[sdiy] Vactrols subbed for dual-gang pot

[Dave Kendall]

Hi All.

Had an idea to replace the dual-gang FREQ pot in the CGS30 band-pass  
filter with a pair of home-rolled Vactrols driven from the output of an  
opamp buffer for CV control of frequency.  It doesn't need to be  
massively stable, as I'm expecting to use it with a modulation source,  
rather than to set a fixed frequency via CV. Details of the BPF circuit  
here ;  http://www.cgs.synth.net/modules/cgs30_bpf.html

To the best of my knowledge, the CGS30 circuit is very similar to the  
Roland SH5 BPF.

By hand-selecting LED/LDR pairs, I can match the off resistance of the  
home-rolled vactrols to within a few ohms, and limit the max resistance  
fairly easily, but between those points, I suspect the LDRs may "drift  
apart" due to different response curves to incoming CV. How would this  
affect the BPF circuit? Does it even matter?

FWIW, I used white or red superbright LEDS with these LDRs and got the  
"off" resistance down to around 130 ohms in the best cases.  (white  
LED)

http://www.rapidonline.com/productinfo.aspx? 
tier1=Electronic+Components&tier2=Optoelectronics&tier3=Photodetectors&t 
ier4=Miniature+light+dependent+resistor&moduleno=34796#techspec

Any and all thoughts welcome....

cheers,
Dave