Why Eurorack?
Since the dawn of time people have been making things, but it wasn't until people started making things that were designed to work with things made by other people that standards were introduced.
Standards are necessary to make sure the plug on your electrical appliance fits in the power socket, and the power supply is just enough to make it work but not too much to make it burst into flames. But what about that power ribbon that connects your module to the power supply with the red stripe on it, what's that all about, and what about the actual dimensions of a module, that kind of imformation would come in handy if you want to build your own case or even your own modules.
As with most new technologies, especially those involving electricity, standards can vary quite a bit depending on what format you choose to use, and when it comes to modular synthesizers there are several formats to choose from, each with their own set of standards.
EuroRack
The Eurorack format is arguably one of the most popular modular synthesizer formats, due in part to the compact size of the modules and the availability of different modules.
Specifications:
Height: 128.5mm (5 inches)
Power: +/-12v and +5v
Jacks: 3.5mm<
Moog Unit (MU)
The large format Moog modules comprised the original System 55 instrument released in 1973. Made popular by electronic music pioneers such as Wendy Carlos and Keith Emerson the Moog format is currently available from synthesizer.com and Moog Music.
Specifications:
Height: 222.3mm (8.75 inches)
Power: +/-15v and +5v
Jacks: 1/4 inch
MOTM
Specifications:
Height: 133mm (5.25 inches)
Width: 75.7mm (3 inches)
Power: +/-15v 4-pin mta-156 connector
Jacks: 1/4 inch
Frack Rack
Blacet Research, PAiA Corporation
Specifications:
Height: 133.4mm (5.24 inches)
Power: +/-15v and +5v
Jacks: 1/4 inch
As with most pioneering technologies these standards are often used as a basic guideline and expanded upon, for example Buchla also uses Banana style jacks and produce modules that are shorter and wider than their closest relative the EuroRack. Likewise Serge also use Banana style jacks yet their modues are over a foot wide. Mattson Mini Modular even produces a Micro Mini format with unique dimensions while conforming to the CV and power specs adopted by most other manufacturers.
Here's a rough guide to the heights and widths adopted by most modular synthesizer manufacturers.
HP (Horizontal Pitch) 5.08mm (0.2 inches). Orientation: Horizontal. Format: EuroRack.
3U 133.4mm (5.75 inches). Orientation: Vertical. Format: EuroRack*, Blacet, FracRak, Blacet.
4U 177.8mm (7 inches). Orientation: Vertical. Format: Serge.
5U 222.3mm (8.75 inches). Orientation: Vertical. Format: Moog Unit, Synthesizers.com, MOTM.
6U 266.7mm (10.5 inches). Orientation: Vertical. Format: Wiard.
It's important to note that the vertical 3U measurement doesn't actually apply to the Eurorack module because they are usually mounted to a pair of extruded aluminum mounting brackets, one at the top and one at the bottom, that have a 2.5mm wide lip so the actual height or U of a EuroRack module is 128.5mm.
Another fun factoid to take into account when dealing with these fuzzy standards is the original Moog modules had a width of 2.125 inches (54mm) and were hardwired into the frame, unlike the Synthesizer.com modules that can be hooked up to the power supply using ribbon cable.
