[sdiy] Vactrol VCF rev 2: help needed

[harrybissell]

I don't think that the center tapped ones have a very GOOD
center tap at that... maybe 20% best case.   At least the
response time should be similar in the same cell, and thermal
tracking should be a little better than two cells...

aside from all the poisonous heavy metals in the construction...
there are other reasons for ditching the vactrols (matching, speed,
etc...)

H^) harry

"JH." wrote:

> 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
> >