Charles Eric LaForest, PhD., GateForge Consulting, Ltd.
This circuit is meant as the input buffer for a bass guitar effect pedal. You can reduce the capacitor values to increase the filter cutoff frequencies into the conventional guitar range.
The input is first filtered by a 10pF capacitor shunt to ground to reduce RF interference, which would otherwise be heard as clicks and pops as it gets rectified by any non-linear circuit component. Place this capacitor as close to the input jack as possible.
A 10nF capacitor and 1MOhm resistor then sets the input impedance, biases the input up to Vbias, and creates a first-order HPF at 16 Hz (-1dB at 32 Hz) to reduce infrasonic noise which wastes usable signal range and cannot be reproduced by speakers.
The 10k series resistor has negligible effect on the HPF and serves to limit current into the op amp should a high voltage appear at the input.
The op amp is assumed to have a JFET input which draws negligible current (e.g.: TL072), and provides buffering, gain, and more filtering. The 10k feedback resistor and 10k potentiometer plus the 1k resistor implements a variable gain of 1.9 up to 10 (+5.6 to +20 dB). At maximum gain, we can accept input of up to 160mVp without clipping the output.
The 5.1nF capacitor in parallel with the 10k feedback resistor implements a first-order LPF at 3121 Hz to reduce string and fret noises which lie above the tonal range of bass guitars. If the gain potentiometer was in the feedback path, then the LPF cutoff frequency would decrease as the gain increased.
You can try out this circuit (including the bias supply and signal source) in the interactive CircuitJS1 simulator: input_buffer.cjs1