I wrote this brief guide to provide a detailed description of EURORACK Modular Synthesizers, also known as 3U format (3 rack units). These instruments accompany me in my artistic/musical journey, and in recent years, they have dominated the market of electronic musical instruments.

Modular Synthesizers, also known simply as Synths, are hardware instruments that, when interconnected appropriately, allow the integration of multiple elements of a classic synthesizer. Each module has standard dimensions and is housed in a case that also provides power. Many musicians such as keyboardists, guitarists, percussionists, saxophonists, and others are fascinated by the various potentials of these machines, including the possibility of integration without the use of MIDI peripherals.

This guide is aimed at all enthusiasts and musicians who want to explore these electronic instruments, already used in the 1970s-80s but which have undergone enormous evolution in the last two decades thanks to new technologies available. The current versions, although significantly more compact, often retain many of the original characteristics, sometimes being replicas of renowned modules that are now discontinued.

Those already familiar with classic synthesizers will have no difficulty delving into this vast world with the help of some basic theoretical knowledge. For beginners, in addition to reading this guide, practicing with classic synthesizers, which usually feature the most important elements, is recommended.

Given the number of modules currently on the market, it is increasingly difficult to choose to assemble and customize one's own system. Today, there are over 100 companies distributing Eurorack Modules, each with different technical and sound characteristics. The approach to each module is different due to the use of graphics, sometimes "alienating" or sometimes more refined, and parameters that facilitate the artist to express themselves in different ways.

To discover all the modules available on the market, you can visit the website modulargrid.net, where you can also study their characteristics and even fit the modules you are most interested into a Cabinet/Case, much like in the game of Tetris. Everyone can create their own customized system to meet their needs, as creatively there are no limitations since all the necessary elements can be found: effects, percussion, equalizers, filters, compressors, pre-amps, etc...

In a single Eurorack system, you have all the elements to create a musical piece. It's possible to create melodic parts, chords, atmospheres, bass, drums, voices, and countless other sounds and rhythms that can be combined together thanks to the use of Control Voltage. Finally, all parts can be played by a single musical instrument: the Eurorack Modular Synthesizer.

These systems, despite their great potential, have some limitations that can be overcome by the user choosing alternative technical solutions. For example, it is not convenient to create polyphony with these systems, as creating a 6-voice synth would require many elements, many connections (or "patches"), and specific modules, when it is sufficient to opt for a polyphonic keyboard synth for a better result. Is it possible to achieve polyphony in modular systems? The answer is yes, but in a rather "extreme" way.

You will discover the evolution that these instruments have undergone until today, how to design them, what are the most important elements, the type of case to choose, and much more. You will finally be able to have a clearer and at the same time concrete view of these fascinating and tempting machines.

ORIGINS AND EVOLUTION OF EURORACK MODULES

Starting in the 1960s at the Columbia University Computer Music Center, a model of synthesis was studied that could offer musicians a new means of sound manipulation using analog electronic instruments. Among these was the Mark II Synthesizer, one of the first synthesizers that had these characteristics and which, for better sound processing, used both Subtractive Synthesis and Additive Synthesis.

During the same period, both digital and analog synthesizers with classical, modular, or semi-modular structures were born. Among these, we can remember the Melochord and Harald Bode's Audio System Synthesizer (collaborator of Robert Moog for the realization of modular systems and other electronic instruments).

From the 1960s onwards, thanks to the collaboration between many musicians and researchers, synthesizers became real musical instruments capable of generating infinite sounds through the manipulation of various electrical signals. The issues to be addressed at the time mainly concerned the introduction of Control Voltage (CV) and the improvement of real-time signal manipulation techniques favoring the "analog" mode. Research to improve signal manipulation speed was a priority since initially it was managed by the computers of the time, which could take hours to process. The standard functional block schematization was also introduced for the study and processing of various audio signals.

However, technology progressed, and many scientists seized the opportunity to create electronic musical instruments with increasingly sophisticated and effective sound processing. Those familiar with synthesizer history will have heard of Donald Buchla and Robert Moog, two pioneers of modular synthesizers and protagonists of the evolution of electronic and concrete music thanks to the creation of innovative and unique instruments.

Both made different choices for the design of these machines; on one hand, Buchla's studies focused on an instrument for composing a new type of electronic music, while Moog's idea was to create an instrument accessible to any musician, even without much specific technical knowledge. However, both considered analog Control Voltage (CV) crucial to automate parameter movement on the instrument.

