Modular 101

Whether hardware or software, most synths out in the wild have a fixed architecture. This means that the manufacturer designed it with a set of components that interact with each other in a set way. 

The standard signal path usually consists of an oscillator section will feed into a filter section, which will then feed into an amplifier section, with a few modulation sources that can be applied to specific sections in specific ways.

Modular synthesizers on the other hand, separate these components into individual modules, leaving it up to the user to arrange and connect them however they want to create their own unique synthesizer sounds and behaviours.

Why eurorack? And what does it mean?

Eurorack is a format of modular synthesizer invented by Dieter Doepfer from Germany. The form factor is basically a miniaturized version of the original formats used in the 60s and 70s by Bob Moog and Roland. 

Eurorack is now the most popular format of modular synthesizer, and it has been adopted by a huge number of companies from all over the world.

This format means that users are able to purchase modules from several different companies to create their own custom, one of a kind synthesizer, which they can then patch together however they choose. There’s all kinds of modules out there, and some take their cue from classic technology, while others are on the cutting edge of audio manipulation. Or they’ll often blend the two ends of the spectrum, reimagining the tools of the past with the technology of today.

In fact, calling them synthesizers isn’t entirely correct, because a Eurorack system can be an effects unit, a sequencer, a control processor, or whatever you can imagine.

The magic Control Voltage (CV)

In order to allow these different modules to communicate with each other, modular synthesizers use a format called Control Voltage. 

This is an analog electrical signal ranging from negative 10 volts to positive 10 volts, sent over an ⅛” patch cable from a source to a destination that can be used to control, trigger or modulate various parameters. 

Types of Control Voltage

The most common types of control voltages are Pitch, Gates, Envelopes, and LFOs.

1V/OCT, making things musical

Eurorack uses a standard of 1 volt per octave (1V/OCT) to set the pitch of oscillators in order to generate musical notes. This means that the 12 notes of a scale are divided between the range of zero to one volt, and jumping from 1V to 2V will step up by an octave. 

Other formats use other standards that are not compatible without conversion. For example, the Korg MS20 uses a Hz/V format.

Gates, Triggers, Clocks, and Square Waves are all signals that toggle between high and low voltage states. These types of signals are often used to trigger sounds or events in a system, synchronize timing of elements, or flip the state of something on or off. 

Gates versus triggers

Gates differ from Trigs because they can be held open, or high, whereas Trigs provide a brief high state which otherwise remains off. Clocks and square waves are signals that repeatedly toggle between high and low states.

Envelopes

If you’re familiar with fixed architecture synthesizers, you’ve probably come across envelopes and LFOs. An envelope is a contoured unipolar control voltage. Since they are unipolar, envelope signals will either be positive or negative. 

Typically they have controls for Attack Decay, Sustain, and Release (ADSR) and are commonly used to shape a sound once a triggering event occurs. For example, when a note is struck the attack determines whether the note will start abruptly or fade in, it then uses the decay rate to fade to the sustain level, and once the note stops playing, release determines how long it takes to fade to silence. In this case, the envelope would be controlling the amplitude or volume of the sound, but they can control any parameter available in a modular synth.

Low Frequency Oscillators are not UFOs

LFO stands for low frequency oscillator. Oscillators are one of the most important basic building blocks of synthesizers, because most of the time they are generating the sounds that you hear. Oscillators repeatedly fluctuate back and forth between positive and negative voltages, meaning that they are bipolar signals. These fluctuations are called cycles, and the size of fluctuation is referred to as amplitude. The rate or speed of these cycles is called the frequency, which is measured in Hertz, or cycles per second. 

If the frequency is fast enough, above 20 Hz, it becomes audible to the human ear. Low frequency oscillators generally inhabit the subsonic range below what humans are able to hear, but most can be sped up enough to be audible.

LFOs are commonly used as sources of modulation in a Eurorack synthesizer to make a parameter rise and fall. For example you can use it to control the pitch of an oscillator, the cutoff point of a filter, or the amplitude of a sound.

