[sdiy] All things Lowpass gate

[Scott Stites]

Self propelled Buchla threads are what I *love* to see.

Anyhoo, I measured the leakage on mine tonight.  Right in line with what you
report - guess the way I use it, I never really noticed it before.

I've worked up a 24 dB Vactrol filter based on two LPG's with a resonance
loop (an idea Mark Verbos touched on in the archives quite a while back).  I
did put an expo sink on that in place of the Buchla circuit, and it did make
the response much more 'normal'.  However, I find I do prefer the original
Buchla control for the LPG itself.  Once again - that's just me....=0), and,
like you say, it's not necessarily bad.

I agree also with the 'personality' of the modules - most Buchla adaptations
are always going to have a bit of that, due to the Vactrol 'angle'.  It's
part of the fun actually, at least for me.  I always breadboard the Vactrol
before comitting it to PCB, just to make sure it 'plays well with others'.
In fact, someone once told me that the modules should be given names due to
their individuality.  Let's see, which LPG am I going to patch to - Eugene
or Beatrice?  =-D

Thanks for all the info!
Cheers,
Scott


----- Original Message -----
From: "Peter Grenader" <peter at buzzclick-music.com>
To: "Peter Grenader" <peter at buzzclick-music.com>; "Scott Stites"
<scottnoanh at peoplepc.com>; "Peng" <peng3002 at yahoo.com>;
<synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, August 10, 2004 10:22 PM
Subject: [sdiy] All things Lowpass gate


> OK, I might as well make a self-propelled string out of this.
>
> The issue of LPG bleed through has been nothing short of an obsession with
> me.  I've tried a myriad of things, even contemplated adding a rather
> complicated noise gate after this circuit to kill it.  I've set one up
using
> a comparator driving an amplitude VCA in a patch and it works, but it's a
> major pain in the butt to find one noise gate circuit which works with
both
> fast and slow attack signals through the LPG.
>
> Garry Chang has a software plug-in that does the trick, but it can only
work
> with recorded passages as it looks into the future 10ms before the sound
> occurs and makes all necessary corrections before hand.
>
> Some things you can try is dicking around with the .001 (i think Don has
it
> has a 920 picofarad or something like that) going into the buffer. Adding
> about 200 pf there will attenuate the bleed, but it comes at a cost of
> increasing the filtering which in turn kills a bit of your high end.  The
> bottom line, if you look at the databook you'll notice vactrols don't shut
> all the way down and there lies your problem.  So to attenuate the bleed
> you've got to try other things and  without going into the details of how
I
> finally got it to what I consider  manageable level (.3mv), I'll give you
a
> hint  -  180 degrees and feedback.  there you go.
>
> Another 'cute' personality trait of the LPG you'll notice is that without
> presorting them beforehand, no two vactrol have an identical curve and
this
> effects the amount of  'ring' in combo mode.  Some ring more than others.
If
> you look at the datasheet you'll realize Perkin Elmer is well aware of
this
> as their dark resistance spec is listed as something like 'off resistance
> after ten seconds'.  This time delay compensates for the variance between
> the devices which P. E. explained to me if the nature of photo resistors.
>
> If you want to standardize the curve across multiple LPGs, I recommend you
> purchase more vactrols than you need and sort them before hand.  At 5
bucks
> a pop, this is a tall order however.  The fact that they are diffewrent
form
> one another doesn't bother me - I actually embrace that part of its
> personality.
>
> Another thing - Don Buchla's  LEDdriver circuit, his sort of quasi expo
via
> a zener trick is on the near side of crappy and you'll lose a bunch of the
> energy in long attacks. Take note that  the Buichla EGs are linear. but
> instead of a nice linear slope on long attacks you'll get more of a
> 'nothing nothing nothing - there There THERE' sort of thing which isn't
> necessarily bad, but it doesn't track the control voltage it's getting.
The
> 292 rev B's led driver is slightly better in this regard.  You can however
> improve the response using other methods of turning the vactrol led on.
>
> hope this helps,
>
> P-
>