Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

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tatou
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Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

Post by tatou » Sat Sep 12, 2020 9:34 pm

I'm a longtime DIY pedal builder who is now exploring Eurorack. One of the deals I made with myself is that 50%+ of my modules need to be DIY (inherited rule from the pedal library), and that I should pass on effect modules (for now anyway), since I already have dozens of pedals. So I'm interested in building a useful interface to bridge the two, in particular a 9v-powered pedal I/O of sorts.

Turns out what I've been thinking about does exist: the INTRFX! However, in the DIY spirit I would prefer to build a workalike myself (and I probably don't need 3 channels). The circuit for this SYINSI Pedal IO module seems like a good starting point...

In particular, the schematic is here:
Image

But 3 changes are necessary:
* add a switch to toggle the direction of audio signal (EURO >> PEDAL or vice versa)
* convert from 12v bipolar to 9v unipolar power supply
* modify the "WET/DRY" mix to be CV-controlled (and use the pot as an attenuator when CV is supplied?)

The toggle seems simple enough with a 4PDT. As for the power, it seems to me that swapping in a standard 9v pedal power supply (with proper filter caps & polarity protecting diodes) should be fine? I don't see anything here requiring a bipolar supply and the TL072 should run fine on 9v. I'm asking here I guess as a sanity check... but about general best practices for adapting other modular circuits to guitar effects as well (filters?).

The biggest head-scratcher for me is CV control. Presumably one could use an LDR, OTA chip, or JFET as some sort of variable resistor, but I'm not advanced enough to figure that out... or how to repurpose the "WET/DRY" pot as a CV attenuator when voltage is supplied.

Any pointers or guidance is welcome, thanks!!
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Re: Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

Post by nigel » Sun Sep 13, 2020 12:03 am

Welcome to the madhouse, tatou!

Can you explain why you want those changes?
- aside from the wet/dry mixer, the circuit already consists of separate synth to pedal and pedal to synth sections, what are you trying to switch?
- if you run it from the pedal 9V supply instead of the synth +/-12V supply, you won't be able to provide a decent Eurorack level signal.
- look up VC mixer or VC panner circuits for the wet/dry mixer section.

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Re: Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

Post by Jay F. » Sun Sep 13, 2020 2:06 am

Hello,

I'm also a bit confused about the need for the DC/DC converter. If you intend to implement that circuit as a eurorack module, you will not need the 12V to 9V conversion : it will run fine on -12V/+12V.

I do understand the need to Eurorack >> Pedal +> Eurorack, which the SYINSI IO covers nicely. ( +> is the mix between the two signals)
I'm a bit puzzled by the Pedal >> Eurorack +> Pedal need. You might simply want to do that with separate modules and use cables as the switch.

Granted, the CV control thing is the tricky part here.

Here are my takes on the same problem.
I adapted a guitar FX circtuit fo Eurorack: https://growyoursynth.blogspot.com/2017 ... odule.html. I needed the 12V to 9V conversion because that was required for the guitar FX PCB.
My output module also has a send/return FX section : https://growyoursynth.blogspot.com/2018/04/output.html

You'll notice that I use a panner circuit in order to avoid too much attenuation in the middle position.
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Re: Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

Post by SPoT_D » Sun Sep 13, 2020 11:16 am

I'm a long time pedal builder myself, but mostly use a bunch of pedal adapters like the ai006 (check the diy section from AI synthesis) to mix the 2 formats..

If you want eurorack modules as a pedal, I think there are multiple options depending on the module you are trying to convert.
A charge pump could boost voltages from a 9v power supply (but not really as high as +-12v) and a vactrol could do some (but not all) CV stuff. Check the Vac Pak from synthrotek for example. If you want to go deeply into what CV options are out there.. https://northcoastsynthesis.com/news/vo ... fications/

You could use 2 VCA's (LM13700) for a wet dry mix I guess. using an inverter in one to go lower in volume as cv is raised..

Please share the build you are going to make as I'm very interested

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Re: Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

Post by tatou » Sun Sep 13, 2020 11:33 am

Thanks for the quick replies, everyone!

To clarify — the three changes I'm asking about, which seem to confuse everyone, implement the features of the INTRFX, for which I'm trying to think about a DIY workalike.

1. It runs on standard +9v guitar pedal power (negative ground). Now, it's possible that internally there's a step-up conversion to a +/-12v bipolar supply and the circuit itself runs on Eurorack specs. (Seems like that's how the Hungry Robot modular boxes work, according to their general info page.) The workflow I'm thinking about would have this circuit in a box rather than in a rack, so powering it like pedals makes more sense to me. Is there a "known" recipe for this?

2. The SYINSI schematic above assumes you only ever want to run from Eurorack > Pedals +> Eurorack. But you can imagine also running from Guitar > Eurorack +> Pedals/Amp. Hence the switch, to offer flexibility in both directions. This feature isn't so important to me (right now) since I don't plan to have (m)any effect modules anyway... (although I can imagine wanting to run my guitar through a step-sequenced filter or RINGS) :) Basically, this would allow the unit to function both as a pedal IO for Eurorack, or as an inst/line-level Eurorack loop for external gear.

3. As for VC mixer/panner circuits — does someone have a link to a good starting point? I've googled around a bit but most of what comes up for me either don't have schematics, or they're so large I'm having trouble teasing out the relevant bits... ¯\_(ツ)_/¯

Thanks!

PS: Jay F. — I actually landed on your blog last night when I started trying to research this! I imagine it will be a great resource as I'm learning... thanks for that!
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Re: Adapting Eurorack circuits as guitar pedals? (12v > 9v power, CV control, etc.)

Post by Bartimaeus » Sun Sep 13, 2020 11:46 am

if you want to do a lot less work converting from bipolar to unipolar, you can just get a MAX1044/TC1044 to generate +/-9V. you'll lose a little headroom compared to the original +/-12V, but that's probably not an issue when guitar signal levels are so much lower anyway.

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