[sdiy] Time fer a new string: Low Pass Gate Update

[Peter Grenader]

Those of you who possibly added a Buchla Low Pass Gate to your list of
projects that were experiencing the same problems I was having with lack of
high end, will appreciate the fix I received today from a modular
manufacturer I will leave nameless unless he wishes to be credited (which I
would surely like to do).

In brief:  My LPG sounded great (or so I thought), until I listened to some
of the old Buchla music I did 20 years ago and realized, even after 20 years
of the AMpex 456 destroying the high end, that it was a lot brighter than
what I was currenty experiencing real time.  Plus, I was getting sharp
attacks then that I couldn't now.

too weird.

I then took my gate over to a neighbor who owns a Buchla and noticed it
sounded  exactly like his orginal LPG's he had and had all of the high end I
could not pull out of it at home

also too wierd.

I suspected was I wasn't giving the vactrol enough juice to sufficiently
raise the resistance in the filter to allow passage of the higher harmonics
of my gated signal.  I suspected this due to the fact that the voltage
limits of my system for both AC and DC were marketably different from those
in Donny B's box.

If you look at the schemo of a 292C or D, you'll notice a 3.9 volt zener
limiting the low voltage going into the vactrol's LED supply.  It was
suggested to me that this is where the problem may lie and I went over to my
local electronics shack to find zener's of a lower value.

They didn't have any.  Shucks.

So I went about it another way. You'll also notice that what's actually
going on is there is a divider formed before the anode of the vactrol's LED.
A 4 volt zener on top and a 4.7 K on bottom to ground. So I cut the
resitance down by about 60% and vwa-la, it did the trick.

Not only did I gain high end, I stiffened up the attack response of the gate
(which some of you mentioned before) AND I raised the signal level coming
out of the gate as a whole, which before was easily 10 DB lower than my
normal system level.

So there you go - if you make on of these jewels and aren't using it on a
Buchla,  cut the resister which is parallel to the zener to about 2.7 K and
you wil be good to go.

BTW, Allied has the VTL5C3/2 vactrols for about 4 bucks each.  Tons of them.

-Peter