Modulators

VCA

The voltage-controlled amplifier - the gate between sound and silence in every synth voice.

What is a VCA?

A voltage-controlled amplifier - VCA for short - is the gate between sound and silence. It is the part of a synth that decides, moment by moment, *how much of a signal makes it to the output*. A VCA takes an audio (or CV) input, multiplies it by a control voltage, and sends the result onward. At 0V the output is silent. At full CV the input passes at full strength. In between, the audio is scaled smoothly, sample by sample.

Without a VCA you can hear an oscillator, but you cannot make it start or stop cleanly - turning a knob to fade a sound in and out is fine for a soundcheck and useless for music. The VCA is what turns a continuously-running drone into a *note*: an envelope opens it on a key press, closes it on key release, and what you hear is a played-and-released sound rather than a permanent hum. Every analog and digital synth in the world has a VCA somewhere in its signal path - even if the panel does not say so.

VCAs come in two flavours: linear and exponential. Linear means the output is directly proportional to the CV - useful for CV processing, ring modulation, and any time you want the math to behave predictably. Exponential matches how the human ear perceives loudness: doubling the CV roughly *doubles the loudness*, not the amplitude. Audio sounds more natural through an exponential VCA; control voltages mix more cleanly through a linear one. A good VCA gives you both.

Our VCA

Ours is a single-channel VCA in 4 HP, with a switchable linear / exponential response curve and one CV input. The exponential mode uses (exp(k*x) - 1) / (exp(k) - 1) with k = 6.9 - that gives the classic ~1000:1 (60dB) dynamic range that audio VCAs are supposed to have. Linear mode is simple multiplication, which is exactly what you want when the CV is a sequencer step or another audio signal you want to ring-modulate against.

The Gain knob has the proper Eurorack dual behavior: when CV is patched it attenuates the CV; when CV is unpatched it sets a fixed gain. We detect cable connection state from the engine itself rather than thresholding voltage, so leaving a 0V cable patched still counts as patched - the way it should.

There is no VCA bias knob and no offset - this is a clean, neutral VCA, the kind you reach for when you want the envelope to do its job and nothing else. For colored, overdriven amplification with simultaneous expo + linear CV, see Reval. For eight channels of attenuation and DC sources, see 8vert. Webrack tries to keep small modules *small*: one job, done well.

In a patch

The classic subtractive voice is VCO into filter into VCA into output, with an ADSR driving the VCA's CV input. The oscillator generates harmonics, the filter sculpts timbre, the VCA shapes amplitude. Without the VCA the note has no beginning or end - the envelope-VCA pair is what makes a sound feel *played*.

VCAs are also signal-mixing utilities. Patch an LFO into the CV for tremolo. Patch one audio rate signal into the input and another into the CV (in linear mode) and you get amplitude modulation - a metallic, bell-like family of timbres. Stack VCAs in series to multiply two envelopes together. Run a VCA between every modulation source and its destination to get full dynamic control over each one - the Make Noise Maths philosophy of *VCA on every voltage*.

Mixing is just a row of VCAs in parallel feeding a summing point. Many mixers are literally a bank of VCAs with a master sum output. The same module that shapes a note can level-match a stem.

Inputs

Audio In (audio) — Signal input. Accepts any audio or CV signal. The VCA multiplies this signal by the control voltage level.
CV (cv) — Control voltage input (0-10V). Determines how much of the audio signal passes through. 0V = silence, 10V = full volume. Typically driven by an ADSR envelope or LFO.

Outputs

Audio Out (audio) — The input signal scaled by the CV and Gain knob. Patch this to a filter, mixer, output module, or another VCA.

Controls

Gain — Master gain (0-100%). When CV is patched, this attenuates the CV signal. When CV is not patched, this directly sets the output volume.
Mode — Response curve toggle. Linear (left) = proportional, good for CV signals and ring modulation. Exponential (right) = matches human hearing, more natural for audio volume shaping.

Inspired by

A classic single-channel VCA topology with switchable linear/exponential response. Linear mode for clean CV processing and ring-style multiplication, exponential mode for ~60dB perceptual range matching how the ear hears loudness. Gain attenuates the CV when patched and acts as a fixed level when unpatched - standard Eurorack convention.

Doepfer A-130
Mutable Instruments Veils
Intellijel Linix

← Back to all modules