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Snaper
Joined: Feb 28, 2014
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Location: Hungary
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Snaper
Joined: Feb 28, 2014
Posts: 217
Location: Hungary
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 Posted: Tue Mar 04, 2014 3:13 am Post subject:
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JovianPyx
Joined: Nov 20, 2007
Posts: 1988
Location: West Red Spot, Jupiter
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| Posted: Tue Mar 04, 2014 7:56 am Post subject:
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I think it would "sort of" work. In fact, this circuit will act quite a bit like a lowpass filter (single pole). In terms of VCA action, it will reduce the output when the Vactrol is darker, but it will never completely cut off the sound. Also - IIRC, a Vactrol has a LDR as the control component. The schematic shows a bipolar phototransistor. If it really is a transistor, you will also likely get severe clipping distortion.
I understand why some people like passive circuits. They are usually quite simple and need no power supply. However, IMO they all perform poorly. The investment in a power supply and use of active circuits will perform orders of magnitude better than passive systems.
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PHOBoS
Joined: Jan 14, 2010
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| Posted: Tue Mar 04, 2014 8:35 am Post subject:
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apart from the 'Amplifier' section I think the CV input wouldn't really work either. They way you have it drawn now the pot can short the CV and
whatever the souce of it (CV) is might not be happy with that. Also to control the LED you're much better of with either a current source or PWM (with PWM
you would most likely need a lowpas filter if you use a phototransistor).
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JovianPyx
Joined: Nov 20, 2007
Posts: 1988
Location: West Red Spot, Jupiter
Audio files: 224
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| Posted: Tue Mar 04, 2014 8:47 am Post subject:
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| PHOBoS wrote: | | ... I think the CV input wouldn't really work either. They way you have it drawn now the pot can short the CV and whatever the souce of it (CV) is might not be happy with that. |
Heh - I didn't notice that, but yeah.
Another way to light the LED besides a (true) current source would be to simply put the variable resistor in series with the CV. Also another fixed resistor in series to force a maximum possible current amount.
However, all that said, I still think it will perform poorly. A way to make a small improvement is to replace the 0.1 uF capacitor with a resistor. I'd suggest trying 1K for a start. The size of that resistor will control how close to cutting off the signal you can get with smaller being closer. Still, it will never cut off completely, but possibly to an acceptable amount.
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FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
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diablojoy
Joined: Sep 07, 2008
Posts: 809
Location: melbourne australia
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| Posted: Thu Mar 06, 2014 6:58 pm Post subject:
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look up scott stites mutant vactrol filter , check out the cv drive circuitry
used for the vactrol its a very good method and works really well for what you want to acheive.
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