[sdiy] Low Pass Gate passing DC - forgedabouydit

Peter Grenader petergrenader at mksound.com
Tue Jun 25 06:01:19 CEST 2002

Hark, the possiblity of yet another string on this already  over-cooked
subject (me being the primary chef involved):

But i thought you guys would like to know that if you try and gate a contant
DC level through a Buchla low pass gate circuit, it just cocks it's ear at
you like a dog and does nothing.

I was hoping to replicate (foolish me) that famous (in my world anyway) Mort
Subotnick percussive slap used on Until Spring and Sky of CS.  He did it
using a control track which was fed through a Buchla Envelope Follower
(which i believe is vactrol based) which was not only trigger the envelope,
but creating the frequency transcient as well.  The EF was set  to a very
fast decay which was going through a voltage processor and processed to
barely make a tiny upward  frequency spike to the sounding oscillator right
at the beginning of the attack.

No use arguing how he did it, I dropped him a note a couple of days ago and
this is straight from the butterfly's mouth, so to speak.

Why i  suspect the B Env Follower to be vactrol based is if you listen to
those trancients he created, they are very breathy.  I assumed this was due
to a vactrol's slow response awakening to the control track bleep. It
created a timing thing that gave you barely a hint of the transcient.

So, since I don't have a Buchla Envelope follower, I figured I could get the
same result if I used a trigger as the control signal in my Low Pass Gate
clones,  gating a pot-variable constant DC level.  By gating a constant
level, I figured I'd be able to adjust it until I would hit the timbral
sweet spot

But ooooooh-nooooooo, this circuit deosn't respond to contant DC levels at
it's input (makes sense since-in the intrument for which is was designed
there was no way of getting that DC to the gate signal input anyway).

If anyone knows if the Buchla Ebnvelope Follwer used a vactrol, I'd be much

let me know and thanks,



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