Buchla was supported by musician Morton Subotnick, who helped him in the realization of the first modular systems like the Buchla Music Box, or Buchla System 100 (used to create Subotnick's 1967 album "Silver Apples of the Moon"), consisting of 25 modules controllable in real-time. Later, Buchla became known for creating control surfaces to replace traditional keyboards, made with touch-sensitive plates and subsequently also pressure-sensitive, which were first presented with the Buchla Model 400 (1982). Later on, he continued to create modular systems and more portable systems like the Music Easel, continuing to offer musicians an instrument with a less canonical and more experimental approach.

Donald Buchla passed away at the age of 79; however, a new company called Buchla U.S.A. now takes care of producing his most renowned products like the 200e series.

Moog, unlike Buchla, focused instead on creating a traditional keyboard electronic musical instrument that could be used in a song like a classic acoustic instrument.

He conducted experiments in his laboratory, making available to various musicians a series of elements for the timbral generation of a sound. He realized that many of them used only some of the elements within the huge synthesizer, thus maturing the idea of creating a portable Synth with a traditional keyboard but which exploited analog synthesis to create various sound timbres. After experiments with CEMS and Moog Modular (used by Keith Emerson and The Beatles on the album "Abbey Road"), the first and most famous portable analog synthesizer in history was born: the Minimoog.

To this day, Moog Music boasts numerous high-profile instruments and remains one of the best synthesizer manufacturers in the world.

It should be considered that these brands adopt two different types of synthesizer approaches: West Coast and East Coast.

The East Coast synthesis, based on Subtractive Synthesis, characterizes Moog Synths and most electronic instruments. This synthesis model uses one or more oscillators generating a signal rich in harmonics that is filtered, subtracting frequency bands. The signal passes through a VCA (or amplifier), in turn controlled in voltage by an Envelope Generator (EG).

We can already distinguish some of the most important elements of a synthesizer: VCO (Voltage-Controlled Oscillator); VCF (Voltage-Controlled Filter); VCA (Voltage-Controlled Amplifier); EG (Envelope Generator).

With the West Coast approach, used by Buchla, we work with Additive Synthesis and Frequency Modulation (FM). There is also the possibility of greater parameter control thanks to multiple Control Voltage (CV) inputs. Complex envelopes are also used to manage amplifiers, and the classic filter is replaced by Lowpass Gate (LPG) circuits, which we will discuss later.

In Eurorack modular systems, we find many elements with these two types of synthesis that can be integrated with even more interesting results. Today on the market, there are cutting-edge new synthesizers such as the Make Noise 0-Coast, which, as its name suggests, is not inspired by either of the two approaches, and the SOMA Laboratory Lyra-8 structured with 8 self-modulating analog oscillators in FM (frequency modulation) in addition to Delays and dedicated distortion circuits.

In the same period when Moog and Buchla studied, designed, and increased their business, some lesser-known brands produced modular and semi-modular synthesizers. Among these was the Electronic Music Studios, which launched the VCS-3 on the market, the first semi-modular synthesizer in Europe (used on Pink Floyd's "The Dark Side Of The Moon" album). The VCS-3 did not use cables for connections but implemented a matrix system to connect various elements, like playing "Battleship" with a futuristic instrument.

Serge, which came a few years later, also proposed modules that were less expensive than Buchla's but adopted his philosophy, sometimes considered more creative and unusual. These modules are still in production and available for purchase on the official website.

With the evolution of technology, there was also room for the development of digital synthesizers, but there were still those who preferred analog machines. In the early 1990s, Dieter Doepfer, already known in the market for the production of MIDI keyboards, created the A-100 system, a modular system composed of a 3U (units) case/cabinet with standard measurements.

The Doepfer modules were clones of some of the famous Moog modules but differed in size and connections made through unbalanced mini-jack cables. The major innovation was their size and flexibility, as until a few years prior, customizable and especially portable modular systems did not exist. Consequently, brands emerged that produced modules with different functions and characteristics but could fit into 3U cases using the same power supply.

Livewire and Plan B served as examples for current brands, being among the first to maintain the format characteristics introduced by Doepfer. This marked the birth of compatibility and integration among different brands thanks to the same power supply system and the use of signals not exceeding 10 volts DC or 10 volts peak-to-peak AC.

In the Eurorack standard, control signals and audio signals are not differentiated, and understanding how to treat them is crucial for effectively managing the instrument. It's worth noting the possibility of controlling the system via MIDI peripherals using MIDI to CV interface modules, synchronizing the instrument with a DAW or external synthesizer.

In the following years, new brands invested in these equipments, redefining the use and approach to modular synthesis. Make Noise, Intellijel, Mutable Instruments, Verbos Electronics, Noise Engineering, Addac System, Xaoc Devices , Erica Synths, 4MS Company, Tiptop Audio are just some of the most important ones producing innovative products to meet the needs of many users.

