I'm designing a headphone amp, and one thing I need is volume control. I'm shooting for excellent fidelity to the digital signal, so left-right channel matching is keeping me from using a two-section potentiometer. I'm also hoping to avoid needing to rely on resistance matching on silicon, so those audio amp ICs with gain controllable by one pot are out. What I'm hoping to do is come up with an analog circuit that will match the gain of two channels based on the position of one potentiometer. I found this article on how to accomplish what I'm after, but the schematic has 404ed and I'm having a hard time understanding what's going on from the text only. My thought process is this:
The gains of the first stage don't change at the same rate with potentiometer turns, as you can see in this graph.
So I'm stumped. I don't see any way to do some analog work on them and wind up with their gains in lockstep. But it seems like it must be possible, as Woodward wrote an article on how to do it. Any thoughts?
- Need to use the wiper as common ground to get two linked resistors
- Common ground requires that each of the resistors be used as the resistor from inverting input to ground of a non-inverting amplifier, as no other configuration makes sense
- Turning the pot increases the gain of one non-inverting amp (A) and decreases the gain of the other (B). Use a differential amplifier to do something like subtract the signal from A from the maximum gain of the stage. For instance, maximum gain is 2, minimum is 1, with the potentiometer at halfway it's 1.5. Turning it a quarter turn gives 1.75 and 1.25, subtract the input signal from the 1.25 and subtract that from twice the input signal.
The gains of the first stage don't change at the same rate with potentiometer turns, as you can see in this graph.
So I'm stumped. I don't see any way to do some analog work on them and wind up with their gains in lockstep. But it seems like it must be possible, as Woodward wrote an article on how to do it. Any thoughts?