This brings up an important fact that is a big part of what makes modular synths so exciting. Audio can be used as a control source and some CV can be audible. 

A big part of what makes a Eurorack system fun is experimenting with connecting things and discovering what happens. Just be sure you’re connecting outputs to inputs and inputs to outputs!

Analog versus Digital

Analog Audio is an electrical representation of the sounds that we hear. Fluctuations between positive and negative voltages in turn cause a speaker to move back and forth in order to reproduce any sound imaginable.

Digital Audio on the other hand, is composed of binary data, and needs to be converted into analog audio (an electrical signal) before it can be reproduced by a speaker. Digital audio is made up of tens of thousands of samples per second of the volume level of a sound. Once these samples are played back, and converted into analog audio, we can then hear the result.

MIDI Interface

MIDI stands for Musical Instrument Digital Interface, and is a polyphonic digital control protocol. It can be sent between hosts and devices like keyboards, drum machines, sequencers and Digital Audio Workstations (DAWs) over 5-pin MIDI cables, or USB. A MIDI signal is able to transmit information regarding when a key is pressed and released, which key or keys are pressed, how hard they are pressed, and other things like the use of a sustain pedal, pitch bend and modulation wheels, and a number of other controls which are referred to as continuous control (CC) messages. Because MIDI is control data, it doesn’t sound like anything on its own, but it can be used to control a wide range of software and hardware instruments.

Midi can be converted to CV in order to control a Eurorack synthesizer, but CV can also come from a number of other sources.

Cases, the 3U body 

In order to start a Eurorack system, you will need a case and a power supply for your modules. 

Cases are measured horizontally in HP, which stands for horizontal pitch, or hole point. The two most common widths are 84HP and 104HP. 

84HP is the same width as a standard audio gear rack, and some manufacturers provide basic rails and ears to mount your system in a rack which can be a very cost effective way to get started.

Vertically cases are measured in rack units, or U, and Eurorack modules take up 3U of height. 

Some manufacturers also utilise a 1U size format, and all of our cases come with a 1U row. 

For that reason, some cases come in 4U and 7U configurations while 3U and 6U options are more common. This smaller 1U space is great for utility modules and frees up space in the rest of your system.

Sliding nuts vs threaded rails

Modules are mounted in cases with screws that attach either to sliding nuts or threaded rails. 

Sliding nuts mean that you must position the nuts in the rail by sliding it into place before you attach your module, and make sure you don’t trap the nuts in an inconvenient place. 

Threaded rails are strips with evenly spaced threaded holes. Meaning there’s always a mounting location available wherever you place your modules. We would recommend to only use threaded rails in your case.

Power it up

Modules are powered by connecting their ribbon cables to the header pins on a power supply. 

Power supplies come in a number of different configurations. Some have a power entry module, others are integrated into the case. Some use flying bus cables which are cheaper, but prone to clutter and noise issues, while others use solid bus boards. 

Make sure your case comes with high quality power supplies using bus boards, these are usually also sold separately if you want to install one of our power supplies in an empty case or build your own.

It’s also worth mentioning that Meanwell Power Supplies can be used around the world with different power voltage standards.

It’s important that your power supply sends enough juice to feed all of your modules. If you leave them hungry, they might not work properly or you could even damage them.

Planning your rack

Modulargrid is a great resource for planning your dream system. You can create a virtual rack as big or as small as you want, and then fill it with your modules of choice. Once you’ve arranged your rack, you can view a data sheet that breaks down and adds up the power requirements of all of your modules. Once you have a sense of how much power you need, compare that to the manufacturer’s specs on their power supply and make sure that it exceeds your requirements.

You should also make sure that your power supply has enough headers to connect all of your desired modules. If you need more you can use a flying bus cable, however you may experience more noise through those modules. 

For this reason it is best to reserve the bus board for your audio processing modules, and use the flying bus cable for CV processors and other non-audio utilities.

