Based on the classic Baxendall tone control circuit, this provides a maximum cut and boost of around 6dB at 10K and 50Hz.
The first BC109C transistor (left hand side) is acting as a buffer. It gives the circuit a high input impedance of around 250k, and has a voltage gain of slightly less than unity. The first stage is biased so that the emitter voltage is approximately half the supply voltage; this allows maximum input voltage swings. As the Baxendall tone control circuit is passive, all audio frequencies are attenuated. Rotation of the potentiometers controls the audio response. A bode plot of frequency versus gain is shown below:
To overcome losses in the circuit the right hand BC109C provides a slight voltage gain of about 6dB. The output is designed to feed an amplifier with medium input impedance of 10k to 250k. The bass and treble controls are designed to cut and lift 50Hz and 10kHz. Both controls should be good quality linear type potentiometers.