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Simple "passive" VCA idea?
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Joined: Feb 02, 2010
Posts: 258
Location: California

PostPosted: Mon Jun 07, 2010 1:44 pm    Post subject: Simple "passive" VCA idea? Reply with quote  Mark this post and the followings unread

Would this work? Would it "leak" too much signal when in the "off" state? Cause impedance issues?

The value of the LDR is a real life LDR, which is what the resistor to ground is sort of guestimated from. The LDR and LED would of course be closed off from other light. The LED's current limiting resistor value is a guess based on my experience with getting a bright enough LED to open up an LDR to it's minimum resistance.

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joshco



Joined: Nov 14, 2008
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Location: germany

PostPosted: Mon Jun 07, 2010 2:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

nice idea! the forsell opto sidechain is similar to this design. if it leaking depends on the ldr. concernig impedances i dont know...
will defenately breadboard one.
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Danno Gee Ray



Joined: Sep 25, 2005
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Location: Telford, PA USA

PostPosted: Mon Jun 07, 2010 3:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Strictly speaking, (picking at nits you know), a VCA is a Voltage Controlled AMPLIFIER. What you are showing is a Voltage Controlled ATTENUATOR. They both could use the same acronym, but VCA usually refers to the amp.

So a passive amplifier...perpetual motion? Wink

Voltage Controlled Potentiometer..?

Just a couple o' pennies worth...
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Joined: Feb 02, 2010
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Location: California

PostPosted: Mon Jun 07, 2010 3:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Danno Gee Ray wrote:
Strictly speaking, (picking at nits you know), a VCA is a Voltage Controlled AMPLIFIER. What you are showing is a Voltage Controlled ATTENUATOR. They both could use the same acronym, but VCA usually refers to the amp.

So a passive amplifier...perpetual motion? Wink

Voltage Controlled Potentiometer..?

Just a couple o' pennies worth...


Attenuator... yea... yea, that's what I meant. Voltage Controlled Attenuator... VCA

Laughing

I was aware of the discrepancy, but thought it could slide because it serves the same general function if we have a sound source which is already loud enough to be attenuated for audio use (which most oscillator circuits, etc, running at 9+ volts are).

Anyway... I plan to try this out tonight... I've been kicking it around in my mind but had not put it into practice. I have a simple percussive envelope generator and some oscillators set up on my breadboard right now, so all I need to do is throw these few extra parts on there and see see see...
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cbm



Joined: Oct 25, 2005
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PostPosted: Mon Jun 07, 2010 4:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

In general VC Amplifiers don't really amplify. The generally don't have gain.

The LDR idea is essentially a Vactrol, as used in many circuits, notably many designs by Don Buchla.

Vactrols work pretty well, but they don't always go all the way off.

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Danno Gee Ray



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PostPosted: Mon Jun 07, 2010 5:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

cbm wrote:
In general VC Amplifiers don't really amplify. The generally don't have gain.

The LDR idea is essentially a Vactrol, as used in many circuits, notably many designs by Don Buchla.

Vactrols work pretty well, but they don't always go all the way off.


Good points.
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adamon



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PostPosted: Mon Jun 07, 2010 9:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Not sure about exact values, but I've got basically the same set up in my system already, and they work great (for my needs at least)!
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PostPosted: Tue Jun 08, 2010 2:32 am    Post subject: Reply with quote  Mark this post and the followings unread

I bread-boarded it and it definitely works. It is sort of a trade-off though in the level you get out of it vs. the amount of bleed through. No bleed through for me meant much lower audio level. I ended up using much lower value resistors to pulldown to ground though - between 380 ohms and 4.7 K was the best range in my test - would probably depend on what you have going through it.
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adamon



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PostPosted: Tue Jun 08, 2010 9:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Indeed, I've found the same thing with trying to reach a balance between appropriate range and levels using these kind of homemade vactrols. I have a lot of them in my system, and I've started using different ldr/led color combos for different applications. I'm using this "vca" of sorts at the output of my mixers, and currently I've been trying some super bright white leds with some 8006 photocells, and they seem to behave over a pretty good range (a little biased on the high end though as the led is so bright).
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Sound



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PostPosted: Tue Jun 08, 2010 10:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello, why do you want it passive? If you have already power in the module, will be better add two op-amp, one at its input -inverting amplifier- with standard 100K input impedance, process the signal with the LDR and a fixed resistor configured as a voltage divider and send it to the output op-amp -inverting amplifier- with its 1K output impedance or lower.
Also you could substitute the voltage divider placing LDR as a feedback resistor of the op-amp output.
I think.
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PostPosted: Tue Jun 08, 2010 10:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Sound wrote:
Hello, why do you want it passive? If you have already power in the module, will be better add two op-amp, one at its input -inverting amplifier- with standard 100K input impedance, process the signal with the LDR and a fixed resistor configured as a voltage divider and send it to the output op-amp -inverting amplifier- with its 1K output impedance or lower.
Also you could substitute the voltage divider placing LDR as a feedback resistor of the op-amp output.
I think.


The goal for my purposes is to make this particular part be super cheap, super simple, minimal parts, minimal space. This is just one little piece of a project to achieve one simple little task. The rest of the synth that it is going into has only two ICs in the design so far, and the role of this in the synth as a whole does not warrant putting another IC in there. That is why I was going for something so simple.
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parasat



Joined: Aug 13, 2009
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PostPosted: Tue Jun 08, 2010 5:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Very cool, waiting on parts to try this. I'm thinking about an all vactrol synth Wink

In the meantime has anyone tried using a MOC3010M?

http://www.moltenvoltage.com/downloads/Application_Note_1_for_MV-52_-_v1_1_-_Molten_Voltage.pdf
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Ojd



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PostPosted: Sun Jan 30, 2011 3:40 am    Post subject:   Reply with quote  Mark this post and the followings unread

I cant believe it works! It hangs out of my modular, on alligator clips. It even accepts +/- voltage!!! Thank you for the design! Must go to forum top arrow
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richardc64



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PostPosted: Sun Jan 30, 2011 10:15 am    Post subject: Reply with quote  Mark this post and the followings unread

As long as we're picking nits about VCAs not usually Amplifying, I'll point out that an LDR plus LED isn't exactly "passive."
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Ojd



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PostPosted: Sun Jan 30, 2011 3:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Can anybody tell me how to mix few CV ins - when CV in pot is set to min resistance it starts to buzz. if its max its ok ? I guess i need some resistors on input. (i simply connected all of the CVs to LED). What value? 33K, 68K, 100K? I want minimum headroom loss, if possible.
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jean-louise



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PostPosted: Mon Jan 31, 2011 10:32 am    Post subject: Reply with quote  Mark this post and the followings unread

i think it doesn't matter that much as long as they have the same values. i saw a passive audio mixer somewhere that had 4K7 resistors. but it might be better to use an active mixer anyway(?).
some info:
http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
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