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 Forum index » DIY Hardware and Software » Developers' Corner
A Voltage Controlled Distortion
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drakfluga



Joined: May 09, 2012
Posts: 49
Location: Gothenburg SE

PostPosted: Sat Feb 17, 2018 11:06 am    Post subject: A Voltage Controlled Distortion Reply with quote  Mark this post and the followings unread

Hi,

I'm sure this question has been asked before but a search of the forum yielded nothing, so in case I'm treading a beaten path here, do feel free to point me in the right direction. Wink

I've been building and breadboarding circuits for a while now, I've learned a lot but I still have a ways to go. I'm currently stuck trying to work out a particular problem: how to effectively tune parameters using CV.

In the past I've built a few digital circuits which use ARM microcontrollers, where I've simply used an ADC to read input voltage. Easily done and a very simple circuit to build; just an attenuating buffer to bring the input CV into the proper range.

However, my latest project is constructing a simple distortion/fuzz module for my synth, and I want to control the amount of drive using CV. I thought this would be fairly straightforward, but it's apparently not.

Basically what I want to do - I suppose - is to set the gain of an op amp using incoming CV. What are my options here? I've seen solutions using a differential pair of BJT's, a FET being used as a variable resistor, or an OTA used as a VCA. Am I correct in assuming that this is the whole idea, basically build a VCA as a submodule of my circuit and use it to attenuate the audio signal?

I'm currently studying schematics of VCFs, VCAs etc, but it's left me with more questions than answers as there seem to be numerous solutions depending on the particular situation, and I haven't really found an instance which mirrors my particular application.

Very grateful for any input!

Cheers!

Last edited by drakfluga on Wed Mar 07, 2018 1:10 pm; edited 1 time in total
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drakfluga



Joined: May 09, 2012
Posts: 49
Location: Gothenburg SE

PostPosted: Sat Feb 17, 2018 11:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Answering my own question, to a degree, I ran across this article which gave a lot of useful information, along with practical examples:

http://sound.whsites.net/articles/vca-techniques.html

Not sure if it left me with more or less questions though. Very Happy
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AlanP



Joined: Mar 11, 2014
Posts: 525
Location: New Zealand
Audio files: 32

PostPosted: Sat Feb 17, 2018 3:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Another option is a vactrol, or a LED/LDR pair.
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drakfluga



Joined: May 09, 2012
Posts: 49
Location: Gothenburg SE

PostPosted: Tue Feb 20, 2018 12:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Right, so this is what a few days of research has yielded:

The conclusion seems to be that for any voltage controllable parameter on a module, you basically need an on-board VCA.

There are several methods to build one; a Vactrol or LED/LDR pair is one, like AlanP suggested (thanks for input!).

After looking at schematics and pouring over datasheets and example circuits for a few days I think I've concluded that going the OTA route is the easiest solution with the least drawbacks. A lot of very good standalone VCAs use OTAs.

I've seen an example of an OTA used in a feedback loop to control the gain of an opamp, but I haven't attempted to build the circuit (yet). This seems to be the better option for me, if I can get it to work.

I have a few LM13700 ICs laying around, time to start breadboarding!

I really thought there was some "standard" way of doing voltage control. I'm fast learning that in analogue synth construction there is no such thing. Very Happy
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drakfluga



Joined: May 09, 2012
Posts: 49
Location: Gothenburg SE

PostPosted: Wed Mar 07, 2018 12:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

The adventure continues!

Over the past few days I've been making steady progress, by now I have a fairly competent circuit that does the job passably well. It could use some improvements, however, as I'm uncertain how exactly the setting of the VCR resistance works.

I played with resistor values until I found something that worked, basically, and the result is... it works, but as the CV goes through the roof and gets amplified on top of that, I get silence. Working theory is the CV biases the amplified signal to one of the rails.

I'm attaching a schematic of the circuit as it is right now. It's very basic, but a good start I think. Once I have a circuit that works well, I plan to add a soft/hard clipping switch, and that's basically the entire thing.

Cheers!

Posted Image, might have been reduced in size. Click Image to view fullscreen.
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