The mh31 vc modulator is a ring modulator of a different cast. Contrary to other common circuits, the mixing of frequencies is realized by using operational amplifiers. The result differs audibly from other ring modulators and one must not expect the classical functions of a standard ring mod. Here we come into noisy and dirty terrain. Moreover, the module is DC-coupled, so it is usable for control voltages too.
The module offers one VCA for each signal input, including an initial amplification control and a control voltage input with attenuator. If you drive the cv input with another audio signal you can even dial in a third signal.
The incoming carrier signal switches the sum of both incoming signals (carrier and modulator). The result gets mixed with a kind of rectified carrier signal.
Heres a video we did overviewing the module: