This is a simple circuit to control the high and low tones in an audio amplifier, it is simple an easy. There is always a little place left where this tiny circuit can be added. It features only, 2 resistors, 2 capacitors and a single (linear) variable resistor (linear!).
The operation is just as simple as the circuit. On the left is low-pass network with the cut-off frequency of 1/(2*pi*R1*C1). On the right-side is a similar circuit but this time as a high-pass network with the the cut-off frequency of 1/(2*pi*R2*C2). If both RC sections are equal and the variable resistor is in the middle it will receive equal parts high and low tones and the pass-curve will be straight, eq. the signal is attenuated but all frequencies doe have the same strength. If the variable resistor is on the left side the low tones will be stronger, if it is on the right side the high tones will be stronger.
That's thus why it is important to have a linear variable resistor, otherwise the neutral will not be exactly in the middle.
With the current values the center frquency is circa 1 KHz as can be seen in the diagram below.