How do I create a subtractive or dffferential two channel audio mixer?
A very simple solution would be to use a pair of 600 Ω line isolating transformers.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Subtractive transformer method.

Transformers would be 1:1 type, typically 600 Ω if your circuits can work at low-Z levels.

Alternatively an electronic solution is possible.







simulate this circuit

Figure 2. A is inverted twice and comes out the same way up. B is inverted once and is subtracted from A. Positive and negative power rails and decoupling capacitors have been omitted.

If your mixer is fancy enough you could do two sub mixes and use the phase invert if it has that feature or invert the balanced input.

L ---&gt; Stereo mix left
R ---&gt; Stereo mix right
L+ --+
     +---&gt; Aux mix left
R- --+
L- --+
     +---&gt; Aux mix right
R+ --+