Voltage controlled amplifier
VCA is an abbreviation of Voltage Controlled Amplifier. Many VCAs do not actually provide any amplification, but actually 'turn down' a signal until control voltage is applied, i.e. maximum CV into a VCA control input will set the amplifier to fully open - no attenuation. Effectively most VCAs are actually Voltage Controlled Attenuators.
Since the traditional use of a VCA is to turn down a signal until control voltage is applied, max CV into a VCA will set the amplifier to unity gain or 0dBv (no gain). 0v CV into the VCA will cause it to fully attenuate i.e -80dBv. Attenuators do the same thing with no voltage control - they rely on a manual knob turn.
VCAs dont just serve to control volume of an audio signal - to change anything on a module without turning a knob in real time, you need it to have a cv input and to apply a cv signal to it. But what if you want to control how much cv goes in and the module has no knob to adjust this? you need an attenuator or a VCA!
Attenuators are usually passive and hence can reduce the amount of signal out of the cv source, effectively acting as a simple fine control knob for the modules that dont have them built in, but of course these lack cv control so have no dynamic capability. since VCAs can be cv'ed. i.e. they can respond to cv's themselves, a VCA can be used as a cv'able way of controlling modulation sources, hence be a dynamic attenuator or control knob by applying an LFO or Envelope to make the amount of modulation vary. As a result, expensive modules with more cv options and more knobs, though basically having built in VCAs, may not necessarily be better.
VCAs have differing sounds depending on components, circuit design etc. - and more importantly, depending how hard they're pushed or how hot a signal they're fed. therefore sometimes you can get a totally different sound by choosing the right VCA to modulate your audio signal. likewise, different VCAs have different 'response slopes', sometimes adjustable with a switch, knob or different input socket and so changing their effect on other cv's as well as audio signal character.
This is why people will always be telling you: You can never have too many VCAs!
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