Minimoog D with expression pedal controlling modulation

In a Moog Mood? Here's a forum for discussion of general Moog topics.
Post Reply
willstring
Posts: 10
Joined: Wed Jan 28, 2015 3:35 am

Minimoog D with expression pedal controlling modulation

Post by willstring » Mon Jun 15, 2015 3:20 am

Hey all,

As the title suggest, i'm trying to figure out a way to add an expression pedal to control the modulation of the Minimoog D, while also being able to control it with the actual wheel, and switch between the two in real time, and having their current values basically add together. (I.e., when the wheel is zeroed it is controlled entirely by the pedal and vice versa, and if the wheel is at 50% then the pedal controls the remaining 51-100%)

Is there a simple way to do this on the Minimoog? Per my calcs it isn't as simple as just adding a three lug pot in parallel with the current mod wheel pot; this won't give the right resistances, due to the way resistances add. But maybe i'm wrong.

Thanks in advance!

User avatar
thealien666
Posts: 2791
Joined: Mon Nov 14, 2011 8:42 pm
Location: Quebec, Canada

Re: Minimoog D with expression pedal controlling modulation

Post by thealien666 » Mon Jun 15, 2015 10:41 am

You're right. It's not as easy as one could think. Especially when you take into consideration that the Mod Wheel on a Minimoog doesn't work in the "conventional" manner. On most synths, the potentiometer controls the amount of modulation going into a circuit, on the Mini the modulation amount is always at maximum, and the Mod Wheel shuts it off gradually. That's one of the causes behind the well-known modulation bleeding artifact exhibited on most Minis (a faint modulation still present even with the mod wheel all the way down, but not when the modulations switches are off) because of the very minimal resistance left on those Allen-Bradley Type-J potentiometers at both ends of the travel (the wiper inside is actually a carbon brush and not simply a metallic arm, for industrial strength longevity, but adding this usually insignificant drawback).

I suppose that a relatively simple circuit with op amps could be devised to accomplish what you're looking for, and could even possibly correct the bleeding issue as a bonus ? But I'm not an electronics engineer and my experience as a hobbyist isn't good enough for me to conceive such a circuit. Maybe someone else could have a go at it ? I'd be interested. :D
Moog Minimoog D (1975)
DSI OB6
DSI Prophet REV2
Oberheim Matrix-6
Ensoniq SQ-80
Korg DW8000
Behringer DeepMind 12
Alesis Ion

willstring
Posts: 10
Joined: Wed Jan 28, 2015 3:35 am

Re: Minimoog D with expression pedal controlling modulation

Post by willstring » Tue Jun 16, 2015 2:45 am

thealien666 wrote:You're right. It's not as easy as one could think. Especially when you take into consideration that the Mod Wheel on a Minimoog doesn't work in the "conventional" manner. On most synths, the potentiometer controls the amount of modulation going into a circuit, on the Mini the modulation amount is always at maximum, and the Mod Wheel shuts it off gradually. That's one of the causes behind the well-known modulation bleeding artifact exhibited on most Minis (a faint modulation still present even with the mod wheel all the way down, but not when the modulations switches are off) because of the very minimal resistance left on those Allen-Bradley Type-J potentiometers at both ends of the travel (the wiper inside is actually a carbon brush and not simply a metallic arm, for industrial strength longevity, but adding this usually insignificant drawback).

I suppose that a relatively simple circuit with op amps could be devised to accomplish what you're looking for, and could even possibly correct the bleeding issue as a bonus ? But I'm not an electronics engineer and my experience as a hobbyist isn't good enough for me to conceive such a circuit. Maybe someone else could have a go at it ? I'd be interested. :D
Actually, in looking into it a bit more it is indeed as simple as I thought. According to the service manual, only 2 of the 3 lugs on the pot are used, implying that it essentially functions as a rheostat. I took the mod wheel assembly out and confirmed this with a multi-meter; its just a 50K pot but only actually 1.2K of it is actually used due to the limited rotation of the mod wheel, and indeed only 2 out of the 3 lugs are used implying it is a purely variable resistor.

SO, the solution is pretty simple actually-- just add another pot in SERIES with the mod wheel pot (again only need 2 out of 3 lugs), that generates 0 ohms when the pedal is up, and ~1.2K ohms when it is pressed down. That way, you can use either the wheel or the pedal to control the modulation. Let me play around with a few different pots/expression pedals and i'll report back, hopefully with good news :D

User avatar
noddyspuncture
Posts: 467
Joined: Wed Sep 16, 2009 7:57 pm

Re: Minimoog D with expression pedal controlling modulation

Post by noddyspuncture » Tue Jun 16, 2015 1:25 pm

Interesting thread... your theory is sound - on paper - however I think adding another pot "in series" would in fact create a larger resistance (larger than the half ohm maximum required) when both are in the "off" position which in turn will probably give you the unwanted mod-bleed mentioned earlier...!?

Cheers,
Tom

User avatar
thealien666
Posts: 2791
Joined: Mon Nov 14, 2011 8:42 pm
Location: Quebec, Canada

Re: Minimoog D with expression pedal controlling modulation

Post by thealien666 » Tue Jun 16, 2015 1:58 pm

Exactly my thoughts, Tom. This would definitely add to the modulation bleeding issue.

