Welcome to the Rabbit Hole (You Have Been Warned)
So you stumbled across Eurorack modular synthesizers and now you are wondering what all those cables, knobs, and cryptic labels actually mean. Trust me, I have been there. The first time I looked at a fully patched Eurorack system I thought someone had wired up a small spaceship. But here is the beautiful thing: once you understand the underlying standards and logic, everything starts to make perfect sense.
In this post I want to walk you through the Eurorack standard from the ground up. No prior experience needed. Whether you are a complete beginner or someone who already makes music and is curious about going modular, this guide is for you. Let’s dig in.
What Even Is Eurorack? A Quick History
Before we get into the nitty gritty, a little bit of history never hurts. The Eurorack format was developed by Dieter Doepfer, a German engineer and synth visionary, back in the late 1990s. His company Doepfer Musikelektronik released the A-100 system in 1995, and that became the blueprint for what we now call the Eurorack standard.
What Doepfer did was essentially create an open standard. He said: here are the dimensions, here is the power supply spec, here is how signals should work. And then he let the whole world build on top of it. Fast forward to today, and there are thousands of Eurorack modules from hundreds of manufacturers all over the world, and they all play nicely together because they follow the same rules.
That open, community driven nature is honestly one of the things that makes Eurorack so exciting. You are not locked into one ecosystem. You can mix a filter from a tiny boutique maker in Berlin with an oscillator from a well known US company and a sequencer from a Japanese manufacturer, pop them all into the same case, power them from the same supply, and patch them together. Magic.
The Mechanical Standard: Let’s Talk About Physical Size
Alright, let’s start with something you can actually see and touch: the physical dimensions of Eurorack modules.
Height: 3U and 128.5mm
Every single Eurorack module is the same height. This height is referred to as 3U, where U stands for “rack unit.” One rack unit equals 44.45mm, so three rack units come out to 133.35mm. However, in practice, Eurorack modules themselves are slightly shorter at 128.5mm, because the remaining space is taken up by the rails and the mounting hardware inside your case. When you see a Eurorack case advertised as “3U,” this is the standard you are dealing with.
This consistent height is what makes Eurorack so satisfying to look at. No matter which modules you buy, they will all line up perfectly in your case. Clean, uniform rows of gear. It is genuinely pleasing.
Width: HP (Horizontal Pitch)
This is where it gets a little bit more nuanced and also a lot more fun. The width of Eurorack modules is measured in a unit called HP, which stands for Horizontal Pitch. One HP is equal to exactly 5.08mm (often rounded to 5mm for simplicity).
Modules come in all kinds of widths. You will see tiny utility modules that are just 2HP or 4HP wide, and you will find complex synthesizer voices that stretch to 30HP or even more. Most cases are sized in HP too, so for example a common beginner case might give you 84HP or 104HP of space to fill with modules.
Why does this matter? Because planning your rack is a huge part of the Eurorack hobby. You are constantly solving a fun little spatial puzzle: how do you fit all the modules you want into the HP space you have? There are even websites and apps dedicated entirely to helping you plan your rack layout. It is genuinely addictive.
The Rails and Mounting Screws
Modules are mounted to the case using M3 screws (3mm diameter metric screws) that thread into special Eurorack rails. These rails have a series of T-slot nuts or threaded strips that allow you to mount modules at any position along the rail. The spacing of the mounting holes on a module follows the HP grid, so everything lines up perfectly.
If you ever build your own case (and you might end up doing exactly that one day), understanding this rail system is essential. But even if you buy a ready made case, it is good to know what is going on inside.
The Electrical Standard: Power Supplies and Rails
Now we are getting into the electrical side of things. Do not worry, I will keep this approachable!
The Eurorack Power Supply: +12V, 12V, and +5V
Every Eurorack case has a power supply that delivers electricity to your modules. The Eurorack standard uses a dual rail power system, which means it supplies both a positive and a negative voltage simultaneously. Specifically:
- +12V (positive 12 volts)
- 12V (negative 12 volts)
- +5V (positive 5 volts, optional but common)
Most modules run entirely off the +12V and 12V rails. Some digital modules, especially those with microcontrollers or digital signal processors inside, also draw from the +5V rail. When you are shopping for a power supply for your case, you will see specs like “2000mA on +12V, 500mA on 12V, 200mA on +5V.” Those numbers refer to how many milliamps of current the supply can deliver on each rail.
Each module has a power consumption spec listed in its documentation, usually in milliamps (mA) for each rail. Add up all your modules and make sure your power supply can handle the total load, with some headroom to spare. Running a supply right at its limit is a recipe for noise in your audio and other strange behavior. A good rule of thumb is to stay at 70 to 80 percent of maximum capacity at most.
