PLAN B analog blog

I just hit the wrong button and zapped a post here instead of
approving it.  Here's the original text:



is there a way to shorten the gates?

I compare both of the LPG's outs and one is more open than the other
with exactly the same sound, cv input and setting

My reply:

Vactrols vary greatly from one another.  It's the photo resistor.  As
fact to this,  looking at the PerkinElmer data sheet,  the
manufacturers of Vactrols, they list the 'off' resistance as 'off
resistance @ 10 seconds'  This wasn't a typo.  Vactrol are SOMETIMES
(keyword there) so slow they take 10 seconds to calm down to the dark
state, meaning their resistance it rests at then the internal LED i off.

Go here to download the PE datasheet/ One the pge loads, click
DATASHEET on the righthand side:

http://optoelectronics.perkinelmer.com/catalog/Product.aspx?ProductID=VTL5C3%2f2

In short,  it's the nature of the beast.  In that light (pardon the
pun), your DTG is operating properly because the vactrols in both
halves are operating within their specs.

If we've ever shipped a Model 13 in which both halves are exactly the
same it was purely coincidental.  The chances of that happening are
slim.  If PerkinElmer is unwilling to presort their vacs, its even
more of a daunting task for little 'ol Plan B.  btw, you will not find
this problem on Doepfer LPGs, because he uses a completely different
vactrol which is designed to react quickly.  The good part:  they are
pretty much the same from one to another.  The bad news,  as a result
of them being fast, they don't ring as ours do, and that is 'the
thing' with low pass gates.  They have that wonderful timbral ringing.
 Without that what do you have?  You have a Model 12 lowpass output
with resonance turned to minimum.

A solution - something Buchla 200 owners were used to doing because
the 1970's edition of the 292 (Quad lowpass gate) also reacted
differently from gate to gate:  This variance you're speaking of only
becomes an issue with short percussion-type events. if you ever
construct a patch which uses two halves of the DTG this,  simply run
the output of the slower one through a standard VCA afterward and use
an Envelope Generator to open that VCA which is triggered the the same
source triggering the EG that's controlling the slower DTG. Simply set
the EG controlling the standard VCA so it fades the signal out of the
same time the faster one does it by itself.

Is this a band-aid to correct a bad module design?  No.  It's a fix to
compensate for natural and expected variances introduced by the
manufacturer of the very part that makes this a great sounding module.

- P