Vactrol VCA

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igorrr
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Vactrol VCA

Post by igorrr »

Hi!

I'm searching for vactrol based vca schematic.
I've tried design by Silonex (figure 10), but it seems that rising voltage closes the input signal and not opens it as it should be in vca.
Image

Single vactrol designs are way too nonlinear and slow.
Can someone help me with vactrol vca?
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Luka
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Post by Luka »

add an inverter?
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meridic
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Post by meridic »

You sure you have the LEDs wired with proper polarity?

It looks to me that it would work as expected, and increase in CV = and increase in volume.

With no CV presant OP1 is high resistance and OP2 is low resistance, so any audio that makes it through OP1s high resistance would have a very easy path to ground, a near short. The output is essentially shorted to ground. As CV increases OP1s resistance decreases allowing more signal through and OP2s resistance increases removing the short to ground and allowing the signal to pass right by and out.
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igorrr
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Post by igorrr »

I will try to breadboard it again, maybe I missed something last time.
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fonik
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Post by fonik »

do you need amplification? gain >1? if not you could use a factrol for building a simple VC attenuator (voltage divider). would even be a passive module...
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gwaidan
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Post by gwaidan »

igorrr wrote:I will try to breadboard it again, maybe I missed something last time.
I think the circuit is performing as it should-as I see it with no CV the current flows thru both LEDs equally, so the resistances are equal and you get 50% attenuation. As the CV increases the voltage across the first LED drops, so the first vactrol's resistance goes up, eventually pinching off the output voltage.

So yes, it looks like increasing CV should attenuate the signal.

Try the figure 15 circuit on this page-it looks like it would give better response.

http://www1.silonex.com/audiohm/levelcontrol.html
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igorrr
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Post by igorrr »

No, I do not need amplification. I used vactrol in passive mode and in active.
Image
Both give very nonlinear response, which makes using ADSR fo VC almost useless.
The schematic from my first post should give more linear response according to Silonex, less distortion and more dynamic range. Will try to make figure 15 too.
I just want additional vca, somewhat close to linear, and made with vactrols. I already have Oakley 3x vca and Blacet dual.
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meridic
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Post by meridic »

with no CV the current flows thru both LEDs equally
How did I miss that.

Oh well.
thermionicjunky
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Post by thermionicjunky »

Thomas White is working on a vactrol VCA as part of his mini PCB project. I believe that his layout is complete. I don't know about the performance of the circuit, but it's an option.
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daverj
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Post by daverj »

How about just swap the input and ground on the two vactrol resistors?

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igorrr
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Post by igorrr »

I’ve decided not to complicate things and to use simple passive design. It appeared that VTL5C6 is quite fast (faster than NSL32SR3S).
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