Eurorack is an incredible tool for any electronic music producer looking to color outside the lines. The possibilities are endless, but therein lies the difficulty. While there is nothing wrong with exploring and having no clue what you’re doing, as that can lead to extremely satisfying results, a more structured approach is often a better way for beginners to learn the basics.
If you are finding your way into Eurorack, you’ve likely used a subtractive synthesizer before. Why not just start by recreating a classic subtractive synthesizer patch? This will teach you the basics of envelopes, filters, volt per octave, gates and triggers. So, that’s what we’re going to do in this article.
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The Oscillator (VCO)
Let’s start at the source: the oscillator, in this case often called a Voltage Controlled Oscillator (VCO). Most analog VCOs feature multiple outputs like sine, triangle, saw and pulse. The idea behind a subtractive synth is that you start with a harmonically rich waveform like a sawtooth, and then attenuate frequencies with a filter.
Filter (VCF)
On a regular synthesizer you’ll often find a switch to choose different types of filters like low-pass, high-pass or band-pass. In Eurorack, just like a VCO, the Voltage Controlled Filter (VCF) often features multiple filter outputs which you can patch simultaneously.
The low-pass filter passes low frequencies, while rejecting or attenuating higher frequencies. This is used in 90% of classic synthesizer sounds, so let’s start with that.
Envelope (ADSR)
Now, let’s add a third module: an Envelope. This is a voltage modifier; it takes a gate input and converts this into Control Voltage (CV). It doesn’t modify any audio signals, so there are no audio inputs or outputs. It runs in parallel to the audio path.
Let’s start by connecting the envelope output to the filter’s CV input. This will stop the sound. Now in order to hear the sound again we need to connect the gate input to a gate output. Most often this is a MIDI keyboard, but it can also be a sequencer.
In VCV Rack you’ll use the MIDI to CV module to convert MIDI signals into control voltage. Most envelopes offer 4 controls: attack, decay, sustain and release, or ADSR for short.
Once you’ve connected a MIDI keyboard or sequencer, a high gate signal is sent out, starting the envelope. The time it takes for the signal to rise to an audible level is the attack time. Make sure the CV amount on the filter is turned up clockwise, as this is often referred to as envelope amount on traditional synthesizers.
After the attack time, we enter the decay time, which is the amount of time in order to lower the voltage to the next stage: the sustain level. The sustain level is the intensity at which the note is held. It stays put at this level until you release the key from your keyboard or until the next step from a sequencer is activated.
Finally, the release time is activated after the key is released, so the note slowly trails off. This is useful to emulate a reverb, for example. Keep in mind that Eurorack is monophonic, so you can only play one note at a time.
Volt per Octave
Speaking of notes, it would be nice if we could also play different pitches with our keyboard or sequencer. That’s what the Volt per Octave (V/Oct) input on our oscillator is for. A voltage of 0V is often indicated as middle C on a piano. +1 V would increase the pitch by one octave and -1 V lowers the pitch an octave.
Divide this note by 12 and you get all other chromatic notes on the keyboard. If you’re using a sequencer, you can actually get pitches in-between the regular notes of a piano. That’s the beauty of analog signals, as there are infinite values, meaning you can create all kinds of exotic tuning systems outside the western equal temperament system.
Gate vs Trigger
What’s the difference between gates and triggers? A gate has a sustain phase, while a trigger does not. In reality a trigger is not infinitely short, it still has a time, but it’s often 1 to 10 milliseconds long, so barely audible.
Triggers are used for drums. If you imagine an acoustic drum kit, a drummer hits the skin of the drum with a drum stick, but doesn’t have any means to control the length of the drum. Connecting a trigger to a traditional ADSR envelope only works when you crank the release time, because the attack, decay and sustain phases are bypassed as soon as the voltage drops.
A gate works more like a violin bow or electric organ. You can sustain the note for as long as you have the will-power. By opening the gate all signal will pass through, but as soon as you close the gate, the signal stops.
LFO
We’ve already made a very simple subtractive synth patch. But let’s add some more life to it with a Low Frequency Oscillator (LFO). An LFO is a modulator, just like an envelope. It controls another module without producing or altering any audio signal.
It works just like a normal oscillator but way slower with wavelengths several seconds long. Some LFOs have a range fast enough to also produce audible frequencies, although I wouldn’t recommend sending an LFO directly to your speakers.
A practical example of an LFO is to send it to a second filter CV input to slowly modulate the filter cutoff up and down. Or you could send it to an additional VCO pitch input to create vibrato effects. If you don’t have any spare CV inputs you can insert a mixer in-between. Just make sure it’s dc-coupled otherwise it will block extremely low frequencies.
VCA
A third option is to send an LFO to a Voltage Controlled Amplifier (VCA) in order to create tremolo effects. A VCA is just a fancy word for volume control. Many traditional synthesizers come equipped with two envelopes, one controlling the filter and another controlling the VCA. This allows for more creative sound shaping options.
In VCV Rack the core modules usually provide single channels, so just one envelope or one VCA, but most hardware Eurorack modules usually offer two modules into one to save some space.
Try to experiment with the module order as well. Patching a filter into a VCA allows you to stop the sound when using self-oscillating resonant filters. It depends on the modules as well, as some VCAs will saturate or even distort with high input levels, while others may add noise that you want to filter out. This is not possible with fixed architecture synths, so that’s a cool option right away.
Effects
Finally, no synth patch is complete without effects. So feel free to add some reverb, delay, distortion, waveshaping or any other creative effect module. You don’t even need to use Eurorack modules for this, as maybe you’ve got some guitar pedals laying around, or use some plugins in your DAW.
Whatever you use, just have fun. Modular is all about exploration and trying out stuff for the sake of it. Don’t be afraid to break things. You can’t, as it is all protected anyway.
If you’d like to learn more about modular, download my free modular getting started guide. Join our free Community and check out my YouTube playlist with over 70 VCV Rack tutorials to get you started.
