Modularizing Pedal Circuits

Hi, looking for advice in circuit design. I have a few pedal/lunetta circuits that I love that I would like to play nice with modular. I’ve been looking to do a redesign in KiCAD but wanted to ask some advice in regards to approach. I know ZVEX and 4MS were able to translate some of their pedals to eurorack format successfully.

For adding CV to existing controls, I’ve done some preliminary research and a vactrol in parallel with the pot driven by a transistor or OPAMP seems to be the most common technique. But I feel like this would get expensive pretty quickly, and I imagine works for 0-5v cv signals, but what about 10vpp signals like feeding a VCO in? I’ve also seen digipots used here, but that would require some kind of controller and DACs.

I’ve done a bit of reading using a JFET or OTA as a VCR but my technical knowledge is a bit limited here. Anyway, curious if anyone has taken on a project like this and has any insights into this type of design problem.

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How easy or hard this is depends tremendously on the individual pedals. In the simplest case, some designs would work with 12V power and modular audio signals directly. Similarly in some designs you could add a port and normal it appropriately then have CV control. Some require different power, meaning voltage regulation; also zener diode clipper, opamps for level conversion and buffering to/from the relevant levels and range (potentially negative voltages) at inputs and outputs. Vactrols are handy as they work with negative or positive voltages (on the output side, not input which is a diode) being resistors, so they are often direct drop in replacements for a pot. Sometimes the pot is logically just a potential divider meaning all you need is voltage in which case you can bypass the need for variable resistance entirely. These may or may not be needed, depending on the design but there isn’t really a catch all solution: you need to look at the underlying design more carefully to know.

EDIT: one thing I intended to say but didn’t: Vactrols are usually very non linear with regard to resistance changes with respect to applied voltage and can’t be controlled by negative voltages. In contrast an OTA can potentially be used to provide linear response and is reactive to both positive and negative modulation voltages.

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This might be worth a look https://aisynthesis.com/product/ai006-eurorack-stomp-box-adapter/ for your research

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A few thoughts on the process generally, not specifically towards pedals, but adapting and making more complex circuits in general. This is just my personal experience, so your experience may be much different :slight_smile:

I’m not really sure about your technical background, but I’d encourage you to just go for it on one of the circuits. I wouldn’t mess around with KiCAD or making PCB’s as a first step. With a limited knowledge set it’s really easy to get stuck in a wormhole of technical possibilities and trying to do it ‘right’.

Personal Aside

There are plenty of folks who can do this EDA first approach, but I’m personally not really one who can do it very successfully… After about 3-4 years of making PCBs and designing circuits and a decade+ of DIY electronics experience (no formal training), I just successfully did my first ‘EDA first’ design (for something similarly complex). I don’t want to discourage you from trying it, just offer up my personal context in case it becomes really frustrating (its hard!)

If you just popped the circuit on the bench / breadboard and started trying things one by one (learning each approach individually), in my experience you’ll likely arrive somewhere that works faster (and you can optimize later). Use your research and technical investigation to guide the decisions on the breadboard. It’s really likely you’ll need to systematically adapt each section of the pedal (I/O, CV, etc.) for what you need in the modular world and doing it on a breadboard will allow you to do each modification one by one and verify that it’s working (and not interfering with something else!)

My experience in using expensive components like vactrols:

If you’re making just one for yourself (or a handful) and a vactrol or expensive component saves you a ton of time, energy, and/or frustration, it’s probably worth it! At least, that’s been my experience (especially in a recent project). Don’t forget about the expense of your time and energy too! If you’re trying to optimize for component cost, then that’s a rabbit hole into itself!

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As a quick aside: vactrols don’t need to be expensive. I’ve bought VTL and STL branded versions of the 5C9 vactrol on eBay for £5 to £7 for 5, so about £1.00 to £1.50 each.
They work fine, but aren’t always sealed that well, letting in light. This is easily solved with a bit of electrical tape around it.

A LM13700 OTA costs about £2.20, you get two on a chip, so the cost is similar and vactrols are simpler to work with. I’ve had most success driving the LED with a NPN transistor or, for an exponential / volume response, using a simple NPN->PNP exponential converter.

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Interested in this too, currently trying to make a module based on a pedal which uses op amps powered by a simple voltage divider. MY plan is to initually build a prototype and just try powering it from +/-12, and if this doesn’t sound good (which it might not, since going from 4.5v to 12v is a bit of a leap!) try and add some voltage divider/regulator stuff

It would be pretty straightforward I think to use a regulator to get 4.5v out of 12v depending on your mA usage.

A quick search yields some options:
https://www.digikey.com/en/products/detail/texas-instruments/LP2985IM5-4-5-NOPB/567844
https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TAR5S45UTE85LF/4304967

more:
https://www.digikey.com/en/products/filter/pmic-voltage-regulators-linear/699?s=N4IgjCBcoKwOwGYqgMZQGYEMA2BnApgDQgD2UA2uAAwIwAsATCMWFQBxyPPhgBsMVAJzcwdBGxogAusQAOAFyggAyvIBOASwB2AcxABfFnDjDoINJCx4ipCiAbsYDOtLmLIK9dr2GQAWiYzC3UAVxsySEoYVxAFJQNfXmQQDQATJT9WJDd44nkAT1l8JUxcNH19IA

You’ll have to test these out for noise, but my suspicion is that they would be pretty much drop in.

edit: just adding a note that using a voltage regulator like this to step down from 12V is pretty common on digital modules where they need 5v, 3.3v, or 1.8v for various things. I don’t see why it would be an issue to do it in the case of converting a pedal.

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Here’s a more thorough article about different ways of adding CV control that I found useful myself: Voltage control modifications - North Coast Synthesis Ltd.

(Spoiler: there are a lot of ways depending on what kind of circuit you want to control and how elegantly & well you want to do it)

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Yes was thinking of this sort of option too. Haven’t yet got my head around whether i’d need to use two regulators (and how you put one on a negative voltage)

If you’re circuit does not use the negative voltage you can basically just ignore it in my experience.

You’ll probably only need the negative if you’re using opamps, in which case just find ones that run on ±12v, they wont interfere with your 4.5v circuit since they are offering up ±12v of headroom not directly interfering with the circuit (if that makes sense).

So for example you should be able to use the regulator to bring the 12v supply down to 4.5v to power the pedal circuit as it exists already and then use the ±12v supply on an op-amp to amplify the signal to what ever you need on the way out to get it to modular levels (10vpp aka ±5v. or 16vpp ±8vpp or 20vpp ±10v)

Get some vactrols in the mix for CV control (you’ll get around loads of mixed-voltage issues this way) and you’ve got a start!

edit: didn’t know discourse converted ± to the ligature version, nice!

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This really depends on the use for the op amp. If it’s a voltage follower than that’s fine, the same is true for amplification modulo correct virtual ground voltage, but consider it being used in other capacities where it saturates. As a more concrete example (I just made it up, but it would be a distortion), imagine a circuit that has an opamp as a comparator with a knob to control the comparison voltage (let’s say timbre) and another that controls the mix between the dry signal and the comparatored one (intensity). If you change the supply voltage to the opamp the voltage scale on half the circuit will change, as will the amplitude of the output. It may work, but the behaviour will change.

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Oh yeah, I didn’t mean in terms of just jamming another voltage into the existing circuit, that seems like a recipe for frustration :slight_smile:

I meant if the circuit uses only 4.5v and you regulate that down off the +12v line, you can just ignore the -12line if there’s no -voltage in the circuit and it’s not used for anything.