Anyone Ever Reverse Engineer A Korg MS-01 Footpedal?

[Paul Marossy]

Want to build one if possible, but I can't find a schematic anywhere online...

[slacker]

From the info here http://www.korganalogue.net/korgms/manuals/pp/0101.html it looks like it would be pretty easy to build something that does the same job. The attenuator sounds like it's just a volume pedal, and the voltage generation could be done with a couple of opamps. I can draw something up if you like.

[Paul Marossy]

I found that page already, but I have to confess that I have no idea how this pedal functions.

I also had a thought that the attenuator may just be a simple passive volume pedal. That's the easy part. It's that -4V to +4V part that I don't have a clue about...

EDIT: The reason I ask is because I want to make one for my Korg Polysix which was recently given to me.

[slacker]

Here you go, this should give you two control voltages that go from 0 to +-4 volts, heel down is zero and toe down is + and - 4 volts.



The first opamp just buffers the voltage from the pot, and you adjust the trimmer so toe down gives you 4 volts. The second opamp inverts the voltage from the first, giving you the negative voltages.
The attenuator in the original might be active but in a synth I think a passive one will be fine.

[Mark Hammer]

Most of the "schematic" is really in the wiring of the jacks themselves, since the function of the pedal seems to depend on which jack/s you do and don't plug into.  Keep in mind, there is only one pot in there.

[Govmnt_Lacky]

Correct me if I am wrong but...

Shouldn't the heel down be -4V and the toe down be +4V?

With the explanation of the schematic, the heel down is 0V 

[Paul Marossy]

Shouldn't the heel down be -4V and the toe down be +4V?

I believe that is how the MS-01 was designed.

EDIT: Also, hard to tell for sure from the info on that webpage, but it looks like the original was designed around a single 9V battery.

[slacker]

Ahh, yeah Mark that makes it a bit trickier, I'd assumed a dual pot for what I drew.
I see the real thing only uses one battery as well, but they could be using a charge pump or something to get the negative voltage. Thinking about it you might be able to use one battery using a voltage divider across them to get ground and +-4.5 volts, it depends if your pot has enough rotation to get to 4 volts I don't think most volume or wah pedals would sweep enough.

Shouldn't the heel down be -4V and the toe down be +4V?

No, both voltages are at zero heel down and get bigger towards toe down, that's what this diagram shows anyway.

http://www.korganalogue.net/korgms/manuals/pp/01specs.html

[Paul Marossy]

Yeah, I guess it's -4V at heel down and +4V at toe down, per this illustration:

[Paul Marossy]

I do know that the volume control on the keyboard itself is a 10KB. So I wonder if that pot they used in the foot pedal was also a 10K?

[Mark Hammer]

Chances are excellent that:

a) it uses complicated switching jacks to determine what the footswept pot does and connects to.
b) the battery is not connected to chassis ground, but rather is used to feed an op-amp that creates a floating ground, like you see on page 14 here: http://hammer.ampage.org/files/Device1-9.PDF

Since the voltage swing of the op-amp will not be from rail to rail, that's why you'll only get +/-4v.  The question is which op-amp will do that for you.

[Paul Marossy]

Chances are excellent that:

a) it uses complicated switching jacks to determine what the footswept pot does and connects to.
b) the battery is not connected to chassis ground, but rather is used to feed an op-amp that creates a floating ground, like you see on page 14 here: http://hammer.ampage.org/files/Device1-9.PDF

Since the voltage swing of the op-amp will not be from rail to rail, that's why you'll only get +/-4v.  The question is which op-amp will do that for you.

Yeah, probably so as far as the jacks are concerned. And the floating ground could have been done that way (which is pretty slick BTW). Here's a picture of the pedal. It's a little different than a wah pedal is:



I'm wondering if I could just use my DOD FX-17 to do the same thing? It's a 0 to +5V thing, I don't know if that could hurt the keyboard or not...  

