Amplifiers and Envelopes

The VCA: Voltage-Controlled Amplifier

An oscillator runs continuously. It doesn’t know when you press a key or when you release it. If you patch an oscillator directly to your audio output, you get a constant drone — the tone is always on. To turn it into something you can play, you need a gate: something that opens when a note starts and closes when it stops.

The VCA in VCV Rack (the Fundamental VCA module) has two relevant inputs: the audio input (where your oscillator or filtered signal goes) and the CV input (where the control voltage goes). A common question when first encountering it: “The top input has a line connecting to the yellow meter — what exactly is it?” That top input is the voltage control for level. The audio goes in one hole; the instruction about how loud it should be goes in the other.

By itself, a VCA with a static voltage at its CV input just sets a fixed volume level. It becomes musical when you control that voltage with something that changes over time — an envelope generator.

ADSR: Attack, Decay, Sustain, Release

The ADSR envelope is the standard model for how a sound’s amplitude changes from the moment a note starts to the moment it ends and fades away.

What each stage does, concretely:

Attack is the opening gesture. A short attack (near zero) means the sound snaps on instantly — a drum hit, a pluck, a percussive stab. A long attack means the sound fades in gradually — a swelling pad, a bowed string. The attack time alone changes the identity of a sound dramatically. The same sawtooth wave with a 0ms attack sounds like a lead synth; with a 2-second attack it sounds like an ambient texture.

Decay is what happens right after the peak. The sound has reached its maximum level during the attack phase, and now it settles back down to a lower level. A short decay creates a punchy, percussive envelope — the sound hits hard then drops quickly. A long decay creates a gradual settling. If the sustain level is set to maximum, the decay stage has no audible effect because there’s nowhere to fall to.

Sustain is different from the other three parameters because it’s a level, not a time. Attack, decay, and release are durations — they control how long something takes. Sustain controls how loud the sound is while you’re holding the key. Set sustain to zero and the sound dies away completely during the decay phase even if you keep holding the key. Set it to maximum and the sound stays at full volume for as long as you hold it. Somewhere in between gives you a held tone that’s quieter than the initial peak.

Release is what happens after you let go. Short release: the sound cuts off immediately. Long release: the sound fades out gradually, like the sustain pedal on a piano. Release is what separates staccato from legato in synthesized sounds. It’s also what creates the “tail” on a reverb-like pad even before you add any reverb — a long release lets the sound linger.

Patching It Together

In VCV Rack, the signal flow for a basic playable voice looks like this:

1. MIDI-CV (or a gate source) sends a gate signal and a V/OCT signal.
2. The V/OCT goes to the oscillator, controlling pitch.
3. The gate goes to the ADSR envelope generator’s gate input, telling it when to start and stop.
4. The oscillator’s audio output goes to the VCA’s audio input (or through a filter first, then to the VCA).
5. The ADSR’s envelope output goes to the VCA’s CV input.

When you press a key: the gate goes high, the ADSR starts its attack phase, the voltage at the VCA’s control input rises, and you hear the sound. Hold the key and the ADSR moves through decay to the sustain level. Release the key: the gate drops, the ADSR enters its release phase, the voltage falls, and the sound fades to silence.

This is the basic architecture of a monophonic synthesizer voice. Oscillator generates the tone. Filter (optional at this stage) shapes the spectrum. VCA controls the amplitude. ADSR tells the VCA how to shape the amplitude over time. Gate tells the ADSR when a note starts and stops.

Shaping Character with Attack and Release

Try this experiment: patch a basic synth voice (oscillator through filter through VCA, ADSR controlling the VCA) and play the same note while changing only the attack time.

At 0ms attack: you get a percussive pluck. Instant on. Aggressive.

At 50ms: a softer onset. Less aggressive, more keyboard-like. The note still has presence but doesn’t punch.

At 500ms: a swell. The note breathes in. It starts to feel like a string section.

At 2 seconds: an ambient pad. The note emerges from nothing. You’ve changed the fundamental character of the sound without touching the oscillator, the filter, or anything else.

Now do the same with release. Short release makes the sound clipped and rhythmic — good for bass lines and staccato parts. Long release makes it wash out and blend into the next note. Very long release (several seconds) creates a sustain pedal effect where notes pile up and overlap.

The combination of attack and release defines more about a sound’s identity than most people realize. A piano has a fast attack and a long release. A violin (bowed) has a slow attack and an abrupt stop. A marimba has a fast attack and a medium release. These are the time-domain fingerprints of an instrument, and when you synthesize sounds, the envelope is what determines whether your patch feels percussive, sustained, swelling, decaying, or something in between.

