Quad VCA

The Quad VCA has 4 independent VCAs, made from two MFOS dual VCA PCBs. I have two of these modules currently, for a total of 8 VCAs.

VCAs are the secret weapon of modular synthesis. On the surface they appear to be boring, utilitarian modules. Really they are the heart of the analog synthesizer. Inside VCOs, VCFs, and all your other VC_ devices, there is probably at least one chip or circuit that either is a VCA or could be considered a VCA.

In the basic VCO->VCF->VCA->output patch, the VCA is serving as a "gate" for the audio, and an ADSR envelope can shape how that "gate" opens and shuts to create volume effects. VCAs are not limited to audio, however. You can "gate" CV as well. Perhaps the most well known patch of this type is the delayed vibrato patch. For normal vibrato, a LFO output, usually triangle, is attenuated and patched into the linear CV input of a VCO. Adjusting the attenuator manually controls the amount of vibrato, and the LFO rate adjusts the vibrato's frequency. If you replace the attenuator with a VCA, things get more interesting. You can still manually control the vibrato, just like with the attenuator, but now you can also automatically control the vibrato with CV. If you put an envelope generator on it (triggered by the keyboard), with a long attack, you can create the delayed vibrato effect. Adjust the attack time to the right amount of delay.

Delayed vibrato is just one of an unlimited number of patches you could make with a VCA controlling a CV. You can also create ring modulator type sounds by inputing audio, then adding audio to the CV input. My PAiA MIDI2CV8 will output MIDI velocity data as CV. I interrupted the ADSR envelope CV signal going to the synth voice's normal VCA, and passed it through another VCA. Then I controlled that second VCA with the MIDI velocity CV. VCAs are very flexible for making creative patches.