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Adding a fine tune knob?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Adding a fine tune knob?
deepblackjoe@gmail.com
To a Stroh Pico vco. There's probably several ways to go about it, but I could use some guidance. Can someone tell me a simple way to do this? I looked at my blacet vco schems but got confused. Maybe two different value pots in series somehow?
Orgia Mode
If you can find a schematic for the pico, that would help tremendously. I tried and failed.
papz
I don't know this specific circuit but a pot connected to the + rail should do the job.
This second CV is to be added in parallel to the one coming from the frequency pot via a resistor. Select the resistor on test, the higher the value the finer the tune .
Orgia Mode
papz wrote:
I don't know this specific circuit but a pot connected to the + rail should do the job.
This second CV is to be added in parallel to the one coming from the frequency pot via a resistor. Select the resistor on test, the higher the value the finer the tune .


Yep this, but what if its a thru-zero vco?
A voltage divider pot, pin 1 to -v and pin 3 connected to +v, then a series resistor connected to the wiper and in parallel to the CV like you said.

Maybe even -1/2v and +1/2v, depending on the schematic if we can see one.
papz
I think the Stroh designer is J3RK, you could ask him : https://www.muffwiggler.com/forum/profile.php?mode=viewprofile&u=1931
deepblackjoe@gmail.com
Thanx alot for the ideas, I think I should try both, really just to see. I'll set this up after din. I don't have the schematic, just the board populated, which reminds me I haven't got the tempco yet. Should be here any day tho. I'll let u know what works. Double cheers
ersatzplanet
An external DC voltage mixed to the input would be all that is needed. You don't even have to modify the module. All you need is a DC voltage source that is scaleable to a small range easily made with a pot and resistors off of the power bus, or with a module like the Doepfer A-176 CV source module (which has four of them), and a buffered must to combine that with what you would normally plug into the module for CV control. Basically any decent mixer module that has a unity gain setting will work too. The latest versions of the Doepfer mixers normalize the first input to a DC voltage so that would be all you would need too. Use the other inputs for the other CV in and plug it into the Pico.
Orgia Mode
ersatzplanet wrote:
An external DC voltage mixed to the input would be all that is needed. You don't even have to modify the module. All you need is a DC voltage source that is scaleable to a small range easily made with a pot and resistors off of the power bus, or with a module like the Doepfer A-176 CV source module (which has four of them), and a buffered must to combine that with what you would normally plug into the module for CV control. Basically any decent mixer module that has a unity gain setting will work too. The latest versions of the Doepfer mixers normalize the first input to a DC voltage so that would be all you would need too. Use the other inputs for the other CV in and plug it into the Pico.


Good point
J3RK
To add a fine tune control, simply add a 2.2M or 3.3M summing resistor to the summing op amp in the exponential circuit, and connect a pot the same way the coarse tuning pot is connected. I don't have the schematic in front of me, so I don't have the op amp designation right now. You just connect the resistor to the same input pin of the op amp that the coarse tune is going to.
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