Envelopes Beyond Volume

The ADSR in your signal chain doesn’t have to control only volume. Any parameter that accepts a control voltage can be modulated by an envelope. The most common alternative target is the filter cutoff.

Patch a second ADSR module to the VCF’s frequency CV input. Set a fast attack, medium decay, low sustain, and short release. Now when you play a note, the filter opens briefly (letting the bright harmonics through) then closes back down. The sound starts bright and darkens over time — like a plucked string or a struck bell. This is one of the most-used patches in subtractive synthesis.

By adjusting the envelope amount (how much the envelope modulates the cutoff), you control how dramatic the effect is. A small amount gives a subtle tonal shift. A large amount creates a dramatic “wah” at the start of each note. The relationship between the filter envelope’s settings and the filter’s base cutoff position determines the tonal arc of the sound.

You can route envelopes to pitch (for pitch sweeps or drum synthesis), to resonance, to effects parameters, or to anything else that accepts CV. The ADSR is a general-purpose shaping tool — it just happens to be used most often for volume and filter cutoff.

Multi-Stage Envelopes

The four-stage ADSR is the standard, but it’s not the only option. Some envelope generators offer more stages for more complex shapes.

An AR (Attack-Release) envelope has only two stages: rise and fall. It’s simpler than ADSR and useful for triggered sounds (like drums) where you don’t need a sustain phase. The sound rises during the attack, then immediately falls during the release.

Multi-segment envelopes (sometimes called function generators or complex envelopes) let you define arbitrary shapes with as many stages as you want. You might create an envelope that rises, dips, rises again, holds, drops halfway, holds again, then fades out. These are useful for evolving textures and complex timbral movements that don’t fit the four-stage model.

In VCV Rack, the Fundamental ADSR covers most needs. More complex envelope generators are available in the module library — look for modules with multiple breakpoints or visual envelope editors. Multi-stage envelopes become useful in advanced patching when building evolving FM sounds and complex timbral sequences, but the ADSR is where to spend your time first. Master the four stages before reaching for more.

Envelope Generators in VCV Rack

The Fundamental ADSR module is straightforward. Four knobs (Attack, Decay, Sustain, Release), a gate input, and an envelope output. Patch the gate from your MIDI-CV module (or a sequencer, or a clock) and the output to your VCA’s CV input.

A few things to know about the Fundamental ADSR:

• The envelope output is unipolar — it ranges from 0V to 10V, not from -5V to +5V. This matters when you’re using it to modulate things like filter cutoff, where a bipolar signal would push the cutoff both up and down. Most of the time, unipolar is what you want for envelopes.
• The attack, decay, and release knobs control times, not rates. Turning the knob clockwise makes the stage take longer.
• If you want the envelope to open the filter more dramatically, you can amplify the envelope signal with a VCA or attenuator before sending it to the filter’s CV input. Or, if the filter module has a CV amount knob (many do), use that to scale the effect.

For a second envelope (to control the filter independently of volume), just add a second ADSR module and feed it the same gate signal. Now you have two independent envelopes triggered by the same note — one shaping volume, one shaping brightness. They can have completely different settings.

What to Practice

• Build a basic monophonic voice: VCO → VCF → VCA → Audio out, with ADSR controlling the VCA and a MIDI-CV or gate source triggering the envelope. Play notes and confirm that the sound responds to your key presses.
• Experiment with attack time. Hold a note and sweep the attack from 0ms to 2 seconds and back. Notice how dramatically the character changes. Find the attack time that makes the sound feel like a pluck, a keyboard, a bowed string, a pad.
• Experiment with release time. Play short staccato notes with a very short release, then lengthen the release. Find the point where notes start overlapping into each other.
• Set sustain to zero and play with different attack and decay times. With no sustain, the sound always dies away regardless of how long you hold the key. Try fast attack + short decay (a tick) vs. fast attack + long decay (a slow fade) vs. slow attack + long decay (a swell that fades).
• Add a filter envelope. Patch a second ADSR to the VCF’s CV input. Set fast attack, medium decay, low sustain, short release. Adjust the base cutoff of the filter fairly low, then use the envelope to open it at the start of each note. This is the classic subtractive pluck patch.
• Try routing the envelope to pitch. Patch the ADSR output to the oscillator’s V/OCT input through an attenuator (to reduce the range). A fast attack and short decay on pitch creates a drum-like “zap” at the start of each note — the pitch sweeps down from high to the target note.