# What is a good circuit for recording a piezo contact microphone or an electric guitar pickup?

I'm building a guitar tuner that can be used with a (piezo) microphone clip for acoustic, and from the pickup for electric. In my experiments they have comparable signal levels.

However, the piezo has a very small capacitance and large resistance. So the input circuit needs to have a high input impedance.

So far I've been using a good old non-inverting opamp with a 100k biasing resistor to ground. This seems to work okay, but then I found this collection of opamp circuits that includes a "Amplifier for Piezoelectric Transducer" that I don't really understand. What is going on here? How do I go about calculating the gain and impedance of this circuit? (and what is the 30pF cap doing?)

I've also found this other question about This guitar piezo opamp buffer lacks any sort of bass, why? which has the common problem of having too low input impedance. Their suggestion seems to be to just increase the biasing resistor of the classic circuit, but in my simulation this only goes so far before the input current of the opamp starts to create a noticeable offset.

• Specifically, what op amp are you trying to use. Yes, it makes a difference. – Scott Seidman Jul 27 '17 at 16:29
• Fair point. I've used LM358 and MCP6002 pretty much interchangeably and in LTspice I picked something at random because those were not available. My mouse seems to have landed on the LT1413. – Pepijn Jul 27 '17 at 16:45
• You can immediately see its gain is 1. Then C1 couples its output back to the biassing resistor (R1+R2) to bootstrap the input impedance to a very high value. – Brian Drummond Jul 27 '17 at 17:44
• You need an op amp with very high input impedance to deal well with low freqs with a piezoelement. the MPC6002 has this -- but the other amps you mention don't seem to spec it. the 358 has bipolar transistor inputs, so I suspect Rin is fairly low. – Scott Seidman Jul 27 '17 at 19:09