[sdiy] Vactrols subbed for dual-gang pot

[Dave Kendall]

Thanks Harry.

The single LED idea is a good one. The off resistance will rise with  
two LDRs on a single LED, as with those particular LDRs, the angle of  
the LED gave minimum off resistance with the LED at 90 degrees to the  
flat front face of the LDR, so with a couple of LDRS, this will change.  
However it may not matter in the BPF circuit, except that the lowest  
frequency (I guess) will now be a bit higher.

I've got a dual scope, so will look at the X-Y trace idea.

Thanks for the tips!  :-)

cheers,
Dave


On Jul 9, 2010, at 21:39, Harry Bissell wrote:

> That would be the same as the guitar effect Mutron III. you should
> be able to see schematics on the web that detail how they did it. They
> used a dual photocell vactrol with a single LED.
>
> I'd make up a fixture to measure the cell resistance by having two  
> current
> sources or bias resistors to make a divider, and send the outputs to a  
> scope
> as an X-Y plot. Now you could choose cells to have the best  
> 'linearity' over
> the range instead of just the endpoints.
>
> If you did get dual cells, you might use the technique in National  
> Semiconductor
> AN-20 "Analog Multiplier". I've used that to replace a resonance pot  
> in a
> state-variable filter (to get VC resonance).  It works well, but use  
> an LED rather
> than the lamp. The lamp has hysteresis problems and a lot of thermal  
> lag that makes it
> unstable at really low illumination (high resistance values)
>
> H^) harry
>
> ----- Original Message -----
> From: Dave Kendall <davekendall at ntlworld.com>
> To: sdiy DIY <synth-diy at dropmix.xs4all.nl>
> Sent: Fri, 09 Jul 2010 13:23:20 -0400 (EDT)
> Subject: [sdiy] Vactrols subbed for dual-gang pot
>
> 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
>
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> -- 
> Harry Bissell & Nora Abdullah 4eva
>