Not to mention the difficulty in "calibrating" the foot pedal to create the full amount of modulation, along the full travel of the pedal, depending on the mechanical design of it. Some are linear (rack and pinion style) or logarithmic (swinging pivot style).
Moog Minimoog D (1975)
DSI OB6
DSI Prophet REV2
Oberheim Matrix-6
Ensoniq SQ-80
Korg DW8000
Behringer DeepMind 12
Alesis Ion

willstring
Posts: 10
Joined: Wed Jan 28, 2015 3:35 am

Re: Minimoog D with expression pedal controlling modulation

Post by willstring » Tue Jun 16, 2015 3:31 pm

noddyspuncture wrote:Interesting thread... your theory is sound - on paper - however I think adding another pot "in series" would in fact create a larger resistance (larger than the half ohm maximum required) when both are in the "off" position which in turn will probably give you the unwanted mod-bleed mentioned earlier...!?

Cheers,
Tom
Good point. However, according to the Bourns potentiometer handbook, MR (minimum resistance ratings) are somewhat counter-intuitively given as a maximum value, similar to tolerance (i.e., the real, measured MR could indeed be lower). So there is a chance that when you sum the MR's from both pots in series, it is still less than that which would produce the mod 'bleed' you mention. I don't notice any bleed currently on my Mini so perhaps there is a fighting chance. Maybe a simple experiment would be to just take another pot (5k or so), zero it out and place it in series with the existing one and see if you get any bleed.
thealien666 wrote:Exactly my thoughts, Tom. This would definitely add to the modulation bleeding issue.

Not to mention the difficulty in "calibrating" the foot pedal to create the full amount of modulation, along the full travel of the pedal, depending on the mechanical design of it. Some are linear (rack and pinion style) or logarithmic (swinging pivot style).
My idea was to take a pedal with known pot travel such as the Yamaha FC7 (right around 60 degrees after accounting for the kinematics in the lever mechanisms), and solve for the total resistance one would need with a linear pot to generate around 1.2K of resistance at 60 degrees, and simply replace the FC7 pot with that one. Per my calcs this implies you would need about a 6K pot, this would generate 1.2K of resistance at 60 degrees of rotation (assuming 300 degrees of total travel, as common on most pots). You could then use the calibration screw on the FC7 to fine tune it and get the ranges right. If the max value isn't exactly 1.2K i'd still be happy, I rarely crank the modulation that high anyway.

At any rate, i've ordered a few different pots of the same form factor as the FC7 pot and will try them out. I'll be sure to report back with any successes and failures...

willstring
Posts: 10
Joined: Wed Jan 28, 2015 3:35 am

Re: Minimoog D with expression pedal controlling modulation

Post by willstring » Sat Jun 27, 2015 4:54 am

We have liftoff!!

It worked, and it is better than I expected! There is no noticeable modulation bleed, see several pictures here: https://dl.dropboxusercontent.com/u/294 ... G_1533.zip

After a few design iterations I was able to get everything up and running as planned, here's how it all went down:

1) Swapped out the pot in the Yamaha FC7, with this one: http://www.digikey.com/product-detail/e ... ND/4733110. This pot is quite smooth and has a great minimum resistance. What I found is that a 1K pot will do you just fine; the type J pot in the Moog actually has a non-linear taper, so at 65% throw the pot is at about 400 ohms, so I set that as my target max resistance (this was plenty of modulation for me). In the pictures I intentionally wired it in reverse; i.e., when the pedal is down, the modulation is off, and when the pedal is raised, the modulation kicks in; this felt a bit more natural to me. I calibrated the pot with a DMM, making sure that it was at its "minimum resistance" when the pedal was all the way down. This took a few tries but eventually nailed it.

2) Removed the decay 1/4" jack on the moog side panel (not permanently though, just covered it in insulation and zip-tied it so it was out of the way, leaving it wired), and replaced it with a 1/4" stereo jack with both tip and ring shunts (http://www.digikey.com/product-detail/e ... -ND/109556), made a break in the modulation circuit and wired the jack in series. I jumped the tip shunt to the ring; that way when there is no cable plugged in, the circuit is automatically reverted to how it was before and modulation can be controlled with just the wheel. When the cable is plugged in, the tip and the tip shunt get opened, and the pedal pot is essentially inserted in series.

3) From there, one can control modulation via the wheel OR the pedal. To remove the pedal simply take the cable out and the wiring is back to the service manual wiring.

If anyone else wants to try this out let me know and i'd be happy to help!

synthguy
Posts: 142
Joined: Wed Sep 17, 2014 7:37 pm

Re: Minimoog D with expression pedal controlling modulation

Post by synthguy » Sat Jun 27, 2015 5:30 am

Sounds like a very useful and well-executed mod.
Congrats on your success!
Visit my synthblog...Moogs and more!
Www.synthguy.com

User avatar
thealien666
Posts: 2791
Joined: Mon Nov 14, 2011 8:42 pm
Location: Quebec, Canada

Re: Minimoog D with expression pedal controlling modulation

Post by thealien666 » Sat Jun 27, 2015 1:11 pm

Great ! Glad to know that it worked well ! Congrats ! :D
Moog Minimoog D (1975)
DSI OB6
DSI Prophet REV2
Oberheim Matrix-6
Ensoniq SQ-80
Korg DW8000
Behringer DeepMind 12
Alesis Ion

Post Reply