Fine tune pot implementation

[Dogstar13]

I’m building another 3340 vco. My first one started out as this

I ended up making this

I intend to build the simpler circuit for my latest attempt, but I’d like to build it with a fine tune pot. I just don’t know how to do it. Can anybody help me with this?

[analogoutput]

A fine tune pot is just one that provides a smaller voltage range than a coarse tune pot. I can think of three ways to do that:

• Connect it to, say, 1 V and ground instead of the coarse tune pot’s 12 V and ground. (But you’d need a 1 V reference.)
• Use a fixed resistor in series with the pot, for instance something like 12 V → 100k fixed → 10k pot → gnd.
• Set it up just like the coarse tune pot but use a larger input resistor connecting to the wiper, maybe 1M or 1.5M instead of 100k.

In any case the other end of the input resistor connects to pin 15, just like that of the coarse tune pot.

Example of the third way, from the Kassutronics VCO3340:

(Here the coarse pot is 10k and the fine pot is 100k. They could both be 10k, though that would draw more current. If both were 100k the response to the coarse pot would be a bit nonlinear since the 47k input resistor is smaller than 100k, but that wouldn’t be terrible.)

[Dogstar13]

Thanks Richard! I got the module all built except the fine tune. The third implementation will be the one I use I think. I can easily add it into the circuit. I’m not for sure I will ever need it but I already drilled the hole in the panel and I just like the idea. The module is already running and calibrated. They are so easy to calibrate. The pot I installed was pretty much already set. I barely had to adjust it. I got lucky. I now have two diy 3340 vcos that can use the same cv and play in unison. Or transpose to another key or whatever. Very cool. I’m gonna build a third just like this one to top it off and have the triad!