Installation and the red stripe -12V

When connecting modules it’s very important to make sure that the ribbon cables are oriented correctly. The -12V connection on the power supply must match the -12V connection on the module. The red stripe on the ribbon cable should always connect to the side of the power header marked -12v or Red Stripe. 

Installing a module incorrectly could potentially damage it, so be careful to line things up correctly and consult the manual beforehand.

Monitoring and Recording your System

Modular signal levels are a lot higher than the line level signals used by other synthesizers, drum machines and consumer devices. 

For this reason, you will want to make sure you have some way of attenuating or reducing the signal level before you send it to your speakers or headphones. If you plug a set of headphones directly into the output of an oscillator it will be really loud, and as a mono output will only produce sound in one ear. After doing something like that, you might only get sound in one ear permanently, so don’t do that.

It is possible to connect your modules directly to a mixer or audio interface using ⅛” to ¼” adaptors. 

This is because Mixers and audio interfaces will provide the ability to attenuate the signal down to a level that you can record without distortion and monitor without damaging your ears or speakers. 

Most manufacturers also have a number of modules that provide headphone outputs or balanced outputs that will eliminate potential noise issues in your signal.

Some manufacturers even have modules with audio interfaces or digital connections to send audio or CV directly between the system and computer.

Synthesizer Building Blocks

Envelopes

Envelopes provide a contoured unipolar CV signal which is useful as a modulation source for VCAs, Filters, Oscillator pitch or pulse width modulation, and many other options. The most familiar form they take is ADSR, which stands for Attack, Decay, Sustain, and Release. Sometimes a Hold stage is added between the attack and decay phases, or they may be simplified with fewer steps.

Envelopes take on some pretty funky forms in the Eurorack world that you don’t really see in fixed architecture synths, and they’re often referred to as function generators. For starters, a lot of them, can cycle or loop, which turns them into unipolar LFOs.

Oscillators

Oscillators are a critical part of a Eurorack system, and they can take on a number of different forms. They can be either analog or digital, and many can toggle between audio and LFO ranges, allowing them to perform either function.

As mentioned previously, oscillators fluctuate back and forth between positive and negative voltages. If they cycle with a frequency between 20 Hz and 20,000 Hz they will be audible, below that range they are considered LFOs, or low frequency

The shape that these cycles take is referred to as the wave shape, and this determines how it will sound. This is referred to as the Timbre of the sound. 

A sine wave is the most pure tone as it produces a single frequency with its oscillations. Other wave shapes introduce harmonics above the base frequency, or fundamental, which makes the timbre more complex. 

For example, a sawtooth wave has a harmonic at every multiple above the fundamental, with each harmonic half as loud as the one below it.

Analog oscillators have a warm, musical character that makes them very desirable. 

Digital oscillators have a level of flexibility and recall that analog oscillators cannot offer. Like having a deep range of functions giving it a wide range of tonality. Some oscillators are actually made up of two or more oscillators that can interact with each other in a number of ways. They can even generate chords, and save and load presets.

VCAs – Voltage Controlled Amplifiers

VCAs are another critical component of a Eurorack system. If you haven’t already, you’re going to hear people say, “You can never have enough VCAs”. So why is that?

If you’ve used a fixed architecture synthesizer, you might be familiar with VCAs already. It stands for Voltage Controlled Amplifier, meaning they use a voltage signal to control the amplitude, or volume, of another signal. The most common way that they’re used, and the way that you see them implemented in fixed architecture synthesizers, is to control the volume of the final sound produced by a synthesizer using an envelope as the control voltage. You strike a key and the ADSR envelope of the VCA determines if that note will swell in slowly or hit abruptly, and if it will end as soon as you release the note, or ring out afterwards.

While this is still a common way to use them, in a Eurorack scenario VCAs can be employed in a number of different ways. It’s important to stress that a VCA is using one signal to control the amplitude of another signal, and that doesn’t mean that it always has to be a CV signal doing the controlling, or that it has to be an audio signal being controlled. 

