Author |
Message |
mrand
Joined: Mar 30, 2014 Posts: 56 Location: Yukon and London Ontario
|
Posted: Sat Jun 14, 2014 6:49 pm Post subject:
voltage controlled resistor Subject description: how to implement voltage controlled resistance |
|
|
Hi all,
Can someone recommend some resources for learning how to implement a voltage controlled resistance? I've read various things about optocouplers but am having a hard time coming up with any practical suggestions.
I'm trying to add a cv input to modulate the "tune" of a simple drum circuit. I'm using a transistor as a kind of voltage controlled resistor, to vary the critical tune resistor, which works, except the control voltage I apply goes right through the circuit and is heard at the voice output.
I've posted the same question, more or less, here: http://waldorf.electro-music.com/forum/topic-35059-0.html&postdays=0&postorder=asc&highlight=
This is more of a learning exercise than anything, so any suggestions would be appreciated!
-C |
|
Back to top
|
|
|
gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
Audio files: 1
|
Posted: Sat Jun 14, 2014 8:48 pm Post subject:
|
|
|
The question of how to add voltage control to a circuit is asked often. The reason you don't find a clear cut answer is because there isn't one. It depends a lot on the circuit you're dealing with and requires a good understanding of the components being used, whether they be transistors, OTA's or vactrols (optocouplers).
People often turn to vactrols since the basic concept is simple: the resistor side of it can be dropped in place of a fixed resistor, the resistance varied with a current on the LED side. The difficulties arise in the response being very non-linear and not very accurate. There is a lot of variation from one vactrol to another and it can be tricky to tweak it to get the response you desire.
The simplest way to convert your voltage input into a current for controlling the vactrol is a resistor in series (similar to an ordinary LED current limiting resistor) but you'll have to experiment to find the best value for the response you want and make sure your CV source can supply the current (or buffer it). _________________ My synth build blog: http://gndsynth.blogspot.com/ |
|
Back to top
|
|
|
JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
|
Posted: Sun Jun 15, 2014 9:01 am Post subject:
|
|
|
The H11F1 (also H11F2 and H11F3) ICs are special opto-isolators that contain an LED that shines on a MOSFET transistor. In theory, it works similarly to a vactrol, but the datasheet claims fairly high linearity (99% I believe - whatever that means). _________________ 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
|
|
Back to top
|
|
|
mrand
Joined: Mar 30, 2014 Posts: 56 Location: Yukon and London Ontario
|
Posted: Mon Jun 16, 2014 6:25 am Post subject:
|
|
|
I found an optoisolator in a bag of random parts someone gave me and promptly cooked it... oops
Going to see about the H11F1 or maybe try rolling a makeshift one.
Thanks for the help! |
|
Back to top
|
|
|
L´Andratté
Joined: Sep 23, 2012 Posts: 150 Location: Hamburg, Germany
Audio files: 5
|
|
Back to top
|
|
|
mrand
Joined: Mar 30, 2014 Posts: 56 Location: Yukon and London Ontario
|
|
Back to top
|
|
|
gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
Audio files: 1
|
Posted: Wed Aug 27, 2014 11:24 am Post subject:
|
|
|
Those circuits are in the LM13700 data sheet as well, but the Nus and Volts article is a nice resource for gaining a better understanding of OTA's.
The first VCR circuit using 1 OTA is only good for AC signals and applications where one end of the resistor is connected to ground.
The floating VCR using 2 OTA's is intriguing, but probably more complex (uses more components and higher cost) than what is usually necessary in most synth applications. I've never seen it used, I think I saw someone try it at one point without success and went on to a simpler solution.
I was tempted to use the floating VCR to add voltage controlled resonance to the Yusynth moog ladder filter, but instead just went with a basic single OTA based VCA in the feedback path. Works well and frees up the other OTA to use as the main output VCA.
It would be interesting to see it used. Might even be fun to try and make a small module that can be used more or less as a simple drop in replacement for a resistor. _________________ My synth build blog: http://gndsynth.blogspot.com/ |
|
Back to top
|
|
|
mrand
Joined: Mar 30, 2014 Posts: 56 Location: Yukon and London Ontario
|
Posted: Wed Aug 27, 2014 10:55 pm Post subject:
|
|
|
Heh, you seem to be coming through with answers to all my questions, gdavis, thanks!
Quote: | It would be interesting to see it used. Might even be fun to try and make a small module that can be used more or less as a simple drop in replacement for a resistor. |
Something like this? http://www.doepfer.de/A1019.htm
I saw that a few months ago and was wondering what one might use it for, aside for development purposes. |
|
Back to top
|
|
|
gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
Audio files: 1
|
Posted: Thu Aug 28, 2014 9:41 am Post subject:
|
|
|
mrand wrote: | Heh, you seem to be coming through with answers to all my questions, gdavis, thanks! |
Ha, ya, I've been through these same questions myself while working on my synth project, so I've put a fair amount of thought into it already.
Quote: |
Quote: | It would be interesting to see it used. Might even be fun to try and make a small module that can be used more or less as a simple drop in replacement for a resistor. |
Something like this? http://www.doepfer.de/A1019.htm
I saw that a few months ago and was wondering what one might use it for, aside for development purposes. |
Actually I was thinking of just a small board, like your idea in the small utility circuits thread, that could be hacked into any existing circuit where you wanted to make a fixed resistor voltage controlled. But otherwise ya, same idea except OTA based instead of vactrol.
For a moment there I forgot I was on the modular synth forum and that "module" already has a specific meaning here. _________________ My synth build blog: http://gndsynth.blogspot.com/ |
|
Back to top
|
|
|
|