Peamp circuit extra-deep bass change capacitor The input impedance is the value of potentiometer Rl. If your instrument has extra-deep bass, change capacitor Cl to 0.5 mF. What appears to be an extra part in the feedback loop is a brightening tone control. The basic feedback from the op amp's output (pin 6) to the inverting input (pin 2) consists of resistor R7, and the series connection of resistor R4 and capacitor C3, which produce a voltage gain of almost 5 (almost 14 dB). That should be more extra oomph than usually needed.