How Do I Get Started With Eurorack Modular Synthesizers?
Getting Started With Eurorack: Basic Module Functions
Eurorack modular synthesizers have gained immense popularity over the past decade and for good reason. The potential for exploring all corners of the sonic universe coupled with their ability to be personally tailored to the user’s needs make them an incredible addition to anyone’s studio or performance toolkit. However, knowing where to begin can be daunting to say the least, so we’ll take a look at how one might methodically approach putting together their first eurorack rig.
Before jumping in, we will assume everyone needs a case of some sort. Cases come in all shapes and sizes. Which case you choose will depend on a number of personal factors such as how many modules you plan to acquire, available real estate, budget, and aesthetic preferences. At some point we may do a post about how to pick a eurorack case and all the hardware considerations associated with that, but for now let’s just say you need a place to put your modules so that they’re not just laying around on a table at-risk of shorting against one another.
Also worth noting that while this article is focused on the eurorack format, the same concepts apply to 5U, frac-rac, and any other format of modular synthesizer with the exception of some differences in power supply voltage specifications.
Power Supplies
Power supply selection is a hotly debated topic in the modular synthesizer world and there’s no one correct answer regarding which supply is best. Ultimately we need to get the power coming out of the wall to turn into +12 volts and -12 volts to power our eurorack bus boards.
Switching or switch-mode supplies are most common as they are small, efficient, and inexpensive. Most switching supplies require a DC wall-wart style adapter for power input, although some will utilize a 110 volt AC input. Under the hood this incoming voltage gets regulated and inverted to output the +12V and -12V required to run a modular system. Many will also have a +5V output as well, allowing certain modules to forego having onboard 5V regulators if they require a 5V rail.
Switching supplies are an excellent choice for smaller setups where you may only be running a handful of modules. Most skiffs (short, flat eurorack cases) run less than 3 inches deep so chunky power supplies simply won’t fit, making a switching supply the best option in most cases. However, switch-mode supplies often have fairly low current ratings and can create noise outside the audio frequency range that could work its way into a variety of modules down the line. If you envision a massive setup in your future, it’s possible that you may outgrow a small switching supply as you expand your setup.
The alternative to a switching supply is what’s sometimes called a “brute force” supply or a linear power supply. These require heavy transformers and heat sinks. While linear supplies are often large and expensive, they provide the benefit of higher current ratings and less noise.
Linear supplies require a 110V AC input and will often have adjustable output voltage. The eurorack standard is +/- 12 volts, so be careful not to accidentally run your power supply at +/- 15 volts as some modules could be damaged. Additionally, linear supplies that do have 5V taps are often able to supply far more current than you’ll ever need from the 5V rail.
While both types of supplies will ultimately output the same voltages required of a eurorack setup, it’s important to consider your goals when choosing a supply. Those looking to perform with only a few modules may find that a lighter and smaller switch-mode supply makes more sense, whereas those looking to build a massive studio rack may want to consider a hefty linear supply that won’t be pushed to its limits in the face of several dozen modules.
VCO and Other Tone Generators
After you have a power supply and case picked out, presumably you’ll want to make some sounds. Once upon a time the primary module for sound generation was the VCO or Voltage Controlled Oscillator. For conventional subtractive synthesis practices this is still very much the case, however today there are a wide variety of other modules available that also generate sounds in less traditional ways.
A VCO, while complex inside, is fairly straightforward in its usage: it’s a module that generates and outputs waveforms. The frequency at which these waveforms oscillate is controlled by an incoming voltage. VCOs will typically have an on-board frequency control for setting an initial frequency or pitch (“pitch” refers to waves within the range of human hearing whereas “frequency” refers to waves including those above and below our perception). Voltages from other modules can then be patched to the CV inputs to change the frequency of the oscillator.
Voltage Controlled Oscillators typically output simultaneous waveforms at the same frequency as one another, but with differing shapes and therefore different timbres.
In addition to a VCO’s frequency control and CV inputs, you will often find inputs and controls for pulse width, syncing to other VCOs, and attenuators to adjust the level of the inputs.
VCOs typically output saw, ramp, triangle, sine, and pulse waves, though some oscillators may offer other more complex waveforms. Additionally, oscillators will have a standard for CV scaling, the most common standard being 1V/Oct. This means that for every 1 volt input into the CV input, the frequency of the waveform being generated will double.
