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vactrol control over a pot set up as a voltage divider
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Luka



Joined: Jun 29, 2007
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PostPosted: Sat Oct 24, 2009 8:58 pm    Post subject: vactrol control over a pot set up as a voltage divider
Subject description: + another general vactrol question
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i'm trying to mod my grinder section of my wave multiplier. the vactrol circuit works but im not sure how to attach it to the pot when set up as a divider?

also

i was hoping to have a pot to control this function as well as the VC control

would it be better to use an offset pot in the vactrol circuit to simulate the original pot function or could i attach the vactrol circuit in series with the current pot?


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problemchild
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The Bad Producer



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PostPosted: Sun Oct 25, 2009 4:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Luka, you may have seen this, I found it very useful when starting using vactrols for VC:

http://thesquarewaveparade.com/images/VtoR.jpg



Charlie

edit: I have used the bottom one with offset to replace a pot...

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Luka



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PostPosted: Sun Oct 25, 2009 1:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks
i have tested the circuits on breadboard and i used that to make my vactrol mod pcb


im just curious how to interface it up to pots with the wiring style in the schematic above

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problemchild
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diablojoy



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PostPosted: Sun Oct 25, 2009 2:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

hi luka
i recently did some experiments along these lines
for a WSG for all the oscillator pots plus the freq pot and the supply power
the vactrols were all wired in parallel with those pots and in series with the positive power rail which worked ok those are all 1 meg pots so some small limitation on pot range is to be expected
I used VTL3 and from memory , so don't take this as verbatim ,the vactrol resistance was approx 3 meg when dark dropping to roughly 100 ohms when fully on, so in theory 3 meg in parallel with a 100k pot should give a better result. if you want to do this as per a voltage divder you would need to use 2 vactrols or say a VTL3/2, one of the ones with 2 resistive elements , just put all 3 leads in parallel with the 3 pins of the pot.
ray wilson recently posted another cct on his site using transistors [look in WSG mods ] which i am trying out now, I have designed and made a board, just haven't finished testing it yet. this is so i can retrieve the 8 vactrols for other experiments.
cheers Smile
Denis.
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DGTom



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PostPosted: Sun Oct 25, 2009 6:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

I assumed thats what the 3 pinned vactrols where for? direct replacements for pots? So the V2R cct. replaces the pot. I'm prob. wrong tho, you know what they say about assumptions Laughing
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Luka



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PostPosted: Sun Oct 25, 2009 7:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

if i understand how a pot works correctly, i might be wrong, would the top half of the pot resistnace works in the opposite way to the bottom half, ie as the resistance increases from the bottom wiper to the centre wiper, it decreases witin the centre wiper to the top wiper section

i dont see how a dual vactrol can simulate this, my original guess on this topic was perhaps i could add make a traditional divider layout and use the vactrol to control the bottom resistor.

btw, is that setup actually a divider? i notice that the top wiper doesnt link to anything.

can i just hook the vactrol to the pcb pads for the bottom wiper and center wiper in a traditional vactrol way?

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diablojoy



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PostPosted: Sun Oct 25, 2009 9:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

sorry luka as far as i can see the only way of using these as voltage dividers is to use 2 vactrols and control them so one is dark when the other is light
would be difficult to get them to pan exactly right
the required circuitry is beyond me at the moment maybe someone else here has looked into more thoroughly but from your first cct it does not appear you would need that functionality it looks like you only need to go in parallel with the wiper and pin 1 or 3 of the pot
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Luka



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PostPosted: Sun Oct 25, 2009 10:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

cool thanks denis

the wiring of this pot kind of confused me
ill wire up my wave mult tonight and see how i go

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Mooger5



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PostPosted: Sun Oct 25, 2009 10:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think it´s possible by splitting the CV into non-inverting and inverting signals. With one vactrol receiving an increase in voltage the other receives a decreasing one.
http://www.geofex.com/Article_Folders/VCA%20Applications.pdf see application number 3 and link them like this (not tested):
Posted Image, might have been reduced in size. Click Image to view fullscreen.
Blocks A and B are meant to be VCAs but I think it should work with vactrols too.
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diablojoy



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PostPosted: Mon Oct 26, 2009 5:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

yes but only sort of
vactrol response is not linear
which would need to be taken into account
to emulate a pot as a voltage divider
It becomes quite complex in semi on states [very small voltage changes=large resistance variations and logrithmic to boot] and
to add to that the attack and decay times are also very different
from each other.
i see no easy way to achieve this but maybe i am over complicating
things ? anyone ?
denis
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Adam-V



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PostPosted: Mon Oct 26, 2009 8:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Luke,

In the drive section the grinder circuit, I think you can just put the vactrol across the DE and DC connections as the pot is merely acting as a variable resistance rather than a divider at that point (Only one side and the wiper are connected). You could put the vactrol in parallel (depending on it's resistive value) and normalise it's control such that it is a maximum resistance when there is not CV present.