In addition to classic elements like VCOs, VCFs, VCAs, etc., today's market offers hardware modules with digital controls such as sequencers, wavetable oscillators, analog drum machines, or clones of old Drum Machines (like the classic Roland 909-808, etc.).

Some brands offer cases or portable cases of various types and sizes; an example is Frap Tools  cases (an Italian brand created by Simone Fabbri), which for the first time designed a Eurorack cabinet structure very similar to Buchla, or MDLR Case specializing in the production of portable cases. Without specific technical knowledge, it's possible to create cases, cabinets, or cases with customized designs using PSU (power supply units) in DIY kits or power modules available on the market. 

From 2010 onwards, the production of these instruments has seen exponential growth, and the offering of compatible products by more famous brands such as Dave Smith, Strymon, Moog, Arturia, Behringer, Eventide and others confirms this trend.

Eurorack systems are therefore highly customizable electronic instruments that meet the needs of every artist due to their compatibility and flexibility. However, the large number of modules on the market and the wide range of choices sometimes lead to purchases that may not be advantageous. Therefore, making informed choices based on the instrument one wants to create isn't easy without a good technical understanding of sound synthesis and the specific characteristics of each module.

How doeas It work?

A modular synthesizer operates through a Eurorack system composed of a case that provides power, one or more modules, and 3.5mm mono mini-jack cables for connections. It's important to decide initially if the system should be portable, and to choose the case dimensions before selecting modules to insert. Module height corresponds to 3 rack units (also known as 3U), while width is measured in HP. The most commonly used measurement is 84HP, which equates to 84 holes on the RAILS (perforated bar) used for screwing in screws to secure modules. Today, the market offers cases in various lengths to allow musicians to customize their system. Additionally, cabinets provide power at +/-12V and 5V distributed via Bus Boards, which resemble power strips, to which we connect our modules using their Flight Cable.

We need to pay attention to correctly connect the modules to the bus board, following the -12V indicated by a red line on the Flying Bus Cable. It's important to note that each case provides power in mA (milliamperes) and voltage of +/-12V and 5V. Each module, depending on its technical specifications, will draw more or less current (A), and this power consumption should not exceed the case's capacity. For example, if we need to power modules that require a total of 1000 mA, a case must supply at least 1200 mA, otherwise it may not power up or activate its protection circuit, if equipped.

Next, we'll need to determine the height (3U, 6U, 9U, etc.) and length (84HP, 104HP, 124HP, 168HP, etc.) taking portability into consideration. If we plan to travel by air, we'll need to choose smaller dimensions to comply with airline regulations. If not, we can choose any size case, potentially saving some cost. The market offers various types; for instance, Doepfer produces both "low-cost" studio cases and portable cabinets with a protective lid for modules and connections, akin to classic flight cases used on large stages.

Intellijel and Make Noise offer briefcase-style units with handles and appealing designs, including a 1U rack row for newer 1U modules. Other brands like Arturia, Tiptop Audio, and Behringer have designed cabinets that can be protected and carried using a backpack or suitcase. Additionally, there are "Skiff" cases characterized by a single row (either 84HP or 104HP) and the ability to be positioned horizontally on a surface.

Alternatively, consider building a case or suitcase using power modules like Tiptop Audio uZeus or 4ms Row Power , mounted on rails and distributing power via a Flying Bus Cable instead of a bus board to modules inside. For DIY enthusiasts with good skills, this is an option worth trying! For those less inclined, I recommend purchasing a pre-assembled cabinet.

Once you've decided where to place the modules, you can begin evaluating the possibilities the system will offer. There's no limit to the number of modules you can install (the limit depends on size and power), so if you want 10 identical oscillators for generating drones, you can do that. Thanks to innovations from various brands, we can now use these systems to process acoustic instruments or sample sounds in real-time. Every musician is free to customize their setup to best meet their needs. There are no rules about which elements to include, but to fully utilize the instrument's capabilities, exploring some aspects of synthesis will be beneficial.

Let's analyze how and what constitutes an integrated structure synthesizer by considering the Moog Model D, one of the most esteemed and recognized monophonic synthesizers. Examined in detail, it is made up of various elements: KEYBOARD, OSCILLATOR BANK, MIXER, FILTER, MODIFIERS, OUTPUT, LFO.

These components are already internally interconnected following predetermined paths. There's an audio signal path and a control signal path, as depicted in the figure below.

