Eurorack Modular Synth Primer

How to get started in Eurorack Modular Synths

What’s the crack?

Wave goodbye to your wonderful life! Eurorack has arrived! For anyone who has played a musical instrument, there comes a time when you think “I wish It could do this or that.” Well what if I told you there’s an instrument which, after spending a bit of cold hard cash, or some tinkering in the electronics lab, you can make it do pretty much anything? You’d think I was insane and caught up in the Eurorack hype!

And you’d kinda be right, however when it comes to electronic instruments there’s not a lot modular synths can’t do. Want to create a classic 303 acid line? Easy! Nice creamy Moog type filter. There’s a module using the exact same circuit. What about a gorgeous acoustic sounding bowed violin sound? Don’t even get me started on the awesome Mutable Instruments kit!


So now you we’ve got the hype out of the way, let’s get into the details. What do you need as a first setup? Each modular rig is made up of individual units, called modules. They are mounted on rails and attached to a power supply. The modules are then patched into each other with patch cables.

The real fun comes with experimentation.

Once you’ve got the basic setup it’s time to decide which modules to go for. A simple system would usually have a VCO as the sound source, a VCF to filter the sound, a VCA to shape the volume and some modules to create control voltages, such as a Sequencer, ADSR and LFO.

At this point I could go into an in depth explanation of what a sawtooth wave is or how a 4 pole filter works, but that kinda misses the fun part of modular synthesis. The real fun comes with experimentation. As long as you follow some basic patching rules, you’ll be making squelchy, futuristic noises deep into the night.

The single most important thing to be aware of is all the patch cables carry two types of signal. The first is Audio, i.e. what comes out of the speakers, and the second, and possibly most important, is CV or Control Voltage. VCO, VCF, VCA: all Voltage Controlled. You might also hear the term Modulation. You use Control Voltage to Modulate the Audio.

Take the VCO for example. If you plug the Saw Out into a mixer, you’ll hear a steady saw tooth wave. It has a distinctive rich tone. Turn the Frequency knob and you’ll hear the frequency of the tone go up and down. You could just manually wiggle the frequency knob and make sweet music al day long, but your hand would soon tire and it’s not easy to hit the same notes twice. What you need is a way to automatically vary the frequency and this is where CV comes in. Plugging a control voltage into the Freq CV In on the VCO will cause the tone of the sawtooth to change depending on the control voltage level.

The sound design possibilities are virtually endless.

It’s the same with the VCF, where the CV controls the filter cutoff, and the VCA, where it controls the volume. With these few modules, you now have a way to automatically control the frequency, the filter cutoff and volume of the sound. With just these controls you could play with a modular synth for the next 10 years and never create the same sound twice. The sound design possibilities are virtually endless.

Give me Control!

So what modules create CV? The main CV modules are the Sequencer, ADSR and LFO.

Imagine a riff played on a guitar or synth. It’s a sequence of notes that either go up or down. The CV output of the sequencer simply goes up and down depending on the the riff you’ve programmed it to play. This is usually plugged into a VCO to vary the initial pitch. At this point you’ve got a tone changing frequency in time with the Sequencer. It’s all the same volume with no spaces between the notes.

This is where the ADSR comes in. With a traditional keyboard, you have silence until you press a key, the volume increases, stays while you hold the key pressed, and goes quiet again when you release. We already know the volume can be controlled by the VCA, but it’s the ADSR that creates a CV envelope to shape the sound. There’s usually four controls on the module: Attack, Decay, Sustain, Release. For drum sounds you might want a very sharp Attack; a slowly evolving pad might increase in volume much slower. While you have the key pressed, the note will sit at a Sustain level and the speed at which it reaches that sustain is the Decay. Once you note off, the Release phase controls the fade out. Simples.

So we now have a way to shape the volume of each note, how does the ADSR know when each note starts? This brings us to another form of CV: Gate and Trigger. These are very similar but different. They both signal the start of something, however the Trigger disappears straight away, but the Gate stays for as long as we need it. Hitting a drum pad will cause a drum to trigger. Holding a note on a keyboard will produce a Gate for as long as the note is held. Once we release the Gate, the ADSR enters the Release phase. Cool huh?

control voltage clock trigger gate

So, we have a Sequencer controlling the VCO pitch and outputting either a trigger or a gate for each note, telling the ADSR to create an envelope to control the volume of the sound via the VCA.

Still with me? I took me a few reads of the paragraphs above to actually get it the first time.

Okay, now we want to increase or decrease the speed at which the notes play. This is the last type of Signal: Clock. The clock is simply a Square wave, you can imagine the high level to be on and the low level to be off. Very similar to the Gate and Trigger. Some Sequencers come with an onboard clock, such as the Pure Sequencer, but most also accept an external clock. Any Square wave should do. An LFO produces can produce a square wave, as do VCOs. The main difference between them is a LFO is lower frequency than a VCO, but you can still use them both as CV. The higher the frequency, the faster the clock and the faster the sequencer runs.

Which brings us to the final piece of the jigsaw: we've just talked about the two very different types of signal and that different modules produce them, but the last paragraph above says there's no real difference between the LFO and the VCO, so, what? I can use Audio as CV?? There's only one way to find out...

So there’s a very quick primer in Modular Synths and kit and knowledge you need to get going:

VCO - Voltage Controlled Oscilator
VCF - Voltage Controlled Filter
VCA - Voltage Controlled Attenuator/Amplifier
Sequencer - Provides Frequency and Gate/Trigger CV
ADSR - Creates a CV Envelope
LFO - Produces slowly oscillating CV


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