Expression pedal/CV box

[patrick398]

Salutations,
I had an idea of making a pedal of sorts, but one which contained perhaps 2 LFOs or maybe stompLFOs, buffering those outputs so that each could drive a couple of CV outs at 0-+5v. I also thought it could be good to have a switching option which routes those CV outputs through some kind of transistor set up, thus converting the CV into expression outputs of sorts. I guess the resistance range of these outputs could be adjusted by a switch to select perhaps 3 of the most common resistance ranges. The goal is to have a box with maybe 4 outputs, 2 per LFO/stompLFO, which can each be set to either provide CV or expression control.
I don't actually own many commercial pedals so my experience with CV/expression is very limited. Is there anything else i need to consider? I have a feeling i've over simplified it.
From what i understand pedals with CV inputs provide the expression pedal with their desired voltage, and there is no industry standard. Is it risky to make something that ignores the voltage the pedal expects and instead gives it 0-+5v regardless?
Could be an interesting project none the less

[iainpunk]

cool idea

some pedals route the signal through the expression pedal, maybe a thing to consider. also the plug layout in respect to the pot lugs differs between brands.

the lfo box is a cool idea, maybe for some types of pedal, having a dynamic LFO of which the frequency is controlled by a foot rocker pedal is also an option

cheers, Iain

[patrick398]

cool idea

some pedals route the signal through the expression pedal, maybe a thing to consider. also the plug layout in respect to the pot lugs differs between brands.

the lfo box is a cool idea, maybe for some types of pedal, having a dynamic LFO of which the frequency is controlled by a foot rocker pedal is also an option

cheers, Iain

Yes the different pot lug arrangements was something that ocurred to me. Initially i was just thinking about doing this as a CV box but thought i may as well try and add some expression pedal functionality.
It could be an idea to include an expression pedal into on the box itself, so with a expression footpedal you could control the depth of the cv/expression to numerous pedals...though i'm not sure how useful that would actually be

[iainpunk]

maybe the expression pedal could determine the DC offset of the LFO. the LFO's amplitude would be controlled by a pot. if you set the pedal in the middle, and the amplitude to max, you have the normal LFO CV, if you take the amplitude to 0, the pedal is the CV, and in the middle, you can have some fun. a middle ''detent'' in a rocker can be done by simple rubber bands.

cheers, Iain

[patrick398]

Here's where i'm at with this so far, it's kind of been thrown together so not the neatest schematic but  it should make sense.

Hopefully this will allow me to have 4 CV outputs, sharing two LFOs (so each LFO goes to two outputs). There's a switch after each LFO which allows selection of 0-5v or 0-3.3v so it can be used with FV-1 stuff. I lifted that part from somewhere around the web but i can't find it now can someone can tell me if it will actually work ?

There's also a switch before two of the outputs which inverts the phase of the LFO so you can have 4 outputs doing different things.

I also included the sync input from the stompLFO datasheet so it can be clocked by something else, or tap tempo.

I'm curious to hear from people who have more experience with CV/expression pedals than me...i'm expecting there to be problems that i haven't really considered, like simply using mono outs. It's my understanding that an expression input on a pedal uses the ring the send the expression pedal a voltage. Is there an issue with ignoring that voltage and sending it either 0-5v or 0-3.3v?

[patrick398]

Actually the more i look at it the more i think that the way my op amps are set up is crazy. The first is supposed to buffer and the second invert but the second has 4.5v on the inverting pin but potentially 5v going to the non-inverting pin.
And the buffers aren't biased but that's ok because they're just unity buffers right?
Thinking i may have made a mess of this now

[iainpunk]

Hopefully this will allow me to have 4 CV outputs, sharing two LFOs (so each LFO goes to two outputs). There's a switch after each LFO which allows selection of 0-5v or 0-3.3v so it can be used with FV-1 stuff. I lifted that part from somewhere around the web but i can't find it now can someone can tell me if it will actually work ?

its just a passive volume attenuator, but not AC coupled.

Actually the more i look at it the more i think that the way my op amps are set up is crazy. The first is supposed to buffer and the second invert but the second has 4.5v on the inverting pin but potentially 5v going to the non-inverting pin.
And the buffers aren't biased but that's ok because they're just unity buffers right?
Thinking i may have made a mess of this now

the inverting buffer should have 2.5v or 1.65v on the (+) pin depending on the level of the signal.

the rest looks sane to me, i don't see any obvious flaws.

cheers, Iain

[patrick398]

Thanks man, i thought i'd lost the plot for a moment but on reflection it's not as terrible as i first thought ha.
So the non inverting pin on the inverting buffer needs to be biased depending on whether the other switch is in 3.3v or 5v mode. That makes sense...going to take some switching jiggery pokery though

[iainpunk]

well, the volume switch that switches the amplitude can easily be doubled up and switch between two points in a resistor divider existing of 3 resistors.

[ElectricDruid]

There's a fair bit of confusion because a lot of pedals have Expression pedal inputs that can also be used as CV inputs. That's not necessarily the case though, and the two things are different really.

An expression pedal is just a pot in a rocker enclosure with the three pins taken to a TRS jack. The wiring for that varies, but "wiper to tip, top of pot to ring, bottom of pot to sleeve" is pretty common.

