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Basic Vactrol Circuit Examples
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sine_wave



Joined: Sep 25, 2006
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PostPosted: Mon Sep 25, 2006 4:32 pm    Post subject:  Basic Vactrol Circuit Examples
Subject description: I want to learn!!
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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
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fonik



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PostPosted: Tue Sep 26, 2006 1:17 am    Post subject: Reply with quote  Mark this post and the followings unread

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?


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vtl5c3



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PostPosted: Tue Sep 26, 2006 8:12 am    Post subject: Reply with quote  Mark this post and the followings unread

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


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sine_wave



Joined: Sep 25, 2006
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PostPosted: Tue Sep 26, 2006 9:39 am    Post subject: Reply with quote  Mark this post and the followings unread

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?
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Scott Stites
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PostPosted: Tue Sep 26, 2006 9:54 am    Post subject: Reply with quote  Mark this post and the followings unread

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
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vtl5c3



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PostPosted: Tue Sep 26, 2006 10:38 am    Post subject: Reply with quote  Mark this post and the followings unread

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?
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StephenGiles



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PostPosted: Tue Sep 26, 2006 1:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
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sine_wave



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PostPosted: Tue Sep 26, 2006 5:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

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?
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Scott Stites
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PostPosted: Wed Sep 27, 2006 9:33 am    Post subject: Reply with quote  Mark this post and the followings unread

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


Maybe you should have used germanium Vactrols Very Happy

Cheers,
Scott
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StephenGiles



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PostPosted: Wed Sep 27, 2006 10:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Laughing Laughing Laughing
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bugbrand



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PostPosted: Wed Sep 27, 2006 1:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

sine_wave wrote:
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?


The Low Pass Gate near the top of the post is a simple VCA which also does filtering -- I want to have a go at that too.

One question though -- I'm just wondering if the 20k pot in the vactrol drive circuit is a trimpot (I'm figuring that it is). Had a look at other lowpassgate pictures and there's usually only one control - gain, so I'm presuming that'd be the offset control in the schematic above? Hmm, CV mod depth control would be useful..

I had a quick look over at Scott's Mutant Vactrol filter which also has (naturally) some vactrol control -- thought that'd maybe shed some light on the lowpassgate questions, but it has a different (and useful looking!) driver circuit..

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Scott Stites
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PostPosted: Wed Sep 27, 2006 8:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

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:

Quote:
Below is a sample of the Low Pass Gate, mostly in the 'Both' mode. There are some long, sustained notes so that you can also get an idea of what the thing sounds like when it's not being 'knocked around' by short pulses, which seems to be the content of most LPG samples that are around. Of course, that is a nice effect, and I do knock it around a bit here. For that, the sequencer, which is controlling the VCO's, is also triggering the AR EG, which is set to 0 attack and 0 decay, and that is modulating the Low Pass Gate on the sequenced part. Any other VCA would just register clicks with this type of patch, but the Low Pass Gate 'rings', which gives a very nice, acoustic texture to the sound. On the sequence, two VCO3's are passed through the LPG, along with the Blacet DSC2000, which is triggered on the fifth step of each sequencer cycle. The DSC2000 pitch is controlled by the Fluctuating Random Voltage circuit. Towards the end is a collage of random...err....noise produced by running the VCO's through the LPG in various modes - the VCO's are controlled by the Fluctuating Random Voltage here, and the triggers are produced by the FRV as well for the main sequenced part and the noise part.


I've been told that it's my most 'analog' sounding sample, though all of my samples are analog - I don't have a softsynth or any digital synths Very Happy
It was recorded straight to computer (this was before I had the D8 ).

If one care's to hear it, here's a link:

http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/lpgseq1.mp3

It's 3.9 MB.

Another often overlooked filter is the Buchla 291 BPF - there is nothing in the world like that beast - I love it.

Cheers,
Scott
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Zoomby



Joined: Aug 28, 2006
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PostPosted: Thu Sep 28, 2006 2:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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Blue Hell
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PostPosted: Thu Sep 28, 2006 3:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Zoomby wrote:
and what exactly the Lux unit means...
Bye.


Had to look it up, Wikipedia has a good item about it.

Basically the Lux says something about how bright humans experience a certain lighting condition to be.

It can get quite messy with these weighted units, as the weighing is done with reference to a "standard eye". And of course LEDs and LDRs would know nothing about that, but instead have there own curves for how the light intensity or sensitivity varies with the color of the light.

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germaniac



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PostPosted: Fri Sep 29, 2006 1:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:

Maybe you should have used germanium Vactrols Very Happy


Hey, I resemble that, buster! Wink
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Scott Stites
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PostPosted: Fri Sep 29, 2006 8:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Sat Jul 10, 2010 6:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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adamon



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PostPosted: Sat Jul 10, 2010 7:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Mon Jul 12, 2010 11:16 am    Post subject: Reply with quote  Mark this post and the followings unread

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?

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Tim Stinchcombe



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PostPosted: Mon Jul 12, 2010 3:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

adamon wrote:
Along the same lines, if the vtl's really do have that much decay, how is it done?
It's due to the chemical properties of the material used for the photoresistor - some of the materials simply cannot change resistance very quickly, especially from light to dark I believe (low resistance to high resistance).

There are two documents you should see, if you haven't already, but both are very hard to find at the Perkin Elmer site (and I'm pretty sure they are there - but then when you search for 'vactrol', and it doesn't find anything, there ain't much hope. Still amazed that such big companies can produce such crap websites). Anyhow here they are:

http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.pdf

http://www.tubes.it/Vac-Tec_Optoisolators.pdf
(The second half of the document, after the spec sheets.)

Tim

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adamon



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PostPosted: Tue Jul 13, 2010 9:23 am    Post subject: Reply with quote  Mark this post and the followings unread

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.
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did_poutrator



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PostPosted: Fri Nov 17, 2017 5:09 am    Post subject: Reply with quote  Mark this post and the followings unread

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
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Grumble



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PostPosted: Thu Dec 21, 2017 4:32 am    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Thu Dec 21, 2017 1:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

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


Well, mainly because of the slow fading, non-linear nature of the vactrol. It gives a very musical response. When you 'ping' a vectorial with a trigger you get a very acoustic response - the main thing with the classic 'buchla-bongo' or 'buchla-banjo'. The interesting thing with circuits for musical applications IMHO is that it is not very often the best solutions on an engineering level that are the most interesting from a musical viewpoint - but it can be very weird or lofi solutions that make up the greatest instruments.
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Grumble



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PostPosted: Thu Dec 21, 2017 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

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