VCAs are helpful to shape modulation signals with envelopes. This can let you ease in an LFO to modulate pitch or filter cutoff with a slow attack. Or if you used a slow LFO to modulate an envelope, you could make your envelopes get slowly bigger and smaller. By using gates to open and close a VCA, you can mute and unmute CV signals to toggle their effect on and off.

Amplitude modulation is a technique where audio rate signals are used to control the amplitude of another signal. For example, by fluctuating the amplitude of a sine wave faster than the frequency of the sine wave, the wave shape begins to change and new harmonics are introduced.

Certain VCAs have signal paths that can also act as a mixer to combine audio sources together into one output signal, either stereo or mono.

Filters

Filters are another important part of a Eurorack system. Like its name suggests, a filter removes part of the signal, and lets the other part of the signal pass through. The type of filter determines what part of the signal gets filtered, and what passes through. 

The most common types of filters are low pass, high pass and band pass. 

Low pass filters let low frequencies pass through and filter out high frequencies, which can give things a sort of underwater quality. These are sometimes referred to as high cut filters because they are cutting the high frequencies.

High pass filters do the opposite, allowing high frequencies to pass through while filtering out low frequencies. High pass filters are often used to create a rising sensation during the breakdown of a song to emphasize the bass drum once it kicks back in.

Band pass filters are a combination of high and low pass filters. They cut the signal above and below the cutoff frequency, letting only a narrow frequency band through. Notch filters, or Band reject filters, do the opposite and remove a band from the signal, which can create a swishy effect similar to a flanger.

Cutoff frequency

The filter cutoff frequency is the point where the filtering begins to occur. By sweeping or modulating this cutoff frequency with various sources a huge range of timbres can be achieved. 

How many poles?

The number of poles, or slope of a filter determines how gradually or abruptly the signal is filtered beyond the filter’s cutoff point.

The filters’ Q

A filter’s resonance, or Q refers to the amount of feedback applied at the filter’s cutoff point. This is sometimes referred to as emphasis because it emphasises and exaggerates the filter’s cutoff point. 

Some filters have the ability to self resonate with high resonance or Q. This means that without an input signal connected, the filter is able to generate enough feedback at the cutoff frequency to generate a tone. This tone takes the form of a pure sine wave, which enables self resonating filters to also function as oscillators. 

Some filters out there can even self resonate and will track 1V/OCT.

Filters play a big role in creating the character of a synthesizer and the sounds that it makes. 

Clocks, Gates, Triggers, and Dividers

Clocks, gates and triggers are all binary signals, meaning that they toggle between on and off, or high and low voltage states. Clocks provide a regular on/off square wave signal used to synchronize sequencers and tempo-based effects. Gates and Trigs both remain off until activated, then flip on to trigger notes and other events. The difference is that gates can be sustained on, while Trigs are short momentary blips.

Synchronised but different

Sometimes you want something to be synchronized, but to run at a different rate. This is where clock dividers and multipliers come into play. They do exactly what the name suggests and take an incoming clock and divide it or multiply it to create a synchronized signal at a different, but related rate. 

Drum sequencing is an obvious and easy to demonstrate example of this. We can use divided clock signals to set up a basic kick and snare rhythm and use the original clock signal for a hihat rhythm.

Certain modules have clock dividers built into them, so they can interact in sync with greater flexibility.

Clock dividers can even be used to divide an audio signal and produce sub octave square waves.

Sequencers and Quantizers

As redundant as this may sound, sequencers are devices that play sequences. This can be a melodic series of notes, raw voltages, rhythmic gate or trigger patterns, or some combination of all of the above. The length of a sequence is usually measured in steps, and a step will usually correspond to a musical length of time such as a sixteenth note. The sequencer will advance step by step through the sequence in time with the clock signal and reset once it completes all of the steps. As it passes through each step, it will output whatever parameter has been set for that step. For example, each step might have a note from the major scale, so as the sequencer strides through those steps it will play a melody.

