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Thread: Control Voltage and the Eurorack Ecosystem

  1. #1
    Junior Member
    Join Date
    Jan 2019

    Control Voltage and the Eurorack Ecosystem

    Hello everyone. A bit of a noobish question, but that's why I'm here. I'd like to start tinkering with DACs in order to produce CV, and could use a point in the right direction (suggestions, resources, tutorials, recommendations, etc).

    I'll soon be using a small power supply (FC Microbus) which will provide me +/-12v and +5v. To dip my toes in the world of CV, I figured a fun project would be to control parameters on a Behringer Neutron, which will accept input voltages of -5V to +5V for some parameters, and -9.5V to +9.5V for others. If I understand correctly, this can be done with a combination of the aforementioned power supply, Teensy (a 3.2 in this case), and a DAC of some sort.

    Being so terribly new to this, I'm not familiar with conventions, standards, component selection (12 bit? 16 bit?), and if there are any essential "go to" ways of producing CV such that it can somewhat-universally function in the ecosystem of eurorack.

    Any suggestions greatly appreciated

  2. #2
    If you have a 3.2, it has a built- in 12 bit Dac so you should start with that. 12 bits is fine for CV. Try the audio library out. Connect a sine or waveform object to the dac object, set the frequency very low, and blam! theres an LFO.
    The dacs output range is 0 - 1.2v so youll have to add gain and voltage offset to get it to euro ranges. Take a look at the Neutron Sound Orgone Accumulator schematic for inspiration.
    Also check out the Ornament and Crime schematic, they use a quad dac (which unfortunately isnt an object in the audio library) but is open source so you could customize it to your needs, theres a fellow doing a Hemispheres suite of all kinds of apps for it.

    So basically start simple with the built- in dac and experiment with the audio library. If youre unfamiliar with electronics look up basic op amp uses: gain, simple low pass filters, and voltage offset, inverting and non- inverting.

  3. #3
    Senior Member
    Join Date
    Feb 2013
    fwiw, there's probably no "go to" way, but you'll probably find there's only so many ways you want to do this. it also somewhat depends on what it is you want to do (pitch or LFO/envelope type CV). your basic two options are: use PWM + filter or a DAC + output stage/filter.

    the former has the advantage that it's kind of cheap, but PWM absolutely requires a heavy-duty sort of filter; with many output channels, that easily adds up to quite a bit of PCB real estate. it's ok for LFOs and the like (Xaoc Devices "Batumi" would be a popular module based on this approach), less so for quantized/pitch output, because chances are that you'll end up with a (too) slow slew rate.

    DACs typically make life easier; there's plenty to choose from, obviously, with different interfaces, bit-depths, packages. re DACs your main considerations probably are the following:

    - are you on a budget?
    - are you comfortable with SMD?
    - what's your use-case? pitch and/or continuous modulation?
    - how many channels do you need/want?

    the main question is probably going to be "pitch and/or continuous modulation", that'll have implications re the type of DAC you want to use, and the kind of output circuitry that goes with it.

    for pitch, basically, you want to make sure the outputs are compensated "in the loop" (*), to minimize pitch error; you'll find that kind of output stage in modules meant to output pitch CV, like the above mentioned OC or Mutable Instruments Yarns, both of which are open hardware. there's no good reason not to use a 16 bit DAC, or at least a 14 bit one. it'll make life a lot easier. (on the interwebs, you'll find a lot of contraptions with 12 bit DACs and lots of trimpots. YMMV, but IMO that's a waste of time)

    for non-pitch, slew rate and output impedance / error is less of an issue; more so, sample rate. in this case, high bit depth doesn't hurt either, but it's less of an issue. you can compensate with an output filter. for example, take a look at the Mutable Instruments Tides schematic.

    if you want both, pitch and LFO type modulation, i'd go with "in the loop" compensation; it doesn't really filter the outputs, but with enough bit-depth (16 bit) it'll give decent enough results even for LFOs (in most scenarios)

    * "in the loop" here/for all practical purposes means the output resistor is inside the feedback path (it'll also have a small, 20p or so, cap in the direct feedback path, which is there to pre-empt oscillations). that's easy to get wrong, because the great majority of eurorack modules you'll find have a (typically) 1k series resistor at the output, which will result in pitch errors, because the upstream modules (again typically) will have a 100k input impedance.

    as for output range or gain/offset, that's more of a secondary consideration; you'll probably have to study some schematics + read up on op amp circuits a bit to figure out how that's done.

  4. #4
    Junior Member
    Join Date
    Jan 2019
    thank you both for this info and your suggestions, it's so valuable to me that I couldn't possibly thank you enough.

    The PWM method possibly having too slow a slew rate is important since I'd prefer the core mechanism be optimized for high rates, and allowing for slew adjustments later/elsewhere in the CV chain. DACs seem to be the way I'm gravitating.

    In response to the questions/considerations. Cost is certainly important, though with diy options, investing in better methods seems worth it when the final creation ends up with a lesser number of perceivable limits. As for SMD, it seems that learning this might eventually be necessary to keep exploration options open and cost down (after the upfront additional equipment investment), sometime after this initial exploration of cv. As for channels/wants/needs, I don't yet intuitively know what my needs or wants might be, though more time swimming in a sea of CV will facilitate bumping into some of the not-yet-felt limits. I come from a Tracker (Renoise) background, where if one hits a limit or discovers a need, the solution is to learn LUA scripting

    Interestingly I stumbled across this in-the-loop resistor method while looking into accuracy with buffered mults, for which I've ordered a few TL074 quad op amps. Very excited to see you mention this, I'll spend the weekend getting acquainted with the concept, and also cutting my teeth on an MCP4922 12-bit DAC.

    Before designing something myself, this project by little-scale ( seemed like an appealing (and capable) way to get something up and running. I reached out to him on a whim last night and he was kind enough to send me a PCB

    Somewhat of a side note, finally looked into Musical Applications of Microprocessors by Hal Chamberlin and it's great!

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