[sdiy] Vactrol VCF rev 2: help needed

[JH.]

Not that I would have done this already (no time ...), but I often think
about using dual (center-tapped) vactrols like the VTL5C3/2 in a
servo configuration, where you run DC on one half, and your
signal on the other half. Then you should be able to build as many
tracking Vactrols as you like. 
Only constraint is that this will be very difficult if you don't have
one end (the center tap) of the vactrol grounded.

JH.



----- Original Message ----- 
From: <mirwin at qouest.net>
To: <synth at oldmail.charlielamm.com>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, March 06, 2005 9:17 PM
Subject: Re: [sdiy] Vactrol VCF rev 2: help needed


> Charlie,
> My experience with matching individual LDRs (for homemade vactrols) is 
> that you need at least 50 units to get a matching set of six LDR's - 
> maintaining matching within 20% or so over five or six octaves in a 
> filter. You can also try tricks like covering part of an LDR with magic 
> marker to raise it's resistance range to help it match the others..
> 
> With vactrols, probably the best thing to do is to measure the 
> resistance at a given LED current and select the filter capacitors 
> individually for each vactrol (so the RC product is the same for all 
> units). Also, run the LEDs in series as other have mentioned.
> Regards, Mike
>