VCA/4QMx2

Dual VCA/Four Quadrant Multiplier

Dual LM13700 based VCA switchable between 2 and 4 quadrant multiplier modes with optional clipping/distortion and difference and inverted sum outputs.
I started from the Four-Quadrant Multiplier example circuit in the LM13700 datasheet and only added what I thought was needed to make it work and be fun to use in eurorack.
The two VCA channels operate independently, although the right channel's X and Y inputs are normalled to the left channel's. This makes it easy to set up the module for use as a stereo VCA, voltage-controlled panner, crossfader, or stereo ring modulator (if such a thing is desirable)
X is the carrier input. It is multiplied by the voltage/signal applied to Y.
The Y input features a sign-changing "attenuverter" level control and a BIAS control that adds DC offset to the Y input after the attenuverter.
The toggle switch determines the mode for each VCA:

Toggle switch down is VCA mode. The amplitude of input X is modulated by input Y.
Positive Y+BIAS increases amplitude of X, negative Y+BIAS decreases amplitude of X.
If you're aiming for a standard subtractive synthesis patch with an audio source into X modulated by an envelope generator into Y then start start with BIAS ccw and -Y+ at ~3oclock.
For amplitude modulation with a bipolar LFO start with BIAS around 12oclock.

Toggle switch up is 4 quadrant mode. X is still amplitude modulated by Y but is inverted when the sum of Y and BIAS is -ve.
I have seen this operation called a 'voltage-controlled polariser' in other modules and is also the mechanism behind ring modulation (although this design does not use the titular ring of diodes)
Setting the BIAS pot to approx. 12oclock silences X, turning cw will increase amplitude and ccw increases amplitude but inverted.
For ring modulation patch 2x audio-frequency signals to X and Y, carefully adjust BIAS and -Y+ until you cannot hear X at the output. Then slowly turn -Y+ cw or ccw to taste.

There are shunts/jumpers on the rear of the module that enable zener diode clamping/clipping on each channel's output for when the output signal exceeds approx ±5V.
VCA mode, especially, is capable of more than 1X gain so fitting these jumpers means each channel can also be set up as a VC distortion/overdrive.

DIFF outputs the difference between the two outputs. Left channel minus right channel.
SUM is the inverted sum of the two outputs. The main reason for it being inverted was that it saved me an opamp that I could use for the DIFF output but it is also nice when using this module for processing/combining CV to have a mix output that is doing things in the opposite direction to the main outs. If using as an audio mixer keep in mind this mix out will be out of phase with the individual outputs.
These two outputs do not have any zener clip/limiting so can get quite hot depending on what the main outputs are up to.

Setup: Patch audio to left Y input, nothing in X
Turn Y pots to 3oclock, adjust trimmers R2, R21 (accessible from front panel) until no signal can be heard at either output.

I have to mention this is a very simple circuit designed with low parts-count in mind.
This is neither a high-fidelity audio amplifier nor lab-grade analogue multiplier. If anything I went out of my way to give it 'character' (read: inaccuracy)
Ideally the 'zero' point on the BIAS and -Y+ pots would be at exactly 12oclock but in practice you'll have to find the sweet spot using your ears or eyes+oscilloscope. In the modules I've assembled so far this is just right of centre.
I've found it pretty intuitive and fun to use so far so hope you do, too.

https://isaacbeers.square.site/product/dual-vca-four-quadrant-multiplier/14