In audio amplifiers control of frequency response is usually referred to as “tone control” and is used to attenuate or accentuate the response at the low or high frequency end of the frequency band.
The control can be carried out by incorporating frequency dependent circuits in the coupling network between two stages. Increasing the effective capacitance in parallel with the load resistance of a transistor will reduce the high-frequency response, and by varying the effective capacitance in the coupling circuit the low-frequency response can be controlled. A typical arrangement is shown in Fig. 19.3. Potentiometer P1 controls the low-frequency response and P2 controls the high-frequency response.