VCA

In easy words

A VCA or "voltage controlled Amplifier" is a module which does exactly what it name says: you can amplify signals via voltage control. You can use a VCA to control the Level of every signal in your Modular system via another signal. In radio electronics this is also known als "Amplitude Modulation" as it modulates the amplitude of the signal: AM.

Hands on

Here is a typical (Quad-)VCA:

It has four inputs and four outputs (below) and a CV-Input for each of the four VCA circuits. With the level knob the base level of each VCA can be set (this way it acts very much like the typical mixer). The CV has a little knob to attenuate the incoming envelope and a LED to display it. Additionally the response of the CV input can be changed anywhere inbetween linear and exponential with an additiona knob. Usually linear response is more comfortable when editing CV signals, where exponential response is more useful when editing audio signals.

DIY

There are different ways of implementing a VCA. Eliott Sound Products has a good guide on the various techniques to achieve a VCA called VCA Techniques Investigated.

A small overview over the pros and cons of the different technologies:

Technology	Distortion	CV Feed-Thru	Speed (Attack/ Release)	Example Parts
'True' VCA	low	low¹	fast	THAT2181, SSM2164, ...
Analogue Multipliers	low/medium¹	low¹	fast	AD633, MC1594, AD834, ...
LTP Transistors	medium/high	low¹	fast
OTA	high	medium/ low	fast	LM13700, NE5517,
LED/LDR	low	nil	slow/medium
JFET	high	medium/nil²	fast³
Diodes	high	high¹	fast²
PWM	low	nil	fast

¹ usually adjustable using preset pots
² depends on speed and circuit topology
³ speed depends on implementation and topology