will tell you that a non-inverting opamp normally cannot be regulated
down to 0 dB gain. If zero output is needed then it is usual to employ
an inverting amplifier and a buffer amp in front of it, the buffer
acting as an impedance step-up device. The circuit shown here is a trick
to make a non-inverting amplifier go down all the way to zero output.
The secret is a linear-law stereo potentiometer connected such that when
the spindle is turned clockwise the resistance in P1a increases (gain
goes up), while the wiper of P1b moves towards the opamp output (more
signal). When the wiper is turned anti-clockwise, the resistance of P1a
drops, lowering the gain, while P1b also supplies a smaller signal to
the load. In this way, the output signal can be made to go down to zero.