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Electrical specifications for new designs
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Electrical specifications for new designs
pepijndevos
Inspired by Wintergatan's Modulin, I've been busy building my own little synths and filters.
Since I'm more of an engineer than a musician, I figured I would start building my own modules from scratch, rather than making a large investment in other people's modules or even DIY kits.

So alright, I've been reading up on how to make voltage controlled everything. Mostly using operational transconductance amplifiers it appears.
I'm sure I'll figure that out. But then I started to look for the CV "standard", and it kind of seems there is no such thing. Or rather, a lot of incompatible things.

Wikipedia is not very helpful at all, and is mostly about octave/volt or hertz/volt, and rising/falling edge triggers.

As far as voltages are concerned... every source seems to say something different.
The most authoritative source seems to be the Doepfer DIY page: http://www.doepfer.de/a100_man/a100t_e.htm

This page lists
Audio signals of 10Vpp from -5V to +5V
Control signals of 5Vpp from -2.5V to +2.5V or 0V to 8V??
Trigger signals from 0V to 5V, rising edge

For triggers, it's a digital signal, so you can accept anything.
For audio signals, the exact voltage is not too important, as most modules will most likely be able to set the gain or have unity gain. As long as your modules accept a fairly large swing, this should work out.
But for control signals I'm horribly confused.

Let's say I have an VCA, and I want to feed it either an 0-8V envelope for normal use, but also be able to feed it the +/-2.5V LFO for a tremolo. How is that ever going to work if 0V=off for the envelope.

Something I've read absolutely nothing about is if there is any specification on for example input impedance. I think I read somewhere typical audio stuff is 20k input impedance.

And final question is about the backplane pinout.
There is +/-12V, 5V, 3 grounds, and two bus lines.
5V is convenient for various IC's, but it seems not all cases provide it. Is this common, or should new designs just include an LDO down from +12V to 5V?
Why are there 3 grounds? Just for power, or is for example the 5V gnd floating wrt the 12V?
Do these bus lines see common use?
kassu
The "standards" in modular in general, and eurorack in specific, are very loose and vary between manufacturers and modules. There is not much one can do about this.

As far as I know, the most commonly adhered to standards are:

- Nominal audio level is 10Vpp, +5V to -5V. Almost all VCOs output this level, and although you can attenuate and amplifiy it as need in your patch, I would stick to this as the nominal ("0dB") level.

- Trigger/gate/logic signals are 0V low / at least 5V high in most cases. If your module outputs 5V signals, it will work for most modules but not all; if you output something like 8-12V it will work for all modules. Conversely, if your inputs have a trigger level well below 5V, they will work with all trigger/gate signals. To be sure, I like to put my trigger level somewhere around 1 to 2 V. It is also recommended to add a bit of hysteresis.

- Control signals come in many forms. Envelopes are usually unipolar, 0-5V or 0-8V. LFOs are usually bipolar, 5Vpp or 10Vpp are common. This combination of unipolar and bipolar is not really an issue, since in actual use you will need to scale and offset the control signals anyway to make them musically interesting. Especially on a VCA, you will want to have an attenuator and offset control on the CV input so you can dial in just the right kind of tremolo, envelope or some other exotic form of control that you have yet to invent.

This leads to a general advice: a good modular setup should have loads of "utility" modules for combining, offsetting, attenuating/inverting signals.
Gandalf
pepijndevos wrote:
Inspired by Wintergatan's Modulin, I've been busy building my own little synths and filters.
Since I'm more of an engineer than a musician, I figured I would start building my own modules from scratch, rather than making a large investment in other people's modules or even DIY kits.

So alright, I've been reading up on how to make voltage controlled everything. Mostly using operational transconductance amplifiers it appears.
I'm sure I'll figure that out. But then I started to look for the CV "standard", and it kind of seems there is no such thing. Or rather, a lot of incompatible things.

Wikipedia is not very helpful at all, and is mostly about octave/volt or hertz/volt, and rising/falling edge triggers.

As far as voltages are concerned... every source seems to say something different.
The most authoritative source seems to be the Doepfer DIY page: http://www.doepfer.de/a100_man/a100t_e.htm

This page lists
Audio signals of 10Vpp from -5V to +5V
Control signals of 5Vpp from -2.5V to +2.5V or 0V to 8V??
Trigger signals from 0V to 5V, rising edge

For triggers, it's a digital signal, so you can accept anything.
For audio signals, the exact voltage is not too important, as most modules will most likely be able to set the gain or have unity gain. As long as your modules accept a fairly large swing, this should work out.
But for control signals I'm horribly confused.

Let's say I have an VCA, and I want to feed it either an 0-8V envelope for normal use, but also be able to feed it the +/-2.5V LFO for a tremolo. How is that ever going to work if 0V=off for the envelope.

Something I've read absolutely nothing about is if there is any specification on for example input impedance. I think I read somewhere typical audio stuff is 20k input impedance.

And final question is about the backplane pinout.
There is +/-12V, 5V, 3 grounds, and two bus lines.
5V is convenient for various IC's, but it seems not all cases provide it. Is this common, or should new designs just include an LDO down from +12V to 5V?
Why are there 3 grounds? Just for power, or is for example the 5V gnd floating wrt the 12V?
Do these bus lines see common use?

There is no true specification for EuroRack but many people follow the A100 guidelines from Doepfer who really established what we recognise as EuroRack. I have taken that and others and generated the EuroSynth Specification which tries to define a more electronically/mechanically sensible approach yet maintain compatability with EuroRack
m.o
In your example with a VCA + LFO, that's more or less "how it is", if you wan't to use the lfo's full swing you'll have to add an offset (maybe a good idea to have built in in an LFO, or you could have the input voltage added to the manual set level (with a knob) on the VCA).
Whatever voltages a module makes use of on input, it should be designed to "handle" anything within the power rails (or at least +/- 10V).

Regarding input impedance, I think most aim for something like 100k.
Many people see it as prodent to also have a low value (100 - 1k) in series on the outputs of modules (to prevent a dead short on the other end causing problems).

The power connectors, busses are most often implemented with these IDC connectors + small gauge flex cable + PCB traces (on bus board) which isn't optimal, you want as low resistance to 0v as possible (hence several parallell 0V connections).

I'm not sure how common 5V is in cases, I know many more "professional" modules have their own regulators on board though.
Grumble
Gandalf wrote:
There is no true specification for EuroRack but many people follow the A100 guidelines from Doepfer who really established what we recognise as EuroRack.
I have taken that and others and generated the EuroSynth Specification which tries to define a more electronically/mechanically sensible approach yet maintain compatability with EuroRack

Thank you! very helpful! we're not worthy
mskala
Voltage range has more than one meaning. There's the range of voltages the module is intended to accept or generate, such that it will operate normally if given those voltages; there's the range of voltages over which it will basically work, but may not operate normally (for instance, because it may produce a lot of distortion); and there's the range of voltages over which it may not work at all, but at least won't be damaged. These three ranges can all be different from each other.

I think it's important, given the abuse to which people subject synth modules, that a module should not be damaged if given any voltage within the power supply limits. That means shorting any input or output to any fixed voltage from -12V to +12V (for Eurorack) should not harm the module. But many modules in the wild, especially DIY modules containing microcontrollers, are in practice capable of being damaged by shorts to voltages outside a smaller expected or intended operating range.
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