Cheers,
Adam-V

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diablojoy



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PostPosted: Tue Oct 27, 2009 1:47 am    Post subject: Reply with quote  Mark this post and the followings unread

luka what cct are you using to drive the vactrol ?
when i did my experiments i borrowed a section from scott's
mutant vactrol filter which did an admirable job
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Luka



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PostPosted: Tue Oct 27, 2009 2:44 am    Post subject: Reply with quote  Mark this post and the followings unread

i used one of the ones in the squarewave parade picture posted above
i made a dual pcb for it too

[


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DGTom



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PostPosted: Tue Oct 27, 2009 3:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool Would love to hear it in action once its up & running, VC Grinder is gonna be gnarly!!
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diablojoy



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PostPosted: Wed Oct 28, 2009 1:10 am    Post subject: Reply with quote  Mark this post and the followings unread

nice board luka
a sample would be good to hear
haven't heard a grinder yet
here's a snap for you
WSG VC using vactrols


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diablojoy



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PostPosted: Wed Oct 28, 2009 1:16 am    Post subject: Reply with quote  Mark this post and the followings unread

and to carry on as i've finished with that experiment
rays new cct using 2n5457's
this is the new board to replace that one so I can get those vactrols back
for another use Smile
Denis.


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Luka



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PostPosted: Wed Oct 28, 2009 1:20 am    Post subject: Reply with quote  Mark this post and the followings unread

wow cool stuff !
i might have to try the 2n5457 circut sometime
do you have a link?

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diablojoy



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PostPosted: Wed Oct 28, 2009 2:22 am    Post subject: Reply with quote  Mark this post and the followings unread

not off hand but its just on the MFOS site under WSG mods
easy to find. by the way still haven't got to test that board yet,
been too busy, hopefully will get to it this weekend Smile
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Luka



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PostPosted: Wed Oct 28, 2009 4:18 am    Post subject: Reply with quote  Mark this post and the followings unread

vactrol mod works
thanks everyone
sounds great


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Adam-V



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PostPosted: Wed Oct 28, 2009 7:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

How did you end up wiring it in?

Cheers,
Adam-V

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Luka



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PostPosted: Thu Oct 29, 2009 2:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

i connected the vactrol out to the dc / de
i replaced the original grinder and drive pots with voltage offset pots which control the vactrol circuit
the limiting resister on the vactrols mimics the grinder and drive pot values

i could possibly somehow tune the responce of the vactrol a little, the grinder and drive responce range is all bunched up to one side of the input voltage range. it works well enough at this stage

does anyone know how to do this?

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diablojoy



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PostPosted: Fri Oct 30, 2009 2:25 am    Post subject: Reply with quote  Mark this post and the followings unread

you could try a 1 or 2k trimmer + a 1k resistor to ground before the base of the 2n3904 in the schematic you were using
that may give you some ability to tweak the response a bit i think maybe.
will drop some of the cv to ground which should increase the range before the led in the vactrol switches hard on .which at a guess is what is happening if it bunches up at the low end .
not quite sure if iam understanding this properly though, the cct i used is a bit different
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The Bad Producer



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PostPosted: Sat Oct 31, 2009 5:30 am    Post subject: Reply with quote  Mark this post and the followings unread

This is very interesting...

I'm just wondering if you could use two of those 2N5457 FET circuits from the MFOS site to replace a voltage dividing pot by inverting the CV to one of the FETs. I'd really like to add voltage control to a few boards (like the JH Solina) which have voltage dividing pots, and the vactrol solution eludes me for the time being... also 2N5457s are sooooooo much cheaper and easier to get hold of (for me) than vactrols, even with the cost of adding a little inverter circuit too...

Does anyone have any thoughts on this, before I start to breadboard?

This Vishay doc is fairly interesting, I've not finished examining it yet...

http://www.vishay.com/docs/70598/70598.pdf

also and this mentions that FETs can be used in audio applications (low distortion)...:

http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm

Charlie

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