One common way to use an expression pedal on modern uP-based pedals is to feed a 5V or 3.3V output to the expression pedal so it is wired as a voltage divider, and then feed the expression pedal's wiper output back to an ADC input on the uP. This is where the "can also be used as a CV input" part comes in, since if you ignore the ring connection that's feeding +V out, you can feed a CV directly in to the ADC.

Older gear didn't necessarily do it like this. You can also use a switched TRS socket to make the expression pedal replace a pot inside the pedal so that you can directly twiddle the knobs with the expression pedal. Like this, the expression pedal could be carrying signal not voltage, and it's a totally different story.

[ElectricDruid]

A couple of small things worry me about your circuit.

The LED/4K7 on the StompLFO output might drag the output down a bit and reduce the maximum output. It'd depend on the LED, so watch for that. 4K7 is a pretty big resistor, so it might be ok.

The 33K/68K divider across the passive output filter will certainly drag the output down. Since the point of it is to reduce the voltage, that's not necessarily a problem, but the divider will interact with the filter so the usual voltage divider calculation won't work exactly.

What are the Schottky protection diodes for?

[patrick398]

There's a fair bit of confusion because a lot of pedals have Expression pedal inputs that can also be used as CV inputs. That's not necessarily the case though, and the two things are different really.

An expression pedal is just a pot in a rocker enclosure with the three pins taken to a TRS jack. The wiring for that varies, but "wiper to tip, top of pot to ring, bottom of pot to sleeve" is pretty common.

One common way to use an expression pedal on modern uP-based pedals is to feed a 5V or 3.3V output to the expression pedal so it is wired as a voltage divider, and then feed the expression pedal's wiper output back to an ADC input on the uP. This is where the "can also be used as a CV input" part comes in, since if you ignore the ring connection that's feeding +V out, you can feed a CV directly in to the ADC.

Older gear didn't necessarily do it like this. You can also use a switched TRS socket to make the expression pedal replace a pot inside the pedal so that you can directly twiddle the knobs with the expression pedal. Like this, the expression pedal could be carrying signal not voltage, and it's a totally different story.

Yes it's a shame there doesn't seem to be more of a 'standard', but since when did gear people every agree on much?
As this schematic is at the moment, i see it as a multi- CV output box with a kind of 'it might work with expression inputs too'. I'm not sure if there's anything i can add that would make it more universal. I looked into sending the LFO out to a vactrol but then i couldn't really see what that would achieve...i guess that covers situations where a pedal with expression input is expecting a change in resistance similar to one of it's own pots. I guess that resistance could be scaled with some resistors and a switch to cover the most common pot ranges...but not sure if it's worth it.

A couple of small things worry me about your circuit.

The LED/4K7 on the StompLFO output might drag the output down a bit and reduce the maximum output. It'd depend on the LED, so watch for that. 4K7 is a pretty big resistor, so it might be ok.

The 33K/68K divider across the passive output filter will certainly drag the output down. Since the point of it is to reduce the voltage, that's not necessarily a problem, but the divider will interact with the filter so the usual voltage divider calculation won't work exactly.

What are the Schottky protection diodes for?

This stompLFO block is the one i use in all my pedals, i just copy the schematic block into new designs. In the original block that 4k7 is actually a 10k. It was only this time i thought how enormous that was and dropped it to 4k7. I can't think why i elected to have a 10k in there originally, but there must have been a reason...i'll look into it on some of my other schematics, but all the pedals seem to work perfectly. It's only a 3mm indicator LED so doesn't need to be all that bright.

As for the filter on the pwm output, that's actually not something i've ever included before and never experienced any issues. I think we've spoken about this before. I included it this time as i thought it couldn't hurt, but if it's going to interact with the 36/68k divider maybe i'll see if i can get away with omitting it again.

As i understand it the shottky's are there to stop the voltage ever exceeding 3.3v by more than a diode drop, but maybe i'm missing something

Cheers!

[ElectricDruid]

but since when did gear people every agree on much?

Well, quite!

I looked into sending the LFO out to a vactrol..but not sure if it's worth it.

No, I agree. It'd be quite a bit of mucking about, and it would probably only work for a few things. Better to go for the CV output.

As i understand it the shottky's are there to stop the voltage ever exceeding 3.3v by more than a diode drop, but maybe i'm missing something

Why would it exceed 3.3V at all? The output of the StompLFO can only possibly be 5V, and with the divider it *can't* be more than 3.3V. I think they're unnecessary.

[patrick398]

Why would it exceed 3.3V at all? The output of the StompLFO can only possibly be 5V, and with the divider it *can't* be more than 3.3V. I think they're unnecessary.

Good point. I'll try and dig out the original website page i got it from...but now that i think about it the post was mainly about interfacing fv-1 stuff with modular so i guess this was precaution in case something bigger than 5v got applied. 
That allows me to also get rid of that 3.3v regulator but i'm hoping it's ok to get the 1.65v with a divider off the 5v regulator.

I've just realised what you were saying about the pwm filtering interacting with the 36/68k voltage divider. I think i'll have to get it on the breadboard so i can dial in the divider properly, but i'll leave it as is for now.