests 22 M&Omega; and shows the most basic circuit you could use: one of these big resistors in parallel with a unity-gain buffer op-amp.  If that level is too low, add two resistors and make it into a non-inverting voltage amplifier (figure 9 b):




  The major advantage of a charge
  amplifier, therefore, can be found
  when a long cable is used between a
  piezo film sensor and electronics. In
  addition, it also minimizes charge
  leakage through the stray capacitance
  around the sensor. Otherwise, simple
  voltage amplifiers are sufficient for
  most applications.


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