This circuit is selective for the tuning adjustment between two closely spaced tones audio. The frequency is dependent on the selective value capacitors and resistors in the feedback circuit between the collector and base of Q1. With the values shown, the frequency can be `tuned` to a hundred cycles or so-around 650 Hz Ri and R2 should be grouped. R3 potentiometer transmitter determines the sharpness of the response curve.
All transistor having a beta greater than 50 can be used. Select a value for R 4 so that the circuit will not oscillate when R3 is fixed for a minimum bandwidth (higher tuning).