Adding CV input to stomboxes

a.b.o.z.

Hi all,

Friend asked me to mod him EH memory man with CV input for one of the pots. I found that easy way to do it is with vactrol, solder opto. side of vactrol to ground and wiper of a pot. Is this correct?
Is there another way to do it? I'm looking at Scott Bernardi PT2399 delay voltage control section. Can I apply this very same scheme for memory man or any other pot?
What is the difference between pure vactrol and Scotts implementation?
Thnx 4 da help.

Cheers

Ivan

voltage control.JPG

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cthulu · Joined: Feb 07, 2009 Posts: 56 Location: Göteborg, Sweden

Only if the pot is wired as a varistor with two lugs shorted. Otherwise it could get ridiculously hard to do.
There are various ways to go about depending on what the function of the pot is. Maybe the pot just generates a CV allready?
Which pot is it?
Can you link to the schematic?
Whích model is it?

oskar

a.b.o.z. · Posted: Mon Feb 09, 2009 6:29 am Post subject:

I'm not sure witch model is it. I will know this weekend.
He wants it for "delay time" pot.
Here is schematics for deluxe memory man: http://www.schematicheaven.com/effects/eh_deluxe_memoryman_delay.pdf and on that schematic "delay" pot is wired as a varistor...two lugs shorted. Maybe he got just memory man not deluxe edition but it is probably the same.
And what about Scott Bernardis schematic? I can do this only when pot is wired as voltage divider? Is his schematic this case? Or what?

a.b.o.z. · Posted: Tue Feb 10, 2009 6:34 am Post subject:

Ok I found out that stompbox is EH memory man deluxe, schematic that I've posted is the one. So easy way to do it with vactrol?
Can I do it as Scott Bernardi did it for his delay (attachment in my first post)?

cthulu · Joined: Feb 07, 2009 Posts: 56 Location: Göteborg, Sweden

That should work just fine!

synthmonger · Posted: Tue Feb 10, 2009 11:32 am Post subject:

Couldn't he just stick a cv input through a resistor in between the diode and cap where the 27kOhm resistor? That's where the LFO's output is going so it should work right?

slacker · Posted: Tue Feb 10, 2009 11:49 am Post subject:

I think that would work, except aren't the 1n caps blocking DC?
So a changing voltage like the output of an LFO would modulate the delay time, but a static voltage wouldn't do anything so you couldn't use the CV just to set the delay time.
Or looking at it again is the diode acting as a voltage/current controlled resistor, in which case it might work for static voltages as well.

synthmonger · Posted: Tue Feb 10, 2009 4:05 pm Post subject:

slacker · Posted: Tue Feb 10, 2009 4:19 pm Post subject:

Good point, it's not just a chorus though it's a delay. So I thought he might be wanting to have CV control of the actual delay time. Then you could use an expression pedal or whatever to set the length of the delay rather than just modulating it.

a.b.o.z. · Posted: Tue Feb 10, 2009 4:30 pm Post subject:

Yeah that's it. Plugging in expression pedal or LFO is the goal here.

strategy · Joined: Jan 03, 2009 Posts: 87 Location: portland, oregon

Can you post a picture and/or description of the project and how you did it? I was thinking of modifying my DMM the same way.

I got the Bridechamber VC Resistor board, implementing a VTL5C2 for mod projects like this...Will probably use that, but not sure if it will fit into the enclosure...

Strategy

oldmanfury · Joined: Oct 20, 2006 Posts: 57 Location: CA

Does the Bridechamber board come with a better schematic, with resistor values? I was trying to make sense out of the board - What is the external voltage in? How is it different from the CV in? What voltage does the board run off of, and is pin2 J4 ground? R4,5,6 - do these set the range of the POT, and with all those resistors in parallel, is it still linear?

Maybe I should just buy one to find out if it comes with better docs.

-gerald

strategy · Joined: Jan 03, 2009 Posts: 87 Location: portland, oregon

I corresponded with Bridechamber requesting some guidelines on components and got:

oldmanfury · Joined: Oct 20, 2006 Posts: 57 Location: CA

Ok, I tried out the Bridechamber circuit, and it works fine. One problem for my intended application is that although the VTL5C3/2 looks like a pot, it's two freekin vactrols that track eachother tied together. I'm trying to add voltage control to a stomp-box (all four knobs), and I think that 3 of the knobs need true pots attached.

So, I got creative and tried out the classic VC panner http://electro-music.com/forum/viewtopic.php?highlight=vactrol+panner&t=17019

I tied the two vactrol resistors together, hoping the end result of this one would be pot-like, but they don't cross eachother quite right (i.e., R1 + R2 is not constant as I vary the input CV.), and I'm not sure that I'm going to be able to pull off a VC pot with this circuit. I don't want to go down the MAX digital pot route, but am I SOL with vactrols?

-gerald