[sdiy] Vactrol control ideas wanted

[Scott Stites]

Hi Harry,
I'm using a couple of 2N3904's configured pretty much like Rene Schmitz's
expo control for his VCO 4069

http://www.uni-bonn.de/~uzs159/

I've just got the anode of the 'top' Vactrol LED tied to +15V and the
cathode of the last Vactrol LED connected to the collector of T2.  I've got
10 of them in series (VTL5C2's for economic reasons -If I had deep pockets,
there'd probably be 5 VTL5C3/2's instead ;->), and I have more than enough
sweep range.  Much more than enough - I still have to go back and tame it
down a bit.

I'm using 2N7 caps, BTW, which seem to fit the 5C2's quite well.  I arrived
at 10 stages for two reasons - ran out of room on the s*lder*ss br**dboard,
and I think 10 LED's is about the max for the expo circuit (well, never
tried going beyond that anyway).  I plan some day to use at least a couple
of expo sinks and see if I can approach anything like the 'Tube' phaser (20
stages).  I get carried away sometimes.....

Cheers,
Scott


----- Original Message -----
From: "harrybissell" <harrybissell at prodigy.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, October 19, 2003 6:09 PM
Subject: [sdiy] Vactrol control ideas wanted


> Hiya all...
>
> I'm planning a vactrol based phaser module (yeah, I know...
> how original :^)
>
> I have a couple of ideas for controling the Vactrols.. like to have
> any feedback. Maybe you've been there, done that...
>
> 1)  Control all the Vactrols with a series current (possibly including
> the
> LEDs in an opamp feedback.
>
> pro: its easy
> con: it probably does not cause the interesting part of the sweep to
> occur
> over a wide range
>
> 2) Control the Vactrols by running them all in series, with one
> Photocell fed
> back into the control circuit.  This is an adaptation of the National
> Semiconductor
> "Analog Multiplier" found in AN-20.  However, this version will not have
> one LED
> illuminating multiple photocells.
>
> pro: (subject to matching) the photocell resistance can be accurately
> specified
> con: its more complex
>
> Your thoughts ladies and gentlemen ???
>
> H^) harry
>
>