[wavley]

I'm wondering if I could just use my DOD FX-17 to do the same thing? It's a 0 to +5V thing, I don't know if that could hurt the keyboard or not...  

I knew you had one and was just getting ready to suggest you try it and see if it's close enough for rock and roll.

[Paul Marossy]

Actually I have three of them now!  

The first one I bought off of ebay fully functional. The second one I bought real cheap off of ebay "broken", it only had dirty contacts on the DC switch which prevented it from getting any power - easy fix. The third one was sent to me by a fellow forumite free of charge. I just had to put that one back together and it was good to go.

[Earthscum]

OT: Did you ever put in a second switch to do toe-down switching in any of your FX-17's? I am doing something similar in a different wah build, and was going to mod my DOD to do toe and heel switch. Not many people own those. Pretty neat pedal, really... especially after LP modding.

[Paul Marossy]

OT: Did you ever put in a second switch to do toe-down switching in any of your FX-17's? I am doing something similar in a different wah build, and was going to mod my DOD to do toe and heel switch. Not many people own those. Pretty neat pedal, really... especially after LP modding.

No, never tried that. I use them only as a volume pedal, for which I think they are hard to beat for a variety of reasons.

[wavley]

OT: Did you ever put in a second switch to do toe-down switching in any of your FX-17's? I am doing something similar in a different wah build, and was going to mod my DOD to do toe and heel switch. Not many people own those. Pretty neat pedal, really... especially after LP modding.

No, never tried that. I use them only as a volume pedal, for which I think they are hard to beat for a variety of reasons.

I only ever use mine as a volume pedal too, sometimes I mess around with the wah, but I'm just not a wah guy and I prefer my modded crybaby when I feel the need.  I think the fx17 has a very interesting wah sound though.

[Mark Hammer]

One of the things to keep in mind is that earlier Korg stuff, like a lot of modular analog stuff, was not always all that standard.  For example, their trigger pulses were not like other companies'.  In many respects, it was a bit like CP/M, which professed to being standardized but was far from it ( http://en.wikipedia.org/wiki/CP/M ).  So the -4-to-+4v sweep may have been useful for Korg stuff, but not necessarily for any other brand of product, such as those useing 0-5v or 0-10v control voltages.

I guess the question you need to ask yourself is whether you want to generate a circuit that mimics an MS-01 for the purposes opf controlling Korg gear, or whether you simply want something in an MS-01 form-factor that does volume control and expression-pedal functions.

[Paul Marossy]

One of the things to keep in mind is that earlier Korg stuff, like a lot of modular analog stuff, was not always all that standard.  For example, their trigger pulses were not like other companies'.  In many respects, it was a bit like CP/M, which professed to being standardized but was far from it ( http://en.wikipedia.org/wiki/CP/M ).  So the -4-to-+4v sweep may have been useful for Korg stuff, but not necessarily for any other brand of product, such as those useing 0-5v or 0-10v control voltages.

I guess the question you need to ask yourself is whether you want to generate a circuit that mimics an MS-01 for the purposes opf controlling Korg gear, or whether you simply want something in an MS-01 form-factor that does volume control and expression-pedal functions.

Well I'm going to build that little opamp circuit suggested above just for kicks when I get a little spare time.

[Paul Marossy]

So it's only like almost six years later now, but I'm finally going to do this project. 

Is there an easy way to get a +4V / -4V power supply? I know there's a way to do it with a 741 opamp to get +4.5V & -4.5V with a 9V battery, but how do I knock off another 1/2 volt? With some resistors?

My other problem is that I have no clue as to how much current is involved. I can't imagine it's enough to be hurt by a couple of resistors but I really have no idea. I would be using it to control the VCF section to get wah-wah like sounds, so I think it's just taking the place of a panel mounted control. But you could connect the pedal in other ways, like between the VCF section and MG section, so I don't know what happens in that case. This would be for use with my Korg PolySix.

Anyone have any thoughts?