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Adding CV to potentiometer
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Adding CV to potentiometer
drsm79
Is there a simple way to add CV to pot? I've got a guitar pedal delay circuit that I'd like to modify by adding CV to delay time, feedback etc. so these can be controlled by an LFO.
Is this a normal thing to do? I saw something about using an LED and LDR, which seemed a bit odd...
jhulk
the pot is an adjustable resistor so is cv control so you need some thing that acts the same either by an ota as a adjustable resistor

or a fet based one or an led/ldr or opamp circuit to perform the same function
PWM
What pedal are we talking about?
craigie77
Synthrotek Vac Pak...

http://store.synthrotek.com/Vac_Pak_Kit
drsm79
It's this board http://shop.pedalparts.co.uk/Echo_Blue_Delay/p847124_8651062.aspx

I want to be able to set a value (feedback, time...) with the pot then modulate it via cv.

I did look at the vac pak but that starts getting expensive (in proportion to the board) for multiple parameters.
gasboss775
drsm79 wrote:
I did look at the vac pak but that starts getting expensive (in proportion to the board) for multiple parameters.


They provide the Schematic for the kit, might be cheaper if you just bought your own parts and followed the schematic. NSL32 Vactrol are only around £2 each.
gasboss775
Another idea for driving vactrol from a single supply. Haven't tested this but it should work:

drsm79
Yeah, I think you're right. Will give a homemade vac pak a whirl.
Troubleshooter
In every case that depends how the pot is used in the cirquit.
There is not one method that works for every pot in any pedal...
kassu
For this particular circuit you can use the vactrol directly replacing the Decay pot (since the pot is used as just a variable resistor).

There are also other ways of voltage control, on this page is an example circuit that might work as well: http://www.sdiy.org/destrukto/vc-echo.html
(The circuit would replace the decay pot and associated resistor, and connect to pin 6 of the PT2399)
whoop_john
ersatzplanet
Here are a few more -
drsm79
This is all awesome, thanks everyone
Sandrine
Another way would be to go digital. Use an MCP4018 digital pot (i2c) for 128 steps (or an HV digPot if voltage is bigger than 5V!) off an arduino with it's ADC hooked to an input buffer from the CV jack. A little over the top, but it'd be flexible and could have curves and such, tailored to whatever pot is being replaced. Also the original pot could be fed in as well so it doesn't lose it's functionality as what is was, or an offset etc.
It's all in the math applause
Rob_C
gasboss775 wrote:
drsm79 wrote:
I did look at the vac pak but that starts getting expensive (in proportion to the board) for multiple parameters.


They provide the Schematic for the kit, might be cheaper if you just bought your own parts and followed the schematic. NSL32 Vactrol are only around £2 each.

Vac Pak PCB only is available for $4.99.
Siri
Sandrine wrote:
Another way would be to go digital. Use an MCP4018 digital pot (i2c) for 128 steps (or an HV digPot if voltage is bigger than 5V!) off an arduino with it's ADC hooked to an input buffer from the CV jack. A little over the top, but it'd be flexible and could have curves and such, tailored to whatever pot is being replaced. Also the original pot could be fed in as well so it doesn't lose it's functionality as what is was, or an offset etc.
It's all in the math applause


@sandrine how the original pot will affect the digital pot? offset?
Sandrine
Siri wrote:
Sandrine wrote:
Another way would be to go digital. Use an MCP4018 digital pot (i2c) for 128 steps (or an HV digPot if voltage is bigger than 5V!) off an arduino with it's ADC hooked to an input buffer from the CV jack. A little over the top, but it'd be flexible and could have curves and such, tailored to whatever pot is being replaced. Also the original pot could be fed in as well so it doesn't lose it's functionality as what is was, or an offset etc.
It's all in the math applause


@sandrine how the original pot will affect the digital pot? offset?


In thru another ADC to be an offset, attenuator, and/or duplicate the original in the absence of a CV being jacked in. That's the beauty of digital control, easy intelligent multifaceted routing. I love digital with analog even though the resolution bug always looms
PK808370
jhulk wrote:
the pot is an adjustable resistor so is cv control so you need some thing that acts the same either by an ota as a adjustable resistor


The discussion has mostly centered on vactrols so far. As hulk mentioned, I have heard of OTAs for this application. At $1.50, an LM13700 is much more affordable than a vactrol in general. Is there a reason the OTA method is less desirable than vactrols if you're just trying to use it as a variable resistor?
jahreed
what about the synthrotek vacpak?

https://www.youtube.com/watch?v=S-kFCt5QfCQ
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