Erica Synths EDU DIY Dual VCA Eurorack Module Kit - Manual (Page 5)

VOLUME CONTROL BASICS
First o ff , let’s answer the most essential question here:
what does a VCA do, exactly? Well, the simplest way to
visualize it is by looking at an ordinary volume knob. We
probably all know how these work: turn it to the right to
increase and to the left to decrease the volume. So far,
so simple. But while they have their place even in a
modular synth, they’ve usually got one major
shortcoming: they’re not automatable. You can only turn
them by hand. In the modular world, this is a big no-no
-– the whole point is being able to modulate any and all
parameters using LFOs, envelopes and sequencers.
So what we’re looking for is basically just a volume knob that can be controlled with
a voltage
. That’s what a VCA is: a voltage controlled amplifier. Where a low level voltage
corresponds to a quiet, while a high level voltage gives you a loud output signal. So if we
control our VCA with an envelope, for example, we can give an oscillation a volume curve
that mimics those of certain instruments. A slow attack sounds like a swelling violin. A
super short attack, quick decay and long release sounds like a plucked guitar.
Now in order to be able to design such a VCA, we need
to take a step back and talk about basic, boring volume
knobs again. How do those work? Well, at their most
basic, they’re really just potentiometers set up as
variable voltage dividers. Because as you might know:
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in the world of electronics, sound is just a swinging
voltage. And the bigger the swing, the louder the sound.
Since voltage dividers do just what their name implies,
they’ll divide (or scale) down any voltage level you throw
at them. So imagine we set our potentiometer to
precisely 12 o’clock. This means that the two
resistances inside the device are exactly equal, giving
us a perfect 50% voltage divider that slashes our
sound’s volume in half. Turning it to the left will then
gradually reduce that volume, until we get complete
silence.
Why’s that? Because we’re changing the relation between the two resistances that
make up our voltage divider . And that relation will determine the factor by which our
voltages are divided down. To understand this on a more in-depth level, let’s visualize it
using the water analogy.
Find out more about potentiometers in the components & concepts appendix (page 37). 3
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