Keyed volume headphone amplifier

The volume control principle of one of the signals is now explained. The other one
is exactly the same.
    The circuit is shown in Figure 7-31. The audio signal is coupled through Cl to
the drain of Vl. When the volume increase key sl is pressed, the current is limited by Rl. The current
charges "cl", causing the gate potential of vI to continue to rise. The resistance between the drain and source of V1 decreases, causing the audio voltage divider at both ends of R3 to increase and the volume to increase. On the contrary, when the volume down key S2 is pressed, ci is discharged, the gate potential of Vl decreases, the resistance between the drain and source of VI increases, and the volume decreases. The working voltage of this circuit is DC 6~12V.