> What are the unit dims?
From Doepfer
> What’s the standard supply voltage/voltages?
+/- 12V.
Assuming you’ll get exactly a clean +12V / 12V from the rails is not a good practice actually they can be away from this ideal value or they can fluctuate with temperature, etc… Whenever you need a particular voltage (for scaling/offsetting circuits for example) you’d better go with a voltage reference.
If your module needs 5V (for MCUs, etc.), there are two options:
- either a regulator on your board - taking the +12V as an input. Problems: heat dissipation, digital noise isolation;
- or use the +5V provided on the bus-board. Problem: not all cases deliver +5V - hence the existence of products like this
> What’re the standard connections at the back?
There is no standard as to where you should put them.
If you only use +/- 12V, you can use a 2x5 pin connector:
If you need the +5V, or the main CV/Gate lines, you can use a 2x8 pin connector:
There are holy wars about the use of shrouded headers. I don’t use them, and explicitly label on the PCB where the “red stripe” (-12V) of the power connector should be.
Everything else (calibration trimmers, expander ports, programmer/JTAG ports…) as you wish.
> What’s the norm for jack sizes?
3.5mm. If you decide to have your PCB parallel to the panel (a wise decision), a commonly used part is this. Another common part is this. Both are total pain in the ass when it comes to industrial production.
> How do you callibrate occilators etc. to 1v/8ve?
Not sure if your question is about:
- What is the scale standard? It is 1V/Oct, Doepfer uses 0V = midi note 36 but there’s no consensus about that.
- What is the preferred component for calibration? There is no standard. Multi-turn trimmer, digital calibration procedure, it’s up to you…
- How to make a circuit follow the 1V/Oct standard? For oscillators and filters this requires the presence of an exponential converter (either as a discrete circuit built with a transistor pair, or through the use of a chip with a built-in exponential response like the 2164) for the “V/Oct” part, and the use of appropriate ratios of resistors at the input of the circuit for the “1” part.
> Is there anything else I need to know?
Gate/trigger level was originally (Doepfer) HIGH = +8V, LOW = 0V - though an increasing number of modules use +5V as the HIGH level - mostly due to the proliferation of digital devices 
As a result, your gate/trigger inputs should better have a low threshold (I use a transistor inverter, so my threshold is one diode drop).
Input impedances are expected to be 100k. I rarely deviate from this norm except when the circuit uses some weird normalling paths which can occasionally half it. Corollary: you’ll find it more convenient to put attenuverters (aka “polarizers”) on your CV inputs than mere attenuators.
Your circuit is supposed to behave well when a strong signal source is connected by mistake to one of your outputs; and when a reasonably long stretch of cables is attached to the output. This is often achieved by having a 1k resistor in series with all outputs (protecting from external signals and preventing the op-amp output to directly “see” the capacitance of the cable) - the drawback being that it’ll cause a voltage drop of 1% into each 100k impedance input it is plugged to. Another approach is to have the 1k resistor in the feedback loop of the op-amp (no drop!), with all the associated annoyances.
Your module is supposed to behave well when the power connector is connected backwards. Solution 1 is to have Schottky diodes in series with the 12V / 12V lines. Solution 2 is to use a polyfuse, and schottky diodes to ground in reverse configuration (when power is plugged backwards, all the current is sucked in the path of least resistance through the diodes rather than elsewhere, and they make the fuses temporarily “blow”). Both solutions equally suck in that they cause a voltage drop proportional to the current used by your module that’s why you shouldn’t rely on the rails being exactly/-12V, especially if you make things blink (Use the +5V rail or an onboard regulator to power anything that blinks!).
My modules are ideally designed so that I can run my pinky in the space between knobs without touching them (not always, but I start from there). Modules from other manufacturers sometimes don’t pass this test and I consider them too crowded.
