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.

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. -------------------------

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