Some sequencers are able to be reset through trigger inputs to ensure they start at the beginning of their pattern in tandem with other sequencers. In order for sequencers to remain synchronized once they begin playing together, they will need to share a common clock signal. In order to set this up, one sequencer will be assigned the “Master” and will run on its own internal clock. The Master then outputs its clock to any other sequencers that will then be set as “Slaves”.

Sequencers take many forms in the world of Eurorack as they are a very important part of making electronic music,

A quantizer takes an input signal and adjusts it into a constrained output. This means that any input signal can be made musical.

Sample & Hold & Slew

You may have heard of the term sample rate in regards to audio. This is a big part of how we create digital audio from an analog signal. 

During recording, samples of the audio are taken to measure how much amplitude it has at a given point in time. Then when these samples are played back at the same rate it reproduces what we recorded. 

Most audio is recorded at 44,100 samples per second because this was determined to be the minimum resolution required to accurately reproduce the upper range of human hearing (20,000 Hz) according to something called the Nyquist theorem.

Sample and hold follows a similar principal, only in this case we are able to vary the sample rate, and the samples are output as voltages. Whenever a gate signal is received, a sample is captured of the source’s current voltage level.

Remember that audio usually uses a sample rate of 44,100 times per second? Sample and hold allows you to play with reducing that sample rate, which can achieve lo-fi sounds reminiscent of old samplers and video games. This is because they used lower sampling rates to conserve valuable memory and CPU resources when technology was not as powerful as it is today.

What’s slew?

Slew introduces a lag between incoming signals which makes them slide from value to another. When used with pitch CV, this is often referred to as portamento or glide. Slew is actually accomplished by using a basic low pass filter

Logic

Because CV signals like square waves, gates, and clocks produce binary on/off voltages, they can be used in Boolean logic equations, or operations, which form the foundation of computational data and processing.

While that might sound daunting and intimidating, these equations are pretty simple and don’t require a math degree. Usually they take two inputs and compare them to produce an output which will depend on the logic type being used.

AND

The first logic operation we’ll look at is called AND. All this means is that if all inputs are high, the output will also be high. This is useful if you want something to always trigger in sync, but have some variation.

OR

Next we have OR, where if one input or another are high, or both, then the output will be high. 

This is useful if you have a number of gate signals that you want to combine and send to one source.

NOR

Then there’s NOR, which is high if neither one nor the other is high. While thinking of it as nor is convenient in this case, it really means negative OR, meaning that it is the opposite of OR. 

NAND AND

Thinking of it this way will help make more sense of NAND, which is negative AND. NAND will be high unless all inputs are high, exactly the opposite of AND.

XOR

XOR is exclusive OR, meaning that it goes high if one input is high, but not if all inputs are high. Lastly, XNOR is the opposite of this, and goes high if all inputs are high or if all inputs are low.

As you can probably imagine, these different logic types can interpret incoming gates into a number of different rhythms to trigger events in your system.

Attenuverters and Offset

You may have noticed that many manufacturer’s have modules with little black knobs above many of the inputs. These are called attenuators, or attenuverters if they are bipolar, and they serve to control the amount of signal sent to that input.

You’ll often find that if you use the full range of modulation of a CV signal, the results are quite wild and unruly. By attenuating that modulation signal you can tame it into a more subtle range. Attenuverters can control both the amount and polarity of an incoming signal, meaning you can flip a positive envelope to provide negative modulation. Or you can use a VCA as an attenuator.

Mixers

If you’ve worked with audio in any capacity, you’ve probably come across some sort of mixer at some point. Naturally mixers also appear in the Eurorack domain as it’s often necessary to blend or sum signals together in order to process the blended sound or output it to speakers.

While mixing audio signals together is obviously important and necessary, in a Eurorack system mixing CV signals together is often a critical but also expressive process. Many of our mixers are able to handle both audio and CV signals with some interesting and helpful tricks.

Some modules are able to blend signals, but also invert them, which can be useful for creating interesting timbres with your oscillators or complex modulation.

