A 4 channel audio mixer that requires very few electronic components can be built using this electronic scheme based on SSM2024 chip. To obtain a small offset and a high rejection, the four inputs of the 4 channel audio mixer circuit must have an impedance of 200 ohms to the ground. These impedances are resistances obtained with R5-R8, which are part of the voltage divider at each input.
With values of the components from this scheme nominal input signal is 1 V (0 dBV). At this level, distortions are about 1%, and at lower levels do not exceed 0.3%.
Using electronic diagram below can be designed a simple balance indicator with control function. If it is connected to the amplifier circuit equipped with two potentiometers for balance, you will have, in fact, balance control, and at the level. For this circuit to work properly it is necessary that the two potentiometers used to be double.
IC1's circuit forms a differential amplifier and resistors R5 and R6 creates a virtual ground for LED output as needed to obtain a positive or negative voltage, although power is asymmetric.