DIY form factor

Alright, after reading a lot about the the different “standards”, I wanted to ask if there is maybe also some advice on which way to go when choosing a form factor for a DIY modular system. I know, it’s quite an opinionated topic but since I hesitate to fix I thought I ask you, experts.

I found Modular Form Factors - Analog Modular Synthesizers for Electronic Music by very helpful, it lists the parameters of all popular system but I am a bit confused and cannot decide.

I know, it’s a first world problem and everything can be changed or mixed together later, but still, I am interested in your thoughts about this topic.

For now, my current design decision is 6.3mm jacks, because I like those and the click when you plug then inn; I don’t want to fiddle around with small 3.5 jacks.

As a physicist, I love the metric system and it happens that you can easily buy cheap aluminium plates precut to round values (e.g. 100mm x 200mm), so I thought I just go with 200mm height, but on the other hand – given the small amount of modules I will build in the near future it’s not that important, so I guess many people go for a U-based system (where U=1.75") and have racks with 3U/5U/6U etc.

It’s probably a good idea to stick to this U-standard, just to be able to extend my system with other modules but on the other hand I also found that the Kosmo format is also using the 200mm system: STUFF - LOOK MUM NO COMPUTER and it claims to be compatible with the EuroRack system. How is that possible? Using adapter panels? The EuroRack is 128mm tall, according to that table above.

For the racks themselves the Adam Hall system is I think perfect and cheap. A 1m Adam Hall 6161 costs around 5 EUR. Some plywood or cheap furniture from Swedish companies might do the frame-job. The screw size is M6.
What system is Kosmo using? I read that it uses M3 screws?

A bit more difficult decision is the power supply. I see that there is a mix of +12/-12, +15/-15 and additional +5V and whatnot. Most of the circuit designs I created so far are will work with either 12V or 15V rails but I am curious what you do. An additional stable +5V rail is also a nice idea.
My first power supply is made out of a toroidal transformer (230VAC to 12 VAC) and the usual 7812/7912 regulartors. I try to design my circuits with +12V and -12V as lower/upper boundaries, just to be compatible with 15V systems.

Sorry if this is a procrastination-like thread, I am just a bit indecisive and waiting for my Arturia BeatStep Pro to finally arrive :see_no_evil:

Well, you’re in an LMNC forum :grinning:, so the (informal) Kosmo spec is a good start:


Kosmo is (nearly) electrically compatible with Eurorack — that is, to the extent they have standardized signal and control voltage levels (they don’t really) they’re the same, and they use the same power voltages and power distribution connectors except that Eurorack in principle also uses +5 V, but in practice almost never does. This means you can have both Eurorack and Kosmo modules and plug outputs from one into inputs to the other (you need some way to adapt between 1/4" and 3.5 mm cables), you can use the same kind of power supply for both, and you can take DIY PCBs designed for Eurorack and mount them behind Kosmo front panels, with some kludges if there are board mounted panel components. Most other established formats use different supply voltages or different signal/CV levels which makes it a little harder to mix them with Kosmo (or Eurorack).

Kosmo is not compatible in size with other formats. Kosmo size is incommensurate with rack units — you could build a pair of Kosmo rails into something that could screw into a rack, but there would be gaps or solid panels at the top and/or bottom. Very few Kosmo if any users try to do that, most use homemade wooden cases. So a mixed system with both Kosmo and Eurorack (or other standard) has to have separate cases/racks for the two. Or do this

to have a small number of Eurorack modules in a Kosmo case. (@d42kn355 might have some of those for sale.)

Eurorack is by far the most commonly used format. Its main drawback to my mind is that the module height is small, and while the width can be any multiple of 0.2" the Eurorack community tends to prefer module widths to be as small as possible, with the result that you have lots of panel components crowded into a small area, with very small knobs close together. I like my knobs larger and further apart. That’s the main rationale for the creation of Kosmo.

Kosmo is probably the most DIY friendly format. There are no built-and-tested Kosmo modules on the market, it’s DIY only at least for now. That DIY orientation plus the larger form factor means Kosmo modules tend to be designed to be easier to build than Eurorack ones. The number of Kosmo modules available is very small compared to other formats, but as stated above, it’s quite possible to convert almost any Eurorack DIY module to Kosmo format.

A lot of people have come into this thinking that, and then discovered how hard it is to stop…


Many thanks @fredrik I totally missed that specification post, that’s very useful! I’ll probably sit down and create some drawings, it’s easier to understand for me :wink:

@analogoutput yes, makes perfect sense. Thanks for the nice overview.

One question which is not mentioned in the specs: what kind of rack-rail is used in the Kosmo format? Are you using the Adam Hall rails with M3 screws and some special rectangle nuts? Or is there another format which is M3 by default?

You are probably right :see_no_evil:

hey up!!! yeah m3, you can even use eurorack spec rails just wider apart, knurlies etc.

however a tried and tested method is just wooden rails and wood screws!


I’ve been using Vector rails with M2.5 screws and slide nuts — the latter are hard to find but PulpLogic sells them. The Vector rails are cheaper than any other metal rails I know of when purchased in a package of 4 60" long, but can’t be used with M3 slide nuts unfortunately.

(There are vendors who sell these rails cut to smaller lengths, but at a higher cost per cm of course.)

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Or do like you did, mix kosmo and eurorack rows in the same case :slight_smile:

Those Adam Hall rails are the vertical rails for racks, not what is usually called rails in the modular synth world.
These are much more expensive…

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Ah, I misunderstood the question.

Most of us here build a wooden case, and either screw the modules into wood, or attach horizontal extrusion rails like the Vector ones.


Yes, this is what I meant. So those are called Vector? I will search for them… I initially thought I will misuse the Adam Hall ones but then saw that M3 is the Kosmo thing, I’d like to stick to that :wink:

…or that!

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Vector is one brand of rails, there are others.
But as @analogoutput said, other brands get even more expensive.
Or you could 3d print some :

Ah yeah, 3D printing is also a good alternative, I have not thought about that yet! I’ll probably do that first and upgrade if I feel they are too fragile. I only have PLA filaments…

I printed some in PLA, and they are fine.
Just don’t overtighten the screws…


I actually just printed a second set a few weeks ago since I like the first set I did so much. Just been really busy with life and enjoying the last of our nice weather when I can be outside so haven’t actually built a case for them yet. ( No, I don’t need a 3rd case yet…going to replace my 1st case since now that I’ve tried using printed rails I don’t want to deal with wooden rails anymore :smiley: )