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, how to approximate a logarithmic volume curve, which sounds more uniform to the ear, and how to boost or cut the final output.
The core of the design is a cascade of both halves of a dual-gang 50k linear taper pot, with each half used as a voltage divider, directly taken from Better Volume Control by Rod Elliott of Elliott Sound Products and produces a nearly perfect log response over most of its range (from 30% to 100%), where the output volume increases increasingly rapidly to match the logarithmic amplitude response of the ear, so the perceived volume changes are smooth at all points on the dial. The lowest part of the range (30% to 0) drops very quickly to complete muting at 0% rotation.
However, adding an non-inverting gain stage in between both halves gives us the ability to handle hotter incoming signals without clipping, since they are first divided by the first half of the dual-gain pot, and get the ability to boost or cut without altering the nearly logarithmic behaviour. Here, we left the dual-gang pot at the center position, which we would like to act as the 0db gain point. Since the pot halves are isolated from eachother, we can simply add up their gain, which is 0.5x (-6dB) twice, or 0.25x (-12dB) total. So to make up that loss, we need a gain of 4x (+12dB). The 47k and 15k feedback resistors give us a non-inverting gain of 4.13x (12.3dB), which is close enough. Now, from the midpoint of the pot, we can either add up to 12dB of output gain (which can reach line-level without clipping if the input is around 400mVp!), or smoothly attenuate it, or even mute it.
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 varies little from 50k Ohms. The 1uF output capacitor both blocks the DC bias voltage from the op amp output and forms a HPF with the second half of the dual-gang volume control, with a cutoff frequency of about 3Hz. The 1k resistor before the output 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.
You can try out this circuit (including the bias supply and a pre-biased signal source) in the interactive CircuitJS1 simulator: output_boost_cut.cjs1. (You will have to fake a dual-gang pot by matching the two output pots.)