The Power Connector: The 16 Pin and 10 Pin Headers
Modules connect to the power supply using ribbon cables and headers. The most common connector in Eurorack is the 16 pin IDC header. This connector carries the +12V, 12V, and +5V rails, plus a ground connection and a few gate signals (CV bus and gate bus) that some systems use for communication between modules.
Smaller and older modules sometimes use a 10 pin header, which omits the +5V and gate bus lines. Many modern power buses include both 16 pin and 10 pin headers so you can accommodate either type.
One very important thing to know: the ribbon cable has a red stripe on one edge, and this stripe indicates pin 1, which is the 12V rail. Plugging a module in backwards can damage it and possibly your power supply. Always double check the orientation before powering up. Most modules have a marker on the PCB or the power connector to show which end is pin 1. When in doubt, check the manual. Seriously.
Control Voltages: The Language Eurorack Modules Speak
This is arguably the most important concept in all of Eurorack. Once you understand control voltages, the whole system starts to make intuitive sense.
What Is a Control Voltage?
A control voltage, or CV, is simply an electrical voltage that carries information. Instead of carrying audio, it carries data about how you want a module to behave. Think of it like a remote control signal: you send a certain voltage to a module, and the module responds by changing something about its behavior.
The most classic example is pitch CV. If you send a CV signal to the frequency input of a voltage controlled oscillator (VCO), the oscillator changes its pitch based on that voltage. Play a note on a MIDI keyboard that is connected to a CV converter, and the converter outputs a specific voltage that tells the oscillator what note to play.
The 1V per Octave Standard (1V/oct)
The most widely used CV standard for pitch is called 1 volt per octave, often written as 1V/oct. The idea is simple: every time the CV increases by 1 volt, the pitch goes up by exactly one octave. 0V might equal C1, 1V equals C2, 2V equals C3, and so on.
This standard is extremely well established and followed by the vast majority of Eurorack manufacturers. If you want two oscillators to track together in pitch, you just send them the same 1V/oct CV signal and they will stay in tune with each other across the whole keyboard range. Beautiful.
CV Voltage Ranges: The Wild West
Here is where things get a little more interesting. While pitch CV follows the tidy 1V/oct standard, other types of CV signals are much less standardized. Different modules and different manufacturers use different voltage ranges for things like filter cutoff, envelope depth, modulation intensity, and so on.
You will commonly encounter:
- 0V to +5V (unipolar, positive only): Very common for modulation sources and LFOs running in “unipolar” mode
- 2.5V to +2.5V (bipolar, centered around zero): Common for LFOs that swing both positive and negative
- 0V to +8V or 0V to +10V: Some modules, especially high end VCOs, accept wider CV ranges for better tracking precision
- 5V to +5V: Another common bipolar range
This variety is not a flaw, it is just the reality of an open standard. The solution is to have a few utility modules in your rack. Attenuators, attenuverters, and offset generators let you scale, flip, and shift CV signals so they fit perfectly into whatever range your destination module expects. Building a solid utility foundation in your rack is one of the best investments you can make as a Eurorack user.
Gate and Trigger Signals: Telling Modules When to Do Stuff
Alongside CV, two other signal types are absolutely fundamental to Eurorack: gates and triggers.
Gate Signals
A gate signal is a digital on/off signal. When the gate is “high” (usually at +5V), something is happening. When it is “low” (at 0V), it is not. The most intuitive example is a keyboard: when you press and hold a key, the gate goes high. When you release the key, the gate goes low. The gate signal carries information about duration, because it stays high for as long as you hold the note.
Gate signals are used to trigger and sustain things like envelope generators. An ADSR envelope, for example, uses the gate: the attack and decay phases happen when the gate goes high, the sustain phase lasts for as long as the gate stays high, and the release phase kicks in when the gate goes low.
Standard Eurorack gate voltage is 0V to +5V, although many modules will happily accept gates up to +8V or even +12V without any issue. Some older or European format modules use different logic levels, so again, check the manual when in doubt.
Trigger Signals
A trigger signal is similar to a gate, but it only carries a brief pulse rather than a sustained signal. Think of it like a single tap rather than a held button press. Triggers are used to fire off events instantly: they might reset a sequencer step, advance a clock divider, ping a physical model resonator, or fire a percussive envelope.
Trigger pulses are typically around 1ms to 10ms in duration, at +5V or similar. The key difference from a gate is that the duration of a trigger does not carry meaningful information, only the timing of the pulse matters.
