From the Synthtech website:
The MOTM-190 is a dual VCA in a compact 1U format. VCA #1 can also be used as a Ring Modulator.
Voltage Controlled Amplifier #1 – Ring Mod
The top knob (BLEND) and switch (V/R) control VCA #1. If the switch is in the ‘V’ position, the circuit is a linear VCA. The BLEND control acts as an Initial Gain control, normalized to unity. If BLEND is all the way to the left, the IN position, the gain is unity. The output level equals the input level and there is no modulation. As the BLEND knob is turned clockwise, more and more of the CV modulates the signal. Fully clockwise in the RM position, the CV changes the gain from 0 to 1. This is ‘standard’ VCA operation. If the switch is in the ‘R’ position, the circuit is a Ring Modulator. As BLEND is turned to the ‘RM’ position, the depth of the Ring Mod effect increases.
Voltage Controlled Amplifier #2
The second VCA is switchable between Linear and Exponential responses. The GAIN control sets the initial gain from 0 to 1. The CV MOD control ajusts the depth of AM for the VCA.