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I'm considering making a Modular Synth module which would pan and modulate a stereo signal. So an electronic circuit to pan the audio from right to left, even allow audio signals crossing over, Left to Right, and back. Given it's for a Modular synth module it'd be Voltage controlled.

I'm not even sure where to start. It might be easier to just go digital and have a micro-controller control things. Analog I'd imagine I need to create two copies of Left and Right signal inputs, so you'd have Left_A and Left_B, and similarly for the Right signal. Those four signals would be amplitude modulated and then summed together again to the Output.

So LeftOutput = (? * Left_A) Plus (? * Right_A) and RightOutput = (? * Left_B) Plus (? * Right_B)

I guess I need four voltage controlled amplifier/attenuator circuits and a two adders.

I'm not an electronics expert so is that possible, sane, the easiest solution?

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Concentrate on one of the stereo outputs first, say left. You want, at one extreme for it be be the left input signal whilst at the other extreme you want it to be the right input signal.

So, you need two voltage controlled amplifiers (VCAs) that can attenuate a signal to nearly nothing and those two VCAs need to operate oppositely to each other such that when you sum the two VCA outputs you’ll get left input at one extreme, pure mono in the middle and right at the other extreme.

Ok so far?

Then you need to repeat for the right output channel.

As regards a VCA that is effective, you could use an analogue gate being activated at high frequency (maybe 100 kHz or more) by a pulse width modulated square wave. The duty cycle modulates (turns on and off) the audio input and, because high switching speeds are used, the output amplitude is effectively controlled. This is an example of a triangle wave being switched on and off by a square wave (50% duty). The "gated" signal is then low pass filtered to remove "above audio artefacts" and the resulting amplitude is 50% of the original: -

enter image description here

Here's an example of a PWM circuit using Linear Technologies LTC6992: -

enter image description here

There are other types of VCA but to go into them is really another question.

So draw out (block diagram) what I’ve described and see if it still makes sense to you.

Pictures also covered in this and this answer.

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