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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Single supply VCA?
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commathe



Joined: Jul 26, 2013
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PostPosted: Tue Dec 02, 2014 9:35 pm    Post subject: Single supply VCA? Reply with quote  Mark this post and the followings unread

Has anyone got any good ideas for something along these lines? I've tried making ones using a Vref/virtual ground but it never quite works out. I need something that will not just pass digital signals and won't clip things. I need a "true" vca.

I'm totally lost. It seems the only options left are some sort of OTA as a voltage controlled resistor design or to use a vactrol (not so great).

Log & linear response would be nice, but not essential.
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Cynosure
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PostPosted: Thu Dec 04, 2014 10:57 am    Post subject: Reply with quote  Mark this post and the followings unread

I would recommend an OTA. You can get two VCA's with one LM13700. I have done it by replicating the configuration used in the JH Wasp filter schematic.

You will need to make sure you attenuate the voltage on the input. OTA's distort easily and the distortion does not sound nice. Also, add a 15k resistor to +V from the diode bias input.
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commathe



Joined: Jul 26, 2013
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PostPosted: Thu Dec 04, 2014 11:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks! OTAs definitely appear to be the way to go. I'm worried about how close they will get to the rails though. I'm considering trying them in the feedback loop of an op-amp...
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PHOBoS



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PostPosted: Fri Dec 05, 2014 3:51 am    Post subject: Reply with quote  Mark this post and the followings unread

an OTA will probably give you the best results,.but I have seen some VCA's using a transitor/FET on the input of an opamp.
either in series with the signal path like here:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
from this thread.

or to pull the input to ground like this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
from this page.

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LFLab



Joined: Dec 17, 2009
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PostPosted: Fri Dec 05, 2014 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

THAT4315? No experience whatsoever, but seems appropriate.
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synaesthesia



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PostPosted: Fri Dec 05, 2014 9:47 am    Post subject: Reply with quote  Mark this post and the followings unread

For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?


diode vca and ar.jpg
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diode vca and ar.jpg


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richardc64



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PostPosted: Fri Dec 05, 2014 4:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.

It started out that way, but without the 2nd diode of the 2-diode design, it more resembles the "1-transistor VCA" used in the TR-808, TR-606 and DR-110: The output is taken from the point where the CV controls the amplitude.

Using an opamp instead of a transistor as the active element allows for a larger input amplitude. In the 1-transistor circuit the input must be limited to less than the transistor's base-emitter saturation voltage -- about 850mV for a 2N3904.

(In that circuit Gate In needs a resistor of 2K or so to Gnd, otherwise Release will be longer than it should be.)

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commathe



Joined: Jul 26, 2013
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PostPosted: Sun Dec 07, 2014 12:03 am    Post subject: Reply with quote  Mark this post and the followings unread

I want to be passing tri/saw/sine signals without too much distortion and definitely not clipping so the diode/transistor method is out. Looks like I'm going to have no choice but to go OTA. Bummer because I also wanted rail-to-rail output (was hoping to do some CV processing) but to be honest it serves me right being so fussy.

Hopefully I'll come back with my own solution some time soon.

I've got my eye on this guitar compressor schematic at the moment. I think using the OTA as a voltage controlled resistor in an op-amp feedback loop would hopefully work well.

Posted Image, might have been reduced in size. Click Image to view fullscreen.[/url]
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LFLab



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PostPosted: Sun Dec 07, 2014 8:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Seems like quite a lot of parts. Have you checked out that4315? 3,60euro at mouser.
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richardc64



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PostPosted: Sun Dec 07, 2014 11:56 am    Post subject: Reply with quote  Mark this post and the followings unread

commathe wrote:
I want to be passing tri/saw/sine signals without too much distortion and definitely not clipping so the diode/transistor method is out.

I failed to mention that omitting the 2nd diode from the 2-diode VCA the circuit handles analog as well digital inputs. Replacing the transistor in the "1-transistor" VCA with a single supply opamp makes it even better, as it allows bigger input swings.

Quote:
I've got my eye on this guitar compressor schematic at the moment. I think using the OTA as a voltage controlled resistor in an op-amp feedback loop would hopefully work well.

I agree with LFLab that looks like too many parts for a single-supply VCA, unless you have your heart set on the compression capability.

Here's a couple more core circuits:


syntom_ota.gif
 Description:
what's missing is the V/I converter.
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This image has been reduced to fit the page. Click on it to enlarge.

syntom_ota.gif



3series.gif
 Description:
Same operation as what PHOBoS posted. The 909 circuit is dual supply, but that's just for the opamp. Substitute a '324 or '358 for single-supply.
 Filesize:  30.4 KB
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This image has been reduced to fit the page. Click on it to enlarge.

3series.gif



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commathe



Joined: Jul 26, 2013
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PostPosted: Sun Dec 07, 2014 6:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again guys! It is time to get bread boarding and see what gets closest. Simulations of the MFOS one are looking ok. More distortion than I'd like but beggars can't be choosers!

EDIT: I just discovered this 4007 based VCA that I would have just glanced over if it wasn't for the fact that I have a note that the schematic as it is drawn here is WRONG. The body connections of the internal MOSFETs are tied to Vss and Vdd, meaning that this may actually work!


4007_dual_vca_112.png
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4007_dual_vca_112.png


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Vanwonky



Joined: Jul 24, 2013
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Location: Perth, Australia

PostPosted: Sun Dec 07, 2014 9:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi - I built this and it worked brilliantly for my needs. Output is a little quiter than I would like but I am sure that could be fixed easily by anybody on this forum but me! Very Happy

synaesthesia wrote:
For single supply, there is also this one, mentioned in this thread: http://electro-music.com/forum/topic-42337.html
I believe it is a variation of the super simple diode VCA plus AR described by richardc64 in this thread: http://electro-music.com/forum/topic-23960.html.
Did anybody try it and can let us know how it works?
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elektrouwe



Joined: May 27, 2012
Posts: 50
Location: Germany

PostPosted: Tue Dec 09, 2014 1:26 am    Post subject: Reply with quote  Mark this post and the followings unread

here is my favourite single supply VCA: cheap (50 cent), exponential control, 4,5..18V supply, low component count, inverting or noninverting output, can drive low ohm loads. cons: ac coupled, DIP obsolete (some shops still have it) but SMD in production


TDA7052AsimpleVCA.jpg
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This image has been reduced to fit the page. Click on it to enlarge.

TDA7052AsimpleVCA.jpg


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