Basic Vactrol Circuit Examples

sine_wave · Joined: Sep 25, 2006 Posts: 20 Location: NY

Hey guys. I'm new here. I was wondering if some of you can post some simple vactrol circuits so I can see them in action and get a better understanding of how they work. Maybe some CV applications? How are they used in the Buchla 292? Is it in the audio path? CV path? Why does it have that pluck? Appreciate any info.
Thanks,
Sine

fonik · Posted: Tue Sep 26, 2006 1:17 am Post subject:

vactrols are used to apply voltage control. actually the diode of the vactrol is part of the cv circuitry, the resistor part of the audio path. some samples:

cheers,
matthias

p.s.: i'm not a native speaker, so could you be so kind to explain the meaning of pluck in that context?

buchla 292 clone.gif

Description:

Peter Grenaders Buchla 292 clone (personal use only)

Filesize:

103.54 KB

Viewed:

3756 Time(s)

This image has been reduced to fit the page. Click on it to enlarge.

vactrol crossfader.gif

Description:

marc verbos vactrol crossfader

Filesize:

87.64 KB

Viewed:

1537 Time(s)

This image has been reduced to fit the page. Click on it to enlarge.

tremolo.gif

Description:

the tonepad version of the tremulus lune tremolo (personal use only)

Filesize:

16.31 KB

Viewed:

1571 Time(s)

This image has been reduced to fit the page. Click on it to enlarge.

vtl5c3 · Posted: Tue Sep 26, 2006 8:12 am Post subject:

Here's another one. Pretty simple portamento circuit from the Buchla 218 touch keyboard.

Buchla_218_Portamento.jpg

Description:

Filesize:

263.93 KB

Viewed:

948 Time(s)

This image has been reduced to fit the page. Click on it to enlarge.

sine_wave · Joined: Sep 25, 2006 Posts: 20 Location: NY

Thanks for the applications guys, but can you tell me what they're doing? Also how would you use a vactrol in a cv circuit?

Scott Stites · Posted: Tue Sep 26, 2006 9:54 am Post subject:

The 'pluck' is the effect a Vactrol can render when it's struck by a fast transient, and it's being used to control filter cutoff or VCA level, or, especially, *both* as in the case of the Buchla 292 Low Pass Gate. A lot of it has to do with the On/Off reaction time of the particular species of Vactrol being used - VTL5C3 is generally the one that you want for that. Quite a bit of discussion of it occurred in the two following threads:

http://electro-music.com/forum/topic-10024.html

http://electro-music.com/forum/topic-10615.html

Cheers,
Scott

vtl5c3 · Posted: Tue Sep 26, 2006 10:38 am Post subject:

Vactrols have two components: an LED (Light Emitting Diode) and an LDR (Light Dependent Resistor).

The way they're used is current is pumped through the LED. More current means a brighter LED. As more light shines onto the LDR, it allows more current to flow through it.

Is that what you were looking for?

StephenGiles · Joined: Apr 17, 2006 Posts: 507 Location: England

I thought I would put a link here to my clean up work in progress on the ADA Final Phase circuit diagram - which I have spent hours on so far, when I should have been gardening or doing my tax return, as there has been some reference to phasing here.
http://img.photobucket.com/albums/v517/uncle_boko/ADAfinalphase1087clean.jpg
For some reason I can't get it any bigger in photobucket, but you can get an idea of what is going on, ie PWM controlled by a 4007/4047 combination.

WHAT???????? I hear you say.........little or no interest from the fuzz box brigade over at Aron's site

sine_wave · Joined: Sep 25, 2006 Posts: 20 Location: NY

Hey guys. Those descriptions helped me out a lot. Is there a "status-quo" cv-in vactrol schematic around? For instance something I could tack on the input of a vca design?

Scott Stites · Posted: Wed Sep 27, 2006 9:33 am Post subject:

StephenGiles · Joined: Apr 17, 2006 Posts: 507 Location: England

bugbrand · Posted: Wed Sep 27, 2006 1:05 pm Post subject:

Scott Stites · Posted: Wed Sep 27, 2006 8:49 pm Post subject:

The Low Pass Gate is one of the classics - for me, it has the same status that the Moog Ladder does in the pantheon of analog synths. Of course the Moog and the LPG are miles apart in how they sound. But the Moog Ladder more or less defined the Moog sound, and the LPG more or less defined the Buchla sound. I've still got Mark Verbos' version (using the CD4066 to flip between Low Pass/Both/VCA modes). It's the 'Both' mode that really lets her rip.

A lot has been said about the acoustic, thwacky, plucky, thwippy properties of the LPG, but really I think just the sound of it modulated with less abrupt transients is wonderful. One of my favorite compositions I did was a demo of the LPG being treated more as a 'normal' filter. On my original Birth of a Synth page I wrote this:

Zoomby · Joined: Aug 28, 2006 Posts: 18 Location: Germany

Hi, I'm also interested in vactrols.
Can someone give tips what to consider when using vacrols? For example what LEDs and LDRs to use, and what exactly the Lux unit means...
Bye.

blue hell · Posted: Thu Sep 28, 2006 3:06 pm Post subject:

germaniac · Posted: Fri Sep 29, 2006 1:41 pm Post subject:

Scott Stites · Posted: Fri Sep 29, 2006 8:26 pm Post subject:

I figured that'd get a rise out of someone Very Happy

intellijel · Joined: Sep 10, 2006 Posts: 67 Location: Vancouver, BC

I am looking at the 292 schematic and especially interested with how the vactrol is driven.

I have a few questions:

1. Are the leds in the feedback loop of the OpAmp so that there is a log response to linear input voltage? Is that for musical reasons or or does it counteract the non-linearity of the vactrol to produce a more linear response relative to CV in?

2. What is the function of the Zener diode in the feedback path?

3. Why is that resistor in parallel to the Zener diode (this should be obvious but I am feeling dumb on this one Razz

I am sure it has to do with when the Zener has not turned on yet.

adamon · Posted: Sat Jul 10, 2010 7:16 pm Post subject:

Any one have any luck running this on +/-9V? Any changes necessary for this to work?

adamon · Posted: Mon Jul 12, 2010 11:16 am Post subject:

So I've been messing with home made vactrols for a while now, and I've got several modules that come close to imitating the functionality of the lpg, but the one thing I don't really get is the nice decay. This had lead me to wonder if there is really that much decay inherent just in the vtl's themselves, or if there is more to it than that (in the rest of the circuit).

Along the same lines, if the vtl's really do have that much decay, how is it done? It seems like it should be on the light emitter side of things, but I don't see how this would be possible with an led. Is the lighting element something else?

Tim Stinchcombe · Posted: Mon Jul 12, 2010 3:07 pm Post subject:

adamon · Posted: Tue Jul 13, 2010 9:23 am Post subject:

Thanks Tim, that answers my question perfectly! Great links too; lots of good information in them. Definitely throwing those in my ever-expanding reference folder.

did_poutrator · Joined: Jul 02, 2007 Posts: 7 Location: toulouse

Hello,
I know I am diging out an old topic but I was wondering one thing. I plan to do an LPG vactrol filter and my question is :

did someone try to apply different CVs to the 2 vactrol leds? And what would be the effect on the sound?

Thanks a lot

Grumble · Posted: Thu Dec 21, 2017 4:32 am Post subject:

Why would anybody use vactrols while there is a H11F1 OPTOFET? Rolling Eyes

mush · Posted: Thu Dec 21, 2017 1:17 pm Post subject:

Grumble · Posted: Thu Dec 21, 2017 1:49 pm Post subject:

The fun thing is that you can have the optofet react like a vactrol while the opposite is not possible Wink