As we can see from the image, sound is generated by 3 analog oscillators with selectable waveforms chosen by the musician. These oscillators feed into a mixer on separate channels, allowing volume control for each. Additionally, the mixer includes a Noise Generator volume control, useful for creating percussive parts or enriching the sound. The summed signal then passes through a transistor ladder low-pass filter (LPF) with resonance and finally into a Voltage Control Amplifier (VCA) to manage the duration of each note. The final output exits via the Jack OUT connector, completing the audio signal chain.

On the other hand, control signals don't follow a linear path, making their operation more complex. However, it's essential to note that control signals cannot process or generate sound but only control parameters!

Firstly, let's define how a traditional keyboard works. Whenever a key is pressed, two control signals are generated:

PITCH: the keyboard sends a control voltage to the oscillator to determine which note to play.

GATE: the keyboard sends a control voltage to the envelope generator, which in turn controls the VCA to manage the length of each note.

The Envelope Generator (EG) only generates a voltage when it receives a GATE and It has 4 parameters for time management (ADSR: Attack, Decay, Sustain, Release). It's connected to the VCA and controls it to allow or block sound from passing through the FILTER. In the Moog, there's a second envelope "triggered" by GATE, controlling the Cut-Off frequency of the low-pass filter.

Lastly, there's the Low Frequency Oscillator (LFO), a sub-audio band oscillator that doesn't reach audible frequencies to the human ear. It's used to modulate parameters over time within the synthesizer. In the Moog, the LFO generates a sine wave and can modulate oscillator pitch or filter cutoff frequency.

Understanding the structure of the Minimoog and its internal connections (even omitting some advanced functions) helps us identify the primary components of a synthesizer:

• VCO (Voltage Controlled Oscillator)

• Mixer

• VCF (Voltage Controlled Filter)

• VCA (Voltage Controlled Amplifier)

• LFO (Low Frequency Oscillator)

• Keyboard/Sequencer

These elements form the essential core of a traditional Eurorack modular system. Let's delve into each one:

• VCO: the oscillator is the SOUND SOURCE, the first element in the audio chain! Hence, selecting it based on personal sonic preferences is crucial. Consider its waveform options, whether it's analog or digital, and any unique features it offers. It's undoubtedly a critical element to choose carefully.

• Mixer: the mixer becomes essential when multiple VCOs or waveforms need to be blended in a more comprehensive system.

• VCF: the filter is almost indispensable, especially in subtractive synthesis. Choose based on its sonic characteristics and functionality. Filters can include various modes like low-pass, high-pass, band-pass, and notch. Classic options include Moog-style (transistor ladder), Oberheim (SEM), or newer alternatives. Different filters can generate distinct harmonics and sounds due to resonance and slope considerations. My advice is always to listen to them, as they can offer varied sonic palettes.

• VCA: the amplifier controls the audio signal volume from the filter. Without it, shaping the amplitude envelope of a note becomes challenging. Some VCAs, known as low-pass gates, follow Buchla-style designs using components like "Vactrols". Consider how the envelope interacts with it.

• LFO: the low-frequency oscillator modulates parameters and offers various waveform shapes as set by the manufacturer. Some LFOs accept Reset or Sync signals for synchronized modulation within your patch.

• Keyboard/Sequencer: Both are crucial for PITCH and GATE signals, making the instrument playable for musicians. Keyboards typically offer standard features, while sequencers can provide more complex functions for experimentation and musical expression.

The elements of the Eurorack system described are interconnected via patch cables, as illustrated in the figure:

The VCO (Make Noise DPO) generates a waveform output fed into a filter (Intellijel Morgasmatron ). This filtered signal is then routed into the input of the VCA (Make Noise Optomix ) and finally the output to our audio interface, mixer or speakers. In the image, notice that before entering the final output mixer, the signal passes through a reverb/delay unit (M.I. Clouds). After the audio signal path, we move on control signals. The sequencer (Intellijel Metropolis) sends a Pitch signal to the 1V/oct input of the oscillator being used; the Gate signal, instead, is sent to the Gate input of the Envelope (Intellijel Quadrax).  The Envelope will enter the CV (Control Voltage) input of the VCA to control the amplitude envelope of the note. The LFO is currently excluded from the signal chain.

This "patch" is quite basic but effective for understanding its operation. If you have a Eurorack system or the opportunity to experiment with one, you'll likely find the sound generated by these machines impressive.

This guide covers only some of the capabilities and features of Eurorack modules. If you want to delve deeper into this topic or have questions about these fantastic instruments, feel free to contact me ( dr@davidericci.it )  for a detailed discussion on their functionality.

Ciao!  :)

Davide Ricci

13/01/2022