Modular synthesizer – where to begin

A modular synthesizer can be whatever you want it to be, and that’s the beauty of it. In this article, I’ll tell you what components are must-haves. I’ll cover the most basic and classic modules that once started it all.

Modular case

The modules have to be installed somewhere. They don’t have their own case. A modular synthesizer can have a case of different sizes, in different materials, mobile or stationary, designed for different module formats. As with the range of modules themselves, you will find the greatest choice for the Eurorek format.

When choosing a case, you have to be guided not only by the current need for the space but also by your future plans to expand your set-up. And it is difficult to predict. My personal opinion is that more is better than less. It is better to put blank panels on empty spaces in the case than to search for a new case every six months. Or think about what to sacrifice in favor of a new module you like.

Another important parameter of the case is its depth. The thinner the case of your modular synthesizer, the better the mobility. But the same parameter will limit the choice of modules that can be installed in a case that is too thin.

desktop eurorack modular case
Desktop case

The case size is specified by manufacturers not in centimeters or inches, but in standard units in which the dimensions of the front panels of the modules themselves are measured. Height in units (U). For a Eurorack module, this is usually 3U. Some manufacturers also make modules with a height of 1U. For such modules, it is necessary to provide a separate row in the case. Width is measured in hp (Horizontal pitch). These units are borrowed from the manufacturers of printed circuit boards. Which is quite logical. All eurorack modules are different in width.

DIY eurorack modular case
DIY modular case

If you have straight hands and a light in your eyes, it makes sense to try to build the case by yourself. You can literally make it out of boards. It doesn’t affect the sound. And if you have a laser-cutting workshop at hand, the task becomes much easier thanks to guys from Nonlinearcircuit. They have prepared ready-made enclosure designs for you. All that remains is to glue the resulting parts together and install the rails, which you need to buy separately.

Power supply

A power supply module is another important element without which a modular synthesizer will not sing. Modules usually require a bipolar power supply. This means that the power supply has an active plus, an active minus, and a zero. The voltages are different for different formats. For example, Eurorek modules are powered by power supplies with 12 volt outputs. Also, most Eurorack power supplies have a +5V output for which there is space on the standard power rail.

eurorack power supply
Linear power supply for eurorack with power buses

Power supplies can be linear or pulse power supplies. Pulse power supplies are lighter and more compact. Linear ones require a step-down transformer, which can be bulky and heavy. Its weight depends on the power. But it is generally believed that linear units create less unwanted interference on the common bus under heavy load. Such units, for example, are often used in professional audio equipment.

impulse modular power supply
Impulse modular power supply

But thanks to progress and talented engineers, now you can easily find impulse power supplies for eurorack that are as good as linear ones in terms of noise characteristics. And at the same time retain their compactness and lightness.

When selecting a power supply, you need to calculate how much current in milliamps (mA) all your modules will consume on each output separately. And then choose a power supply considering the maximum current consumption of the whole system. Usually, the positive arm is the most loaded, a little less load is on the negative and very rarely there are modules that consume current from +5V output. There are even power supplies for Eurorecs that do not have a 5V output.

Voltage Controlled Oscillator (VCO)

Voltage Controlled Oscillator
Eurorack VCO module

The voltage-controlled oscillator is a basic start of a modular synthesizer voice. The purpose of this module is to generate an AC wave. It can have different shapes and characteristics. These characteristics can be influenced by knobs on the front panel or by control voltage from other modules and controllers routed to the inputs.

For example, changing the frequency of the generated wave changes the pitch of the sound, i.e. the note. The Eurorek VCO uses the 1V/OCT standard, where changing the input voltage by 1 volt changes the tone by 1 octave.

It is classic for VCO modules to have 4 different output waveforms – sine, triangle, sawtooth, and square (meander). It is called a meander because the ratio of positive and negative phases can be changed, thus making the waveform look like a rectangle.

With only 2 classic VCOs at your disposal, you can already have fun modulating the different characteristics of the first oscillator with the second.

