Eurorack Synthesis: Exploring The Obvious & Less Obvious Uses of VCAs

If you’ve spent any time investigating the world of eurorack, reading through threads on the muffwiggler forum, watching videos from Richard Divine, Mylar Melodies, DivKid and the growing roster of eurorack YouTubers out there, chances are high that you have come across the sentiment that you can never have too many (or enough) VCAs. Depending on your experience with synthesizers, this may seem like either an odd or completely self evident statement. In any event, it’s worth taking a look at what VCAs are, what they do, and why they are so important to any eurorack system.

The Basics

Just to make sure we’re all on the same page from the start, a VCA is a voltage controlled amplifier. Basically this means that it uses a voltage signal to control the amplitude, or volume, of a second signal. The most common use it provides is to shape the volume of a synthesizer voice using an envelope. You strike a key, the key triggers a gate which launches an envelope, and this opens the VCA so we hear the note. The shape of the envelope dictates whether the note is percussive and abrupt, or if it swells in and eases out slowly. Without a VCA, the synth’s oscillators would buzz away ceaselessly at full blast unless you manually manhandled the volume knob.

In a fixed architecture synthesizer like a Roland SH-101 or a Moog Sub37, this functionality is built into the design of the synth so the user doesn’t have to think about this chain of cause and effect. Dedicated envelopes are provided to shape the volume contour of the VCA so the user can either play short percussive stabs or slow swelling pads. In a eurorack system this is not the case and this pipeline has to be patched together. However, this necessity creates some interesting options and alternatives. Furthermore, the various VCA designs available can often provide unique functionality and character not possible with fixed architecture synths. For example, the Intellijel Quad VCA has a cascading architecture, meaning that each CV input feeds into the following input, and similarly each output feeds into the next output. This means that you can use both an LFO and an envelope, or even up to four CV inputs to control the volume of a signal, or you can use one envelope to control and mix together four oscillators.

Intellijel’s Quad VCA maximizes HP with its clever architecture.

It’s important to note that while a VCA is using one signal to control the volume of another signal, this does not mean that the signal being controlled has to be an audio signal, or that the controlling signal can only be a control voltage signal. Using VCAs to control the level of an envelope or LFO can be a very useful technique, and using an audio rate signal to control the volume of another signal can introduce some interesting results.

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