Faderbank Run: See top post for status


Yes. The current build has individual cv outs. It’s magical.


Thanks to @scanner_darkly for pointing to the source thread for the teletype/grid information - I should have thought about quoting it here. Also it might be a question to be asked in the norns thread regarding the available USB power. I just have no idea how much current a eurorack module might be drawing into its CV inputs and how it sums up at the USB.

Also what comes to my mind is that there was an issue with Mangroves damaging an intellijel module because the run input was directly connected to the raw 12V power rail, I think.

To be honest I have no idea what the opamp buffering or an output capacitor do or if they already address these questions. :grinning:


OK - so the issue in the TT thread could be described unexpected consequences: a regulator that is perhaps underpowered for delivering 5V to a grid, suddenly experiencing excess current draw, and failing. And given the TT design, no 5V = not much of a Teletype system.

A grid at full brightness draws much more current than any Eurorack module I can think of through their CV inputs (which isn’t the same as the current draw on a PSU).

However: I have no idea what the Norns power situation is, and am not the person to ask (and this perhaps isn’t the thread for that).

So: the issue there was with Just Friends, and it was with an input being normalized to the -12V rail without an impedance resistor (so as to detect when a jack wasn’t plugged in - picking a voltage likely to be unavailable outside the circuit, I’d guess). As a jack scraped past, it might briefly make a connection to said normalized jack, and without an impedance resistor, -12V at a lot of current shot through. If you’re not expecting that at the other end, that could be problematic.

There are loose opinions around input/output impedance resistors in Eurorack, but there are also reasons to implement alternative opinions, mainly down to context.

But regarding this:

Would it be okay regarding USB power/current if all 16 faders output 5 V to the modular while it takes the power from norns?

the answer is still, technically, “it depends what’s at the other end”. Current, not voltage, is the issue.

I mean, personally, I’m reasonably confident there won’t be any other issues, but if you want an 100% correct answer, what it comes down to is what’s powering the faderbank and if the inputs at the other end have some kind of impedance resistor. Most of the time, both of those aspects will be absolutely fine. But I cannot actually confirm how every single Eurorack module in the world works, and if they’ve been designed correctly.

Sure, but you care enough to ask if it is “safe to plug the faderbank into norns.”, which manages to immediately cast the quality of either object into doubt, and which puts people like me on the defensive immediately. And it feels from the questions that answers involving the phrase “it depends” won’t come over as satisfying.


It was not my intention to cast the quality of anything into doubt and the notion that I “care enough” to pose a question for mere discouragement is way off!

I openheartedly admitted that, being a layman, I did not understand the technical part of your answer (opamps, capacitors…). I struggle a bit to understand how this could have come across in a offending manor. At least I had no hard feelings when I posed the question.

Also I admitted that I have no idea (in the sense of absolutely no idea) how much current a CV input might draw. What is wrong with asking then? To me it seems more cumbersome to just try it out and then getting told, uh, you should not have done it cause it wasn’t intended by the design. I hoped for a framework like: In a common modular environment using all 16 CV outs of the faderbank to arbitrary inputs might typically draw up to xxx mA from the USB power bus - be sure the device you connect it to can provide this and keep in mind that this might vary with the individual design of the used modules. I might be a bit dumb but I still think this would be a useful information.



In which case: I’m sorry for being prickly and, as I say, on the defensive. My goal for the faderbank is that it feels like it should be clear you can just plug it into whatever. But you’re right to call out that the modular environment beyond it is a little bit beyond our control.

I guess by ‘care enough’ I just meant: you are concerned enough that this might be an issue to ask; whilst it may not be the intent of your question, I - as a co-designer - feel a bit defensive as a result. I know I acknowledged this, but I’m sorry that we’re both a little on edge as a result.

To respond to a useful clarification you made -

I don’t think I can provide that clarification. Emphasis on “I” - perhaps other engineers on lines could assist. But it becomes such a generalisation that it ends up being too vague to be useful?

I think, however, some plain language around how the CV outputs work might help - just something to indicate their relation to the USB power circuit. If I were powering it from an adapter, habit would lead me to using a 1A USB adapter Just Because, but I know that the USB out of eg a Macbook Air tends to be around 500-800ma iirc, as a maximum.

So: I think you’re right that I need to find a better answer than shrugging and going “it depends”, but I’m scrabbling for what that might be. And beyond that, apologies for cross-purposes and feeling prickly. Sometimes making things feels… very exposed.


as someone with not much stake in this, i’ll give it a try.

[udpdated] that should limit the current per output to 10mA, total for all channels 160mA, so nothing to worry about at all.

