Charles Eric LaForest, PhD., GateForge Consulting, Ltd.
This circuit demonstrates how to remove the bias voltage from a signal before output to external audio equipment, how to keep the output stable and protected from high current, and how to approximate a logarithmic volume curve, which sounds more uniform to the ear.
The op amp output buffer could be used as-shown to isolate some previous filtering or other impedance-sensitive circuit, or the actual final op amp of a circuit can take its place.
Working backwards from the output, the 1k resistor limits any outgoing or incoming current and isolates the op amp output from any load (cable) capacitance which could degrade its stability when the output volume is at maximum.
The arrangement of the 100k potentiometer and the 10k resistor is directly taken from Better Volume Control by Rod Elliott of Elliott Sound Products and produces a quasi-log response, where the output volume increases first slowly then increasingly rapidly to match the logarithmic amplitude response of the ear, so the perceived volume changes are smooth at all points on the dial. It is assumed the external output load has a high impedance (hundreds of kOhm or more). Thus, the total load on the op amp output from the volume control varies from 9.1k at maximum volume to 100k at minimum.
The 1uF capacitor both blocks the DC bias voltage from the output and forms a HPF with the volume control, with a cutoff frequency of 17Hz at maximum volume (so -1dB at 34 Hz) down to 1.6Hz at minimum volume.
You can try out this circuit (including the bias supply and a pre-biased signal source) in the interactive CircuitJS1 simulator: output_buffer.cjs1