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 MΩ at 50 Hz, for instance, so if you wanted to pick up 50 Hz, you'd need an amp with several MΩ input impedance.
Signal Conditioning Piezoelectric Sensors from TI shows a circuit with 10 MΩ input impedance, for instance. Interfacing Piezo Film to
Electronics suggests 22 MΩ 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