In practice, many modules accept either gates or triggers at their inputs, and the module treats a short gate just like a trigger. The ecosystem is flexible.
Audio Signals: When Voltage Becomes Sound
The last major signal type in Eurorack is the audio signal itself. After all, this is a music making system!
Audio Levels in Eurorack
Audio signals in Eurorack are carried on standard 3.5mm mono patch cables, just like CV and gate signals. There is no separate connector for audio. This is actually one of the coolest things about Eurorack: everything is the same type of signal at the hardware level. The distinction between “audio” and “CV” is mostly conceptual. An LFO running at 20Hz is a control signal. Speed it up to 200Hz and suddenly it is in the audible range and you can use it as an audio source. The system does not care.
Eurorack audio signals typically have an amplitude of around plus or minus 5V peak to peak (often written as ±5V). This is significantly hotter than typical consumer line level audio, which is why you need a proper output module or interface module to convert Eurorack levels down to something your audio interface or speakers can handle.
Sending a raw Eurorack output directly into a consumer line input can work but may sound distorted, and in extreme cases could damage the input. Always use a dedicated output module or Eurorack to line level converter to get your audio out of the rack safely and at the right level.
Polyphony and Mono Signals
It is worth noting that virtually every signal in Eurorack is monophonic, meaning it carries a single channel of information. If you want stereo audio, you need two separate channels and two patch cables. If you want four voices of polyphony, you need four oscillators, four envelopes, four VCAs, and so on, all patched individually.
This is very different from a traditional synthesizer where polyphony is handled internally. In Eurorack, building a polyphonic voice means actually building and patching multiple voice chains. It is more work, but it also means total flexibility and control over every voice independently.
Everything at a Glance: The Eurorack Reference Table
Here is a handy summary of all the key standards and signal types we have covered:
| Signal or Standard | Description | Typical Voltage Range |
|---|---|---|
| Mechanical Height | 3U format, 128.5mm panel height | N/A |
| Module Width | Measured in HP, 1HP equals 5.08mm | N/A |
| Power Supply Rails | Positive and negative supply plus optional 5V rail | +12V, 12V, +5V |
| Power Connector | 16 pin IDC (most common) or 10 pin IDC | N/A |
| Pitch CV (1V/oct) | One volt per octave standard for pitch tracking | Typically 0V to +5V or wider |
| Modulation CV | General purpose control voltage, varies widely | 2.5V to +2.5V, 0V to +5V, 0V to +10V |
| Gate Signal | Sustained on/off signal for note duration | 0V to +5V |
| Trigger Signal | Brief pulse for event timing | 0V to +5V, ~1ms to 10ms pulse |
| Audio Signal | Audio in the synthesizer signal path | Approximately ±5V peak |
Please keep in mind that this table covers the most common standards. Individual modules and manufacturers sometimes have their own quirks and extended specifications. Always check the manual for the specific module you are working with!
Tips for Getting Started with Eurorack
Now that you know the fundamentals, here are a few practical tips to help you get started on the right foot.
Start with a complete voice first. Before buying individual modules, make sure you have the basics covered: an oscillator, a filter, a VCA, and an envelope generator. These four elements form the foundation of classic subtractive synthesis, and understanding how they interact will teach you more than any individual fancy module ever could.
Do not underestimate utility modules. Attenuators, mults, mixers, and offset generators might not be the most glamorous purchases, but they are the glue that holds a patch together. Budget for them from the start.
Think about your power supply carefully. Buy a power supply with more headroom than you think you need. It is much easier to grow into a capable supply than to deal with noise and instability from an overloaded one. A quality power supply is one of the best investments in your rack.
Plan your rack on paper or a planning tool first. Apps like ModularGrid let you plan your rack virtually, check HP usage, and estimate power draw before you spend any money. Use them. They save time, money, and frustration.
Patch everything. The real magic of Eurorack is not any single module. It is the patches. Experiment with using audio signals as CV, modulate your modulation sources, feed outputs back into inputs. The only way to learn is to patch and listen.
Final Thoughts
The Eurorack standard is elegantly simple at its core: consistent physical dimensions, a well defined power system, and a shared voltage language that lets every module communicate with every other module. That simplicity is what makes the format so endlessly expandable and exciting.
Whether you are building a compact desktop system for generative ambient music or a massive wall of modules for experimental performance, the same fundamental rules apply. Learn them well and the whole world of modular synthesis opens up to you.
If you have questions, want to share what you are building, or just want to geek out about patch ideas, drop a comment below or reach out via the contact page at solisynth.com. I love hearing about what people are making with their rigs.