While 1V/Oct is the primary standard for oscillators and controllers alike, it’s worth noting that some VCOs may employ V/Hz or some other standard which could make playing tonal music challenging if not accounted for.
Other sound sources besides the conventional VCO include samplers, such as the Make Noise Morphagene and the Qu-Bit Nebulae, and a variety of “complex” oscillators which mix multiple waveforms, generate tones via wavetable, create noise, or deploy boolean logic to make sounds.
While the concepts, features, and makers of these other types of sound sources are too broad for the scope of this article, they are worthy of mention as you may find something like the Mutable Instruments Rings module (a digital module that uses concepts of physical modeling to make tones) is more in-line with your vibe than a conventional VCO.
Whichever option you go with, the important thing to remember is that, unless you’re planning to use your eurorack rig in conjunction with another complete synth or as simply an effects processor, you need some module in your synth rack that can make sound.
VCF or Waveshaper
This category also straddles the line between convention and innovation. Historically, the next most important module in one’s rack would be the voltage controlled filter or VCF, but there are also a number of modules that allow for sonic manipulation and waveshaping in a variety of ways other than simply low pass, band pass, and high pass.
A traditional voltage controlled filter will cut certain frequencies from its incoming signal. The point in the spectrum at which this occurs is called cutoff frequency and can be altered by an incoming control voltage. Most filters also have a resonance control which creates a certain amount of feedback, creating a bump in the spectrum and emphasizing the cutoff frequency.
For a deeper explanation of filter types, we recommend checking out our post on how envelope filters work.
The purpose of a filter is to shape the incoming waveform in a certain way, leading to a different sound at the output. The low pass filter (one in which highs are cut and lows pass through) is the most common form of VCF found in synthesizers, but high-pass and band-pass filters are also frequently used. While filters are an essential piece of one’s eurorack setup, it’s also possible to use waveshapers, wavefolders, or distortion modules to change the shape and therefore the sound of incoming signals.
A conventional filter requires a harmonically rich waveform to really get the most out of it. Running something like a sine wave into a low pass VCF will not yield the same result as running a saw wave through it. Wavefolders on the other hand create harmonics that aren’t present in the input signal and can change something like a sine wave into a new and interesting shape with all sorts of interesting overtones. Distortion modules, like distortion guitar pedals, are also considered to be a type of waveshaper as they clip a signal and reshape it until it generates new harmonics.
Over time you will likely end up with a number of different sonic processors in your eurorack setup as filters and waveshapers all have distinct personalities and varied uses and there is no one-size-fits-all approach to sonic sculpting. Furthermore, sweeping the cutoff on a highly resonant low pass filter with a square wave running through it seems to never get old.
VCA
“You can never have enough VCAs” is a common phrase in forums and memes related to eurorack and for good reason. The voltage controlled amplifier is one of the most versatile pieces of modular kit available, even if it doesn’t seem overly thrilling at first glance.
A voltage controlled amplifier allows for the changing of a signal’s level (the height of the waveform or other incoming voltage) through the use of a control voltage. So for example, at 0 volts on the CV input, the output might be producing silence and at 5 volts on the CV input, your input signal is fully audible.
VCAs are often used to turn sounds on and off remotely. When running a VCO into a VCA, you can use the VCA to amplify or attenuate the sound, simulating the practice of playing notes on a keyboard or guitar. By using envelope generators, low frequency oscillators, and other CV sources, a VCA can allow for the manipulation of loudness in a variety of ways.
VCAs can also be used for non-audio signals. If you run an LFO into a VCA and then patch the output of that VCA into the frequency CV input on a VCO, the LFO will modulate the frequency of the VCO, but the VCA will allow you to modulate the depth with which that LFO is modulating things. As the VCA’s CV decreases so too will its depth of modulation.
Voltage controlled amplifiers allow for drastic changes like turning sounds on and off as well as subtle changes like slowly altering the amount of modulation between two modules. VCAs will often come in banks of multiple amplifiers on one panel to save on space and power connections.
