Why did Doepfer choose ±12V?

Prompted by an exchange on Twitter: Why did Doepfer choose ±12V power for Eurorack?

While ±12V isn’t unprecedented — Serge used it for instance — it does seem that most previous synth formats used ±15V:

We now have a situation where there are lots of circuits out there which could potentially be turned into Eurorack (or Kosmo!) modules except that they were designed for ±15V and some modification usually is required. So what was Doepfer thinking when choosing ±12V? Are there clear advantages? Was it just a matter of “hey we have a good source for ±12V supplies, let’s use that”?


…or if it was 9 V AC supplies :smiley:


there is another very common product that is exported all over the planet by many manufacturers , cars. they mostly have 12 volt d.c. systems. usually red=positive , black=negative , green=ground . so why not use a system people are used to . I am sure there is a practical electrical engineering reason for their choice though . and as a side note it still baffles me as to why they would put a red stripe on the negative side that seems like it could be confusing to some .


Actually cars are 12.7v at rest… or 13.4V normally when running :wink:


of course the car guy , smart ass LOL :grin:

What does ‘Patch and Tweak’ say about this?

Nothing specific what I can see, except that it’s “based on a common computer and test equipment frame size”.

That refers to Eurocard, where he got the size (a common Eurocard size is a 160×100 mm PCB in a 3U subrack) and probably also the voltages – Eurocard itself is a physical standard, but common Eurocard buses like VMEbus and STEbus use 5 V and ±12 V.

Why ±12 V? No idea, the VMEbus spec says:


interesting , now were did Sam come up with the 20 cm hgt. for his boards ?

He said he found a source for 20 cm aluminum for panels.


and Kosmo was born onto this world …


To try to answer that question, I just entertained myself by skimming through a bunch of 1970s bus specifications, and it seems 12 V makes its first appearances with early NMOS circuits that used ±5 V and +12 V.

Before that, supplies were slightly more obscure, but the advantages of being compatible with TTL’s +5 V (from the early sixties) influenced a lot of designs, often requiring negative rails, positive internal ground, and inverted internal logic to make non-BJT circuits work but still produce TTL compatible output. Early PMOS processors like 4004 and 8008 used +5 V and −10 V rails, for example.

This doesn’t really answer why digital folks went for ±12 V and not, say, the more common ±15 V that you see in all opamp datasheets, but it might be that 15 V (or 15+5 or 15+15) puts you uncomfortably close to some MOSFET limitations. That issue for BJTs is the reason TTL is +5 V: you want a high voltage to have room for internal diode drops, stay away from the noise floor, etc, but not so high that you cause reverse base-emitter breakdown (which typically happens around 6 V).

Another reason might just be that everyone knows that 12 V is a nice and practical voltage – nine 1.5 V cells, six lead-acid cells – and engineers in the early seventies probably remembered cars switching from 6 V positive ground to 12 V (*) negative ground.

*) nominally 12 V, that is, I know it’s higher in circuit but I’m wearing my (amateur) electrical engineer hat here, not the (amateur) mechanic hat, the chemist hat (12.246 V under standard conditions!) or the airplane mechanic hat (14.0 V, obviously!).


Clicking around a bit more brought up a pre-TTL IBM document from ~1960 which uses transistor logic with ±6 V and ±12 V rails, so seems it was a well-known digital voltage long before early NMOS.


In school I was taught that the early transistor voltages were a result of Bell Labs teaching/licensing of transistor manufacturers in the early 50’s.

There was a US Army mil-spec document in 1954 that spec’d 6, 12, and 24 V standards for equipment, but it specified point to point voltages.

By the time NMOS rolled around, +/- 6 and +/- 12 were in common usage by Bell Labs and other computer companies. +/- 5 seems to have sprouted around the time of NMOS.

Needless to say, by the time Doepfer was messing with eurorack, the +12, -12, +5 power supplies were pretty ubiquitous, and standard in a lot of the racks that already existed.

Going to the 1V/octave standard (from the 1.2) from what Moog and others were using seems to be a convenient change…similar range.


I feel like Eurocard and the various 19” Subrack standards are a secret in the Eurorack world.

Nobody realises you can buy extrusions and all the fittings dirt cheap from RS etc, and instead buy overly expensive cases with mediocre components and PSU :man_shrugging:t3:

I’ve also got some beautiful ex-MOD 3U subrack PSU that’s -12/12/5. I wasn’t going to use it for audio thinking it wouldn’t be clean enough etc, but then dawned on me that these are designed for medical equipment, precision signal equipment etc so are way higger spec. So I’ve started building a 6U chassis which the PSU plugs into from behind (on runners with a connector it mates with), filtered IEC socket and then will use some very nice bus board PCBs I picked up.


I’ll also add that I came from Subracks and that kind of kit, and discovered modular synths via that :joy:

Handily I have a few crates of subrack components, as I love using them for all sorts of projects.

What’s RS?

Cheapest extrusions I know of are Vector TS600, at around $65 for a package of four 60 inch pieces.

RS components, I assume: www.rs-online.com

We are the world’s largest distributor of electronics and
maintenance products.

(cannot help thinking of the first minute here when people talk about professional eurorack enclosures…)


Yeah, RS Components. Between them and Farnell, they’re the biggest suppliers in the world.

Mouser is just unbelievably expensive… Rapid is good, much smaller than RS/Farnell, even with them now being owned by Conrad.

Fun fact, my office faces RS’ headquarters, and if I walk 30seconds from my house, I can see the other side of their building :see_no_evil:

I don’t know who Vector is, but the biggest manufacturers of rails are Nvent and Rittal. Between them they’ve bought up most manufacturers. Vero (two companies with one name, one makes PCB the other makes enclosures) are the next biggest.

A pair of 84hp rails are normally about £6-10 each depending on what function they are (there’s lots of options in the subrack world, but Eurorack just needs the most basic).

Doepfer use a German brand of subrack, that I think is owned by Rittal now. They’re pretty cheap too, but that brand is harder to get in the U.K. now.