vactrol VCA? that easy?

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falafelbiels
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vactrol VCA? that easy?

Post by falafelbiels »

Hey folks,

Could this inverting opamp with R1 replaced by a vactrol work as a VCA?

Image

I'm getting some extra vactrols in but I'm just not patient enough to wait for them to arrive and then try it out...
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Post by A Dingleberry Monstrosity »

I dont see why not! :despair:

on paper atleast :hmm:
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falafelbiels
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Post by falafelbiels »

So why aren't any around?
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Post by pugix »

It would have a VCA effect, but vactrol has drawbacks when used as a VCA, such as difficulty fully cutting off the signal. Search for discussions of Low Pass Gates used in VCA mode. There are many simple VCA circuits, if simplicity is what you're aiming for.
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falafelbiels
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Post by falafelbiels »

I see.

Well I've tried my hand at stripboarding an MFOS dual VCA twice and failed and I have real trouble figuring out what's going wrong there. So I'm aiming for something I am confident I understand. So, extreme simpliciy...

I have built a dual LPG so I'm covered there. I am missing some VCAs for modulation really. I think I'll just try this, It doesn't need to work perfectly. If it's fun then that's enough for me.

Thanks!
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Post by DGTom »

I think it would be cool as a CV VCA - especially in front of a VCO FM in, throw a capacitor into the negative feedback, with a switch to take it out, so it has an inverting integrator mode - hey if its gonna be lo-fi & imprefect may as well make it flexiable right? :D

put another inverting amp stage after the first with R1=R2 & you get a non-inverted output as well as inverted :tu:
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falafelbiels
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Post by falafelbiels »

DGTom wrote:I think it would be cool as a CV VCA - especially in front of a VCO FM in, throw a capacitor into the negative feedback, with a switch to take it out, so it has an inverting integrator mode - hey if its gonna be lo-fi & imprefect may as well make it flexiable right? :D
If its that simple, yes, just wontering what to do with it (or what it does). Integrator makes a square wave into ramp right?
DGTom wrote: put another inverting amp stage after the first with R1=R2 & you get a non-inverted output as well as inverted :tu:
Oh yeah that should be good for a modulation VCA, didn't think of that...


Butteh, cool huh?
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Post by jenamu6 »

Yo Niels stop playing wit vactrols and make me one of DGTom's clock dividers ........please?
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falafelbiels
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Post by falafelbiels »

jenamu6 wrote:Yo Niels stop playing wit vactrols and make me one of DGTom's clock dividers ........please?
Well get me a PCB and I'll fix it. I'm having too much success with PCBs lately to go stripboarding now.

Matter of fact, get me 2 PCBs!
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Post by bananeurysm »

would it be better if you replace r1 with the circuit at the top of this page?
http://thesquarewaveparade.com/images/VtoR.jpg

i breadboarded that the other day to get CV into my 40106 oscillators and was SUPER excited to see it work!

would that + that simple amplifier make a good VCA? would it be too slow/slewed for standard VCA duty?

(I just used whatever led and photocell i had sitting around... no fancy vactrols)
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falafelbiels
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Post by falafelbiels »

Well it has more parts so it is likely to work better :hmm:
I'll try that I hope tomorrow. I have all the parts, but have a job too...

I only have 4 more HEs to fill in my DIY rack and this would be really swell I suppose.
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Post by DGTom »

Oh, I assumed a cct. to drive the LED was a given, it wouldn't work that well without it. I'd use the middle one from TSWP VtoR schems so you'd have an offset pot as well as CV in.
falafelbiels wrote: If its that simple, yes, just wontering what to do with it (or what it does). Integrator makes a square wave into ramp right?
Well, in this case it'd smooth / smear any stepped voltage at the input & you'd have VC over it :D
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Post by falafelbiels »

HEY!

Well it took me some effort but I got one running. I've been trying with what turned out to be a busted vactrol at first. Once I figured that out, things started running more smoothly.
Anyway, so it works. It is a bit touchy with the CV you feed it.
The offset voltage is all the way down because it doesn't seem necessary, but I thought it was needed to cancel out the forward voltage drop for the LED in the vactrol. Release tails from an envelope drop a bit too fast for my taste so I think I will need it.
CV has to be attenuated really low to recognize an envelope shape for example.

So what I'm thinking this TL072 in there could do a bit less of amplifying.

I guess I could up the current limiter resistor between the transistor and the vactrol, it is now 2k2 I believe. Otherwise maybe I could lower the resistor in the feedback path of the tl072. It is now 200k.

Any hints?

I'll stick in that Integrator thingy when it runs smooth(-like)!

Image

Great Pic huh? Well at least it happened.


Oh yeah it's the second one from the page Bananeurysm linked up. Thanks Bananeurysm!
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Post by bananeurysm »

nice! i'm assuming it's fairly slow in response? no snappy envelopes?

you should pull out your expensive vactrol - swap in an led+photocell and let us know the difference!
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Post by falafelbiels »

bananeurysm wrote:nice! i'm assuming it's fairly slow in response? no snappy envelopes?
Not very snappy no but still usefull. I included an example of snappyness (using my MFOS ADSR which I so far have found to be snappy enough).

bananeurysm wrote: you should pull out your expensive vactrol - swap in an led+photocell and let us know the difference!
Well I don't know man I just stuck it in a panel and filled my cabinet full at last. It may not come out any time soon...



Oh yeah only the first couple of sounds are set as snappy as possible. Later on I open the attack, just to prove something (namely that it actually works, which may be not too much of a surprise to some, but I'm kind of proud here).

Too bad the panel didn't allow for many connectors and switches and pots. It is now simply a VCA without that integrator, but it may get an inverted output instead of the second, non attenuated input, which is kind of crap. May as well use a mixer before it, right?
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falafelbiels
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Post by falafelbiels »

And just so you know I'm not lying.
Well actually that could be anything couldn't it? One of them fancy 4HP blind panels...
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Post by mono-poly »

Those lovely yellow knobs again :help:
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falafelbiels
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Post by falafelbiels »

Hehe, you don't like em Mono? The colour is kind of yuckie I admit, but you know, style has never been my most refined trait.

Anyway, the thing rings when CV goes down, nicely so!
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Post by mono-poly »

Great to hear about the ringing.
Niels i really like the yellow knobs!
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Post by falafelbiels »

I don't know if I do... But they turn! :party:
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Post by Peake »

Vactrol VCAs ring very organically but often lack extreme punch...
What are they doing, in the dream house
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