You may find that you have a variety of VCA types as some will have logarithmic CV responses and others will have linear responses (and some switch between the two!). The topology of VCAs can also differ in ways that may affect how they are utilized or how they behave when pushed to their limits. OTA chips, discrete transistors, monolithic VCA chips, and even vacuum tubes can all be used in voltage controlled amplifiers with varied characteristics.
Envelope Generators and LFOs
Not counting modules or sources of pitch and gate CV, the most common sources of control voltage utilized in modular synthesizers are envelope generators and LFOs.
An envelope generator creates a changing voltage over time. They can come in different varieties, but all are triggered by some sort of pulse signal. When an envelope generator is triggered it then creates its output voltage according to its architecture and how the controls are set.
AR, ASR, and ADSR are the most common envelope types found in eurorack and other formats of modular synthesizer.
AR or attack-release envelopes are the simplest to use. They have a rise time and a fall time. The release phase will happen as soon as either the attack cycle has finished or the gate signal stops. AR envelopes with no attack time start high and immediately fall and are great for plucky or percussive patches.
ASR or attack-sustain-release will remain high for as long as the gate signal is held and then enter the release phase when that gate signal goes low.
ADSR or attack-decay-sustain-release envelopes begin with an attack phase, the output level then falls for a time specified by the decay control at which point it lands at a level set by the sustain control. Once the gate goes low, the envelope will enter into the release phase.
Where envelope generators create control voltages at the request of a gate or trigger signal, LFOs (low frequency oscillators) generate a continuously changing voltage that can be used to control any number of parameters on other modules. A triangle wave LFO for example, when patched into a VCF’s frequency CV input would bring the cutoff frequency up and then down and back up again and back down again until unplugged or attenuated. A square wave LFO patched into the frequency input on a VCO would create a trill-effect, perceived as two pitches being played rapidly back and forth.
LFOs are best thought of as an extra hand that will turn a knob up and down repeatedly. Their frequency determines how often that hand turns up and down. Any attenuation in the path such as an attenuator built into the module or a VCA, will control how far back and forth that imaginary hand is able to turn that knob.
CV and Gate Controllers
The final piece of a basic eurorack kit involves finding a method with which to provide pitch CV and gate signals for creating notes. This can be done with a keyboard, a MIDI-CV converter, or an external sequencer.
Many analog and semi-modular synths now offer CV and gate outputs such as the Arturia Microbrute and the Korg Arp Odyssey. CV outputs for pitch in the 1V/Oct standard will output 0.0833 volts for each half step on the keyboard. Gate outputs are typically 5V when active, but sometimes more and will remain high for the duration of the key press.
MIDI to CV converters are also commonplace. There are some modules in eurorack format that take care of this functionality as well as a variety of standalone boxes. These allow you to take MIDI from either a MIDI keyboard or an audio interface and control note CV and gates using those devices. If you plan to use your modular setup with a DAW, a MIDI to CV converter is an excellent piece of kit as it allows you to combine the flexible editing of MIDI in a DAW with the flexible sonic palette of a modular synthesizer.
Another common way to trigger and control notes is through the use of a sequencer. There are sequencer modules for eurorack that allow you to program specific voltages for each step as well as external hardware boxes that do a similar thing. The Arturia Beatstep series is quite popular for this as it outputs the necessary analog voltages needed to control the eurorack system, but it can be synced via MIDI or USB to a DAW and can save sequences and patterns.
I personally have a bit of an obsession with sequencers and can never seem to have enough of them in my setup. While sequencers are often used to program note/gate sequences, their CV output is universal and can also be used to modulate filter cutoffs, waveshapes, and any other parameter that can be voltage controlled.
The Journey Into Eurorack
This list is meant to offer some suggestions on getting started. Over time you’ll likely find you want things like multiples, mixers, sample and holds, drum modules, effects, and more. The beauty of modular synthesis is that each new module boasts a number of inputs and outputs that can be plugged into all the other inputs and outputs in your system, giving you the freedom to explore and expand endlessly.
If you’ve already got a semi-modular synth (such as a Moog Mother 32 or Behringer Neutron) you may be able to start with even fewer modules than those described here as those have all the makings for a complete voice already built in. Once you’ve started adding modules to your case, grab some patch cables and see just how far you can stretch your sonic imagination and if you have any questions, feel free to contact us. We’re always happy to talk synths.