Eurorack explained: what to know before buying your first case
Eurorack comes down to a few simple dimensions: 3U tall, widths in HP, a ±12 V supply. The basics you need to choose a case and power a module safely.
Modular is all about freedom: every module is a building block and you decide how they connect. Before the first module, though, comes a less exciting and more decisive choice: the case. It sets your available space, your power budget and, in the end, the size of your system for years.
The format in three numbers
Eurorack is built on Doepfer’s A-100 system, whose technical documentation is still the reference.
- Height: 3U. One rack unit is 44.45 mm (1.75 in), so 3U is 133.4 mm. Module front panels are 128.5 mm tall to clear the rails.
- Width: HP. One HP (horizontal pitch) is 5.08 mm, a fifth of an inch. Modules are sized in HP, from 4 HP for a slim utility to much wider complex oscillators.
- Power: ±12 V. A-100 modules run on +12 V and -12 V; some older modules also need +5 V.
The power ribbon: the one thing that does not forgive
Each module connects to the case’s power bus with a ribbon cable, in two versions: 10-pin, carrying -12 V, ground and +12 V, and 16-pin, which adds +5 V plus shared CV and Gate lines on the bus.
Remember the red stripe: it marks -12 V and goes at the bottom, on the side of the bus board printed -12 V. Doepfer’s own documentation sums it up as “red strip down”. Check the module side too, following the maker’s marking, before powering up.
Choosing a first case
Count HP, not modules
Cases are described by number of rows and usable width per row, in HP. Add up the widths of the modules you are considering and leave room: systems nearly always grow.
Check the power budget
Every module lists its current draw in milliamps on +12 V, -12 V and, where relevant, +5 V. Add them up rail by rail and compare with what the case’s supply delivers. A full but underpowered case leads to erratic behaviour, or worse.
Mind the depth
Module depths vary. A very shallow case narrows your options, so note its depth and compare it with the modules you want.
Learn synthesis first, then build
A modular does nothing until you patch it. The building blocks of a subtractive voice, oscillator, filter, amplifier and envelope, are exactly those of a regular synth. If ADSR envelopes or voices and polyphony still feel fuzzy, a semi-modular or a free software synth is a great training ground before you invest in a case. And when it comes to trade shows, Superbooth is where modular shines brightest.
For the full dimensions, Doepfer’s A-100 mechanical documentation covers heights and widths, and its power bus page describes the 10- and 16-pin ribbons.
My take
The case is the least glamorous purchase in a modular system and the one that shapes everything else. A slightly oversized case with a comfortable supply beats the opposite: modules can be sold on, an undersized power supply follows you everywhere. And the red stripe deserves a thirty-second check on every connection, even on your hundredth module.