The MIXER (MIX) is two independent circuits for mixing audio signals. Each has three inputs with pots for adjusting the input levels, and a fourth input marked "X1", indicating unity gain. This latter input can be patched to the output of another mixer allowing mixers with more inputs to be assembled from several MIX sections.
The R2 MIX mixer is for audio signals only, being an AC coupled inverting summer. CV mixing is done with the R1 PRC Dual Processor (DC coupled and non-inverting).
There appears to have been a lot of variation in Serge mixers over the years. The R2 PCB survived until at least 1975, although by this stage, it was no longer being assembled as per the original design. Instead, mixing resistors were moved to the pots on the panel to save wiring, and the resistor to earth from the non-inverting op-amp input was replaced with a link.
Even in 1975 it was apparent the writing was on the wall for this PCB because on the same panel as my example is a hand built mixer (same panel art) using a single LM1458 instead of two LM307. Also the circuit was improved by including the 330 ohm output resistor in the feedback loop of the op-amp, whereas on the R2 PCB it was not.
A possible modification is to replace the 470n capacitor with something larger, perhaps a 10uF bipolar/non polarized electro, thus allowing a better bass response.
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