But this oscillator generates a signal continuously, there is no volume knob or button that makes it play only when you press it. Other modules are used for that.

Voltage Controlled Amplifier (VCA)

VCA eurorack module

Voltage controlled amplifier is a module without which your VCO will not shut up. The word “amplifier” is a little confusing here. It’s more of an attenuator than an amplifier. Its job is to operate the volume knob, which can be controlled by voltage.

But the beauty of VCAs is that you can use them to affect the amplitude of any signal, not just the wave from the oscillator. You can change the amplitude of some control signal with a second control signal, creating a chain of complex modulations.

On youtube, I once came across a wonderful video demonstrating the power and capabilities of a VCA as a seemingly uncomplicated module.

Voltage Controlled Filter (VCF)

eurorack VCF module
Eurorack VCF module

Voltage controlled filter is the main helper in shaping sound in subtractive synthesis. Its task is to attenuate certain frequencies in the signal coming to the filter input and to amplify frequencies at the cutoff point (resonate at the cutoff frequency).

By subtracting frequencies from the incoming signal we radically change the sound. There are many different circuits and modules that can perform the same task in different ways. For example, affecting the same square wave, some filters make the sound softer, others more aggressive. Different settings at resonance on the same slice can add different subharmonics to the output signal.

Most classical filters involve high-frequency filtering (LP – low pass). But there are filters with low pass-frequency (HP – hi pass) or with fixed bandwidth (BP – bandpass).

Envelope generator (EG)

eurorack envelope generator
Eurorack envelope generator

People already familiar with sound synthesis have probably heard the magic abbreviation ADSR. So this is the module that is responsible for generating a single wave of a certain shape with a specified attack (A), decay (D), sustain (S), and release (R). It is needed to make the individual extracted sounds similar to sounds that occur in nature and can be extracted from acoustic musical instruments.

By energizing the signal from the oscillator output to the CV input of the VCA module. We will affect the amplitude of the signal at the input. Thus giving the endless hum from the generator the dynamic characteristics of a piano or flute sound.

Of course, the signal of this generator can be used not only for VCA control but also for any other parameter that needs to be changed at a certain moment by a signal from a trigger (a signal that represents a short burst of voltage, e.g. from pressing a key on a keyboard).

Low Frequency Oscillator (LFO)

Eurorack LFO module
Eurorack LFO module

Low frequency oscillator is an oscillator used to change various parameters of other modules. The maximum frequency it can generate is usually around 100-200Hz, but the minimum frequency can reach a cycle of several minutes.

The output has multiple waveform options or variable waveform output.


Eurorack mixer
Eurorack mixer

In a eurorack modular synthesizer, a mixer is needed not only for mixing audio signals but also for mixing control signals. The big advantage of eurorack is that both audio and control signals in this format have the same amplitude level. You can experiment, mix everything with everything, listen to control signals, or use the signal at the VCO output for high-frequency modulation of parameters of other modules. The main thing is not to confuse input with output.


modular sequencer
Eurorack sequencer module

Classic synthesizers usually have a piano-like keyboard. But with modular synthesizers, your hands are more likely to be busy twiddling knobs or tapping patches. The sequencer was invented to free your hands and pay more attention to shaping the sound. Its task is to continuously play the same sequence of notes.

A classic sequencer sends at least 2 types of signals – a 1v/oct note pitch and a gate signal that tells you when a note starts and when it ends. Many people prefer to use the sequencer as a standalone device rather than a module.

standalone sequencer
Standalone sequencer

When you put the above components together, you get a classic modular synthesizer consisting of basic modules like Robert Moog’s first systems.

Of course, the minimum composition of a modular synthesizer may be different. Modern modules are quite different from classical modules in terms of functionality, and one modern module can replace several classical modules at once. But as the functionality expands, you get a more complex interface.

Personally, I think that the transition from classic devices to modular devices implies that the constituent modules should be as simple as possible. Yes, perhaps such a system will be more bulky. But yes, it is hard to make a modular synth compact. And a modular synthesizer is not built to be compact. Flexibility and lack of limits for the flight of fancy are important.