The balance control circuit is an important stage of a stereo  amplifier. The function of a balance control circuit is to vary the voltage gain in both the channels by 6dB in opposite directions. This works in a stereo amplifier. The controlling variable resistor is inserted in the feedback circuit. The average gain of the circuit is about 24dB and the total distortion for minimum gain is below 0.2dB and in the maximum gain is about less than 0.3dB. The difference is maintained low because of the large amount of feedback applied in the circuit. The frequency response at -3dB point is from 20Hz to more than 20KHz. The input impedance of the circuit is about 140K Ohms and the output impedance is about 90 Ohms. The detail circuit schematic is mentioned in the attached LINK. The below mentioned curve details are shown about the distortion level.

Bill of material and Circuit Schematic

Electronic Circuits – Simplified