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I am tracing out the preamplifier for a piezo acoustic guitar pickup but what is puzzling me is that the input goes through one capacitor to the non-inverting input of an op-amp AND a capacitor which goes directly to ground. The circuit is surface mount so it is not possible to determine the values of the non-Aluminium Electrolytic caps. Can anybody explain to me what the capacitor to ground is doing please?

The attached image shows the circuit fragment. The buffered op amp output then goes through a tone control and another buffered op amp stage before going through a volume control on the output. I have not shown the rest of the circuit, which I think I can understand OK.

Probably there to help filter out RF interference. Likely something in the pF range.

What is the thing you're tracing? Would be helpful if we knew what exactly it was. Can you post photos of it?

Assuming the input wire is 0.1 ohms, you'll have an approximate150 kHz low pass filter. With 1 ohm input resistance the frequency drops tenfold, well into the audible range.