The filter shown in the figure can have its cutoff frequency adjusted between 2 kHz and approximately 20 kHz. The central frequency of the range is determined by the capacitors, which may have their values changed. Note that a double potentiometer is used which must be connected so that the resistances of its sections vary in the same way. The operational amplifier used is the 741, but equivalents can be used. The source must be symmetrical from 6 + 6 V to 741 or other voltages depending on the operational amplifiers used. The input resistance is of the order of 100k ohms and the gain in the pass band is unitary.
