Used to mix several signals. One of them is an LFO and will be mixed as a CV. All signals in will be 0-5V and will produce a current of 0-700uA for control of 4 OTAs within 2 lm13700 chips. Those OTAs will operate as current controlled resistors to create an active low pass filter (not shown in this schematic). After the 1st stage of the inverting summer, the diode and voltage reference act as a limiter that keeps the voltage from exceeding -5V when summed. This all gets invered through the 2nd stage to produce the output mix between 0-5V. This ensures that the circuit cannot overload the LM13700 cintrol current pin regardless of the number of the number of input signals. Circuit uses a +-9V power supply