“typically,” modular synthesizer circuits do not draw significant power from control voltage busses, and this just isn’t an issue at all. they get their power elsewere; the CV inputs are passed through a resistor and scaled down to a much smaller range than 10v before actually being used in a circuit. 100k ohms is a very common value for input resistance.

the general problem with such pronouncements, is that there are no standards for this. there’s nothing stopping someone from making a module design that is totally insane, like powering lamps from its CV inputs or something. but in general this should not be a concern.

note also that we assume the destination voltage is correct and isn’t, say, accidentally normalled to -12v (which can happen, though it’s not supposed to.) that still shouldn’t hurt the teensy in this design (and i doubt it would hurt anything else.)

the issue you reference with TT and grid is very, different. the grid does take its power from its usb input, and in extreme cases can be a lot of power that the TT wasn’t designed to provide. hence monome’s official recommendation to provide the power from elsewhere (via an ‘offworld’ circuit or something like it.)

even so, the one instance of TT regulator failure was IMHO a fluke, not an expected outcome for that scenario.

anyways, faderbank current draw should not be at all problematic regardless of what you connect the CVs to, because of the high output resistance. (necessary caveat that of course its possible to exceed the ratings of everything - don’t try to jump-start your car with your USB device.)


Thank you for your detailed answer, @infovore . I can see that sometimes you might need a thick skin when you expose yourself and I really appreciate how you share your process.

Also I am very much looking forward to build and use a faderbank. In fact I am a bit excited about it. In my imagination the grid, arc and faderbank are portable, easy to set up, focussed (or clutterfree) and playable control surfaces and complement each other well when controlling something like a modular synthesizer or norns/computer.

Yes, it actually ‘feels like I can just plug it into whatever’ so learning that you can not just plug a grid into anything with a USB jack felt a bit unsettling and I tried to get a better understanding of what is possible with these devices and what not. Or if the setup I imagine would have to be extended with several auxiliary USB power sockets and another power strip for example and how portable and easy to set up it would be in reality.

I did not want to sound sceptical or devaluating and I apologize if I did. It’s more that feeling left alone with a vague “it depends” reminds me at two main classes of experiences when asking modular manufacturers things like this. Either someone does not want to be taken responsible for the bad design or mistakes of other people (which I can understand) or there is an actual issue touched that someone is already aware of but does not want to openly talk about cause there is no fix for that. I am not saying or implying that this is the case here, just want to explain why I got a little more insistent, which might felt offensive or devaluating to you but was not intended to do so.

Thank you @zebra for the insight and education your giving here (and elsewhere)! I understood that there is not much to worry and that this is a totally different case than the grid/tt issue, somethings are still to technical for me to understand and it would be great to get a better feeling for what is possible.

I don’t understand what the 100k ohms mean practical. Could you make any assumptions abut the ballpark of non significant power we are in here? I understood that the faderbank itself needs around 150 mA. With all CV outputs connected to typical and well designed module inputs and all faders up - is it more like 5 - 10 mA per channel or more like 5 - 10 mA for all channels at together i have to add here? I know we are talking about assumptions and common sense here.

Not sure about this - it sounds like it is not recommended to connect the CV out to -12V source, but this might be too obvious? Also I thought the CV outs were not connected to the teensy but directly to the USB power inlet? (also no idea what would happen if someone connects that to -12V source jack).


No, the 150mA, to my understanding, is a current draw the faderbank (like any synth module or desktop device that doesn’t have a zillion lights) can always be expected to pull regardless of what it’s doing.

the 100k input resistor will scale down incoming CV to a point where it’s usable without damaging the module. as @zebra says, this has nothing to do with faderbank. Rather, it’s a typical (but not required) feature of module design. Also, CV outlets would be connected to the teensy, not the power supply

plugging into -12V is a reference to a past incident in Eurorack and not an expected occurrence. theoretically another module could expose the -12V rail on a jack and if the faderbank (again, or anything) wasn’t prepared, things could go badly for it when connecting it to that jack


(also, I realised this morning: the output impedance resistance on a faderbank channel is 1Kohm, not 100ohm.)


Did you say … 1 Kohm?



Used to filter down the number of electrons.


@Leverkusen the TL/DR is that current draw for the faderbank is not something you should have to worry about at all. it will max out at something around 200mA - and that would be in a strange situation where every single connection to the cv outputs was drawing significant power.

in the spirit of “teaching to fish” - to get current (in amps), divide voltage (in volts) by resistance (in ohms.) this relationship is very much worth remembering.


I’m in for DIY or prebuilt!


I am interested in a kit version of this… Be it PCB w/enclosure, just PCB, or full Kit.


so it sounds like it should be safe to power faderbank from teletype. in such scenario, is it possible for faderbank to communicate with teletype over usb as well? this would save a cable (since you wouldn’t need to plug i2c), and this could also mean that faderbank could push updates to teletype instead of tt polling fb from the metro script.


twenty chars of midi ops?!!


we should probably create a separate thread for potential teletype/faderbank ops.
i’m curious if it’s technically possible though


should be fine. either as midi or tty or hid


perfect, so this means faderbank/norns will also require just one cable without the need to go via i2c or midi.


you’re of course right. but since faderbank emits midi over usb, I was just developping further… and dreaming :slight_smile: