This is the purist approach solution to the tinny contact mic sound problem if you want the lowest electrical noise, or the lowest distortion at higher levels. Piezo mics aren't usually considered hi-fi, so distortion isn't usually a problem of the amplifier - at high levels the mic will not be particularly linear. However, they are remarkably low noise. Although they are high impedance, they don't need an outrageously high load impedance - not like an electret mic, where you need gigaohms to avoid losing bass. The calculation in the introduction showed a load of 330k was enough, so 1meg is plenty for a contact mic. The first thing that sprang to mind was a TL071 - the overall schematic is shown below.
Diodes D1,D2 stop you destroying the opamp with large signals from the piezo device if you drop it. You can leave C1 and R4 out if you need less gain, though there is much to be said for the cheap version if you have lots of signal.
The results were good. a 7mV rms 1kHz signal was fed into a 10k:12ohm attenuator, and then coupled via a 15nF capacitor to approximate the piezo source, generating 8uV. A satisfying drop in noise was obtained. However, substituting the TL071 for the usual audio workhorse opamp, the bipolar NE5534 brought the lowest noise of all.
Noise comparison of 2n3819 single fet with opamp version using a NE5534 (left and right channels respectively)
freshwater hydrophone recording using this with an air cell piezo disk hydrophone. At the start of the recording you can faintly hear woodpigeons calling above the surface propagating through the water, and aircraft noise, before the chomping sound.
Matching levels of the 1kHz test tone to correct for the slight difference in gain shows the NE5534 offers a noise floor 10-20dB lower than the 2N3819, in return for extra complexity. At the back of my mind was the wonder whether a piezo mic would not have more self-noise anyway, so I removed the test tone and put in the mic. This is a reasonably quiet location. The hum is because this circuit is on a breadboard with the mic laid on the shelf, and some of the hum is mechanical, from the transformer of the power supply powering the unit, which is on the same shelf unit. About 3/4 of the way in some piece of machinery is started by my neighbours - I did not hear this at the time but it was conducted through the house into the shelf with the mic. You wouldn't normally have to put up with this level of hum - building the circuit into a metal box and using screened cable to the piezo device would fix that. However, the piezo mic has not raised the noise level significantly.
NE5534 at same gain as test circuit but with the contact mic replacing the 1k tone.
This goes to show that the piezo contact mic, for all its uneven frequency response faults, is capable of an excellent noise performance when used correctly. A more practical stereo version of this was constructed in a box, measurements and test results are available.