It's two ins, two outs, so basically two separate circuits. Customer would prefer a single-knob solution. Precision dual gang pots are ruinously expensive.
If it was single-in, split to stereo, I could do crossover controls for the tone and a balance control to make up for intolerance in the volume control, but two circuits has me stumped as far as single gang goes.
Something OTA based?
What type of tone control? That might point toward a solution. Maybe something optical or transistor based with a single gang voltage control.
I managed to convince them to let me just use dual gang pots. Problem solved :P
I mean, digital is the real way to do this, but ain't nobody got time for that.
So, what is the circuit? You got me curious. Precision pot makes me think hi-fi or recording.
I happened to run across this about how the AC15 tone control is for both channels. Not sure if it helps for your case, since it's summed to mono..
http://www.voxshowroom.com/uk/amp/ac15_1959.html
"The "Tone" control was located in the power amp circuit between the phase inverter and output tubes. Here is how it worked.
The signal from the preamp is directed to the phase inverter. The output from the phase inverter is split into two signals. One of these signals is 180 degrees out of phase with the other. When "out of phase" signals are combined, they cancel each other out. The "Tone" control works by combining the high frequency signals from one side of the phase inverter with the other."
Yeah, but that only works because you have the same signal in both paths. Full stereo you two separate signals that may or may not be out of phase with each other in some situations. Like if there was an extra pedal that inverted phase in front of one side and not the other or if for some reason the signals were already out of phase.
Could always go with Chase Bliss's solution and pack 37 vactrols into the thing
I was thinking optical with 2 precision optocouplers with resistors in parallel. Have it driven by a single transistor.
No idea how well this would work, but hey. Obviously some values are guesstimates. Linearity might be an issue - maybe a C value pot instead.