[sdiy] vactrol?

[Peter Grenader]

groovyshaman wrote:

>> Anyway, i could use a little help on where to source these devices, and
> any tips on connecting them.


Yeah, they are pricey, but they perform really really well for musical
applications..so - GET OVER IT (kdding).

The VTL series is made by Perkin-Elmer and available through Allied. As far
as app notes, go to the Perkin-Elmer site and look under the
opto-electronics tab, there is a downloadable pdf which will answer all your
how-to's in this regard.

All Electronics has a bin with some sort of opto-isolated photo resistor in
them as well.  They are not Perkins and I have no idea of their resistance
range, nor if they are available in their on-line catalog as much of the
floor stock is not.  They may sell them if you call - but I'm pretty sure
the guys won't have an idea what a vactrol is.  If it's any help, they are
kept in the same isle and right next to their zeners and chokes.

There's also a Canadian firm making vactrol-type devices (the name escapes
me right now -duh) which I've heard good things about. I want to say
Soltronix or something along those lines, but I'm pretty sure that's off the
mark.  Some of these are very fast though (M&K used them as the primary
ingredient in the limiters within their whiz-bang subwoofer amp which had a
1/2 cycle response time at a 125 hertz - that's pretty damn fast for a
photoresistor.). 

There's also a slew of them made in China, which I believe Dieter is using
in his vactrol devices.  They don't appear to have the same decay time as
the VTL series, which for me is THE thing with Vacs.  The Chinese variety
I've seen are rectangular shaped - about a 1/2 inch long by about 1/4 inch
square.

My feelings - if you don't want a vac to be slow, then don't use one.
You'll spend less money and take far less space using a trans. amp like the
3080 or 13700. Remember that not only are vac's as big as june bugs,  you've
got to drive them and this will require a somewhat linear current or voltage
source - so we're talking an tranny, or an expo pair, an op amp and a
trimmer or at very least a divider for the setting the gain.

hope this helps,

- P