An inventory of expression-pedal input standards

[Mark Hammer]

I'm in the preliminary stages of planning something, but I would like it to be amenable to as many different expression-pedal inputs as possible.  So, what are the "standards" that you are aware of?

I know that the Line 6 standard is a 0-10k variable resistance to ground, and the input on my old MXR digital delay is 0-50k.  I also know that Source Audio pedals use a 0-3.3V control voltage standard.

What are the standards for some of the Boss or EHX pedals with expression inputs?  Do they use a variable resistance, or a voltage divider of some kind?

[Fancy Lime]

Good question, no idea. But putting the same cart before the horse: With the information that will surely be gathered in this thread, we could design a "universal expression-pedal" that will fit most effects with an expression input. Seems like a useful thing to have a DIY design for.

Cheers,
Andy

[digi2t]

Good question, no idea. But putting the same cart before the horse: With the information that will surely be gathered in this thread, we could design a "universal expression-pedal" that will fit most effects with an expression input. Seems like a useful thing to have a DIY design for.

Cheers,
Andy

Wouldn't we have to run that idea past the Competition Bureau, or the the Committee for Ethical Business Practices first? One expression pedal to fit all? That sounds almost... unAmerican.

I guess with retirement looming, Mark must be exploring new ways to stir up the shit. 

[anotherjim]

Most are TRS with pots. Control voltage comes from whatever it's plugged in to. If used where the socket is TS for a variable resistance to ground, TRS type can still work.


Some have a "minimum" heel-back variable resistance in series, this usually for organ swell pedal use as those on original Hammond organs don't go to silence.

The main compatibility issue is the order of wiring to tip and ring.
Some, like the Roland use tip to pot wiper, +supply on ring.
Some reverse this order.
Some have a DPDT to select the connection.
All use Sleeve for ground.
Some add a "safety" 1k resistor in series with the pot wiper.
Most have 10k pots.

[ElectricDruid]

I've seen ones like anotherjim mentions. 10K is a good value since it won't load a ADC input too much. 100K can limit the speed at which conversions can be made (takes a while for the sampling cap to charge up). Obviously this doesn't matter unless you're designing around uPs, but I think their increasing popularity in pedal designs is also why you're seeing more expression pedal inputs on things than previously - all it costs is one input jack, one IO pin, and a bit of software.
I also like the way this design doesn't commit itself to voltage level. You could use it with 3.3V or a 5V or 9V or whatever.
One other point about it is that you can use the normalisation on a stereo jack to switch between an internal pot or a pedal plugged into the expression jack - it doesn't even have to be a control voltage input. Just make sure you design your pedals around 10K pots, I guess...

Tom

[Mark Hammer]

Thanks for that, jim.  Much appreciated.

So if I wanted to have something with as much universality as possible, I would need to have a TRS output jack, whose T and R could be reversed, and whose R could be cancelled/rendered-moot.  I would not need to supply any sort of voltage (unless it was a Source Audio pedal).

What does the EHX controller "put out"?  Is it simply a variable resistance or does it supply a control voltage of some kind?  I can't tell.

[digi2t]

The EHX Next Step single expression/cv pedal instructions state 0-5v output. There's no pot.

The EHX Single and Dual Expression pedals are passive. They use 10k pots.

[Mark Hammer]

Hence my confusion - there are two different kinds of pedals.

A guy I work with has an identical twin who worked in a building a block down from where we used to work.  The brother would park in the facility under our building and walk over to his.  One day we pass each other on the main floor entrance lobby and I gesture hello, eliciting no response from him.  "What a stuck-up jerk", I thought.  That was the day I learned that my co-worker had a twin, whom I had never met.

[ElectricDruid]

That's why there *should* only be one type of pedal - a passive expression pedal, with three connections, TRS. If you *want* a 0-5V CV, you can easily organise that by feeding 5V out of the socket and using the expression pedal as a voltage divider. Likewise with 3.3V or whatever other Cv standard you choose.
Expecting everyone else to have some pedal that needs a battery and just happens to put out the exact right voltage range for your pedal is daft, it seems to me.

Tom

[Fancy Lime]

I think Tom is right. If you design the expression input such that it takes voltages and have a switch that allows to feed voltage to a resistive pedal or alternatively not feed anything for use of a pedal that has its own voltage, that input would be universal for all kinds of pedals. But you need an additional voltage divider in series with the pedal to adjust for different pedal resistances and voltage outputs. I don't see an obvious universal plug-and-play solution here. But if the user can be trusted with a little biasing via a trimpot, that should be fine. LED indicators to show if the voltage coming from the pedal is correct would make that rather easy even for technically unexperienced users.

Very interesting project, please keep us posted. If you get this working, the universal expression input wold make a very useful circuit snippet. Also: community testing with lots of different expression pedals would be great.
Andy