Others are able to use gate and clock signals to trigger muting of its input channels, which can be routed to bus outputs.

Last but not least unity mixers can sum sets of inputs to a dedicated output.

Passive VS Buffered Mults

Often when creating a patch, you’ll want to send a signal to more than one destination at once. This is where multipliers, or mults can be very useful. Mults come in two flavours: Passive and Buffered. Passive mults don’t require any power, but buffered mults will need to be powered, so they can also be considered active mults.

Buffered mults provide a perfect copy of the input signal, however with passive mults there will be a small drop in voltage caused by the input and output impedance of your modules.

For this reason you should avoid using passive mults for more critical and precise applications like sending pitch, or you might find that your oscillators detune.

Sometimes it can be useful to use a buffered mult between modules even if you are not multiplying the signal. This can often resolve issues related to using semi modular synthesizers like the Microbrute and Mother32 with Eurorack systems.

Planning your first system

Before you jump in and start buying modules, it’s a good idea to first stop and think about what kind of system you want to make. I often recommend people take a look at what they already have in their studio, and think about what they feel is lacking. 

Usually when a person is wanting to make the jump into Eurorack synthesizers they already have some experience with software or hardware synthesizers, and may own one or more pieces of gear. In that case, designing a system to complement or contrast what you already have probably makes the most sense.

Deciding on a general concept for your first system can be a helpful way to narrow down the vast selection of available modules. Will it be a sequenced monosynth, a west coast noisemaker, a polysynth, a drum rack, an effects unit, a control processor for another piece of hardware, a generative music machine, a live performance rig, or something else entirely? 

What kinds of music or sounds do you hope to make with it? Which leads to the question: How much space will you need? And the follow-up question: What is your budget?

Deciding on the answers to these questions can provide a helpful starting point which will help you target your research more specifically. 

Spend some time on our forum, ModWiggler, Reddit, Youtube, and on the Facebook Eurorack groups asking questions and reading people’s thoughts, because you’re bound to find some input that strikes a chord with you. 

Use modulargrid.net to plan a system or three and share the images to see what people think and recommend. Just remember to take people’s opinions with a grain of salt, because at the end of the day you’re building your own system and you should make it the way you want it to be.

Of course, your plan will undoubtedly change throughout the lifespan of your system, but that’s another part of the fun of Eurorack.

I would recommend starting small, with just a few modules. If possible, build a basic synth voice of a VCO, VCF, VCA, and a function generator. Or buy a module that can get you a full synth voice in one module. 

Starting small not only makes more sense financially, but it also helps you to better get to know the modules you have without being overwhelmed by the possible options of a rack full of modules. 

Creative limitations can inspire some unique ways of using modules that you might not find if you had all the tools at your disposal.

While it can be good to start small in terms of modules, you don’t want to restrict yourself on your case size. It’s a good idea to buy a bigger case than you think you’ll need, because let’s face it, you’ll probably want the space eventually as your system changes. It’s more cost effective to buy a bigger case to begin with, than to buy a small case and then another case once you outgrow it. 

East Coast vs West Coast Synthesis

You might have already come across the dichotomy between the so-called East Coast and West Coast schools of synthesis, but here’s a quick summary.

East Coast generally refers to Robert Moog’s approach to synthesizer design, which uses subtractive techniques. That is, oscillators are blended to create a rich and complex sound, which is then filtered to refine the desired tone. Subtractive methods are great for creating musical tones that mimic real instruments like brass instruments, flutes, and woodwinds. Moog’s East Coast synthesis was popularized through his instruments and the people that played them, which caused other manufacturers like Roland, Korg, and Yamaha to adopt this method.

West Coast synthesis was largely developed by Donald Buchla. Part of his approach was to take a simple source like a sine wave oscillator, then make it more complex through techniques like wave folding, low pass gates, and audio rate frequency modulation. Buchla was less concerned with emulating existing sounds and instead wanted to explore new timbral territory.

The flexible nature of Eurorack synthesizers allows them to straddle the two approaches and take advantage of both techniques.