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 Forum index » DIY Hardware and Software » The layout factory
Mike Sims LM13700 VCA in Euro
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jbeuckm



Joined: Nov 30, 2008
Posts: 165
Location: Stockholm
Audio files: 9

PostPosted: Fri Jan 22, 2016 11:01 am    Post subject: Mike Sims LM13700 VCA in Euro Reply with quote  Mark this post and the followings unread

http://www.teaser.fr/~amajorel/sims/

I'm working on a layout for 12v. The original article had the name of an NPN instead of PNP and some hard to read values. How do my values look in this schematic?


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Mike Sims VCA Clone for 12V
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jbeuckm



Joined: Nov 30, 2008
Posts: 165
Location: Stockholm
Audio files: 9

PostPosted: Sat Jan 23, 2016 1:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Verified layout here: https://github.com/jbeuckm/Mike-Sims-VCA

I didn't have exact values -

33k1 -> 33k
61k1 -> 56k
33.2R -> 33R
30k1 -> 30k
390pF -> 330pF


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xjn



Joined: Aug 01, 2012
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Location: STL

PostPosted: Fri Jan 29, 2016 9:26 am    Post subject: couple of questions.. Reply with quote  Mark this post and the followings unread

Hi Mike, great work. I have a couple of questions about the circuit.

The notes from the original seem to say that the output is inverted but that this can be changed. It looks like you've built yours the same as the original. Is the output inverted in your version?

Secondly, and somewhat related, I'm wanting to use this mainly to process CV. Is this circuit ok for CV, or is it more for audio? Is it AC coupled? I see the caps near the input, but I'm not knowledgeable enough yet to be sure about their function.

thanks!
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jbeuckm



Joined: Nov 30, 2008
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Location: Stockholm
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PostPosted: Fri Jan 29, 2016 10:21 am    Post subject: Reply with quote  Mark this post and the followings unread

Note that the design is by Mike Sims (read the article linked above) but I am Joe from STL Wink

Yep, looks like the output is inverted. Three inverting amplifiers in the signal path...

This design is for audio - you should probably look for a DC-coupled VCA, and you might want one that can respond logarithmically. I can recommend the MFOS dual VCA #2 (old design) for one-sided DIY:

http://www.musicfromouterspace.com/analogsynth_new/OLDIESBUTGOODIES/VCA/dualvca-2.html
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xjn



Joined: Aug 01, 2012
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Location: STL

PostPosted: Fri Jan 29, 2016 10:56 am    Post subject: Reply with quote  Mark this post and the followings unread

sorry Joe, that's embarrassing.. nice to meet you.

I'm also in STL.

thanks for the tip, the MFOS one does look nice. I was going to make two anyway so a dual version would be great.
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LFLab



Joined: Dec 17, 2009
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Location: Rosmalen, Netherlands

PostPosted: Mon Feb 01, 2016 3:05 am    Post subject: Reply with quote  Mark this post and the followings unread

The inverting output configuration of the last opamp could be changed to non-inverting, that would solve it right?
Any reason why this last stage is inverting?
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Electric Druid



Joined: Mar 13, 2012
Posts: 44
Location: UK

PostPosted: Mon Feb 22, 2016 10:30 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
The inverting output configuration of the last opamp could be changed to non-inverting, that would solve it right?
Any reason why this last stage is inverting?


The last stage is a an I-to-V convertor. It doesn't have the input resistor of a typical inverting amp. I don't know of a non-inverting op-amp I-to-V stage, so that part has pretty much got to be like it is.

However, the original article says "Interchanging pins 13 and 14 of IC1B realizes a noninverting VCA.". That is, swap the -ve and +ve inputs of the middle OTA stage. This is so simple to do, I'm slightly surprised they didn't publish the circuit that way round instead.

Tom

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dougcl



Joined: Feb 08, 2008
Posts: 33
Location: Portland OR

PostPosted: Tue Mar 22, 2016 4:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

The first OTA does not invert. The input is tied directly to the output, so the voltage must be the same. The second OTA and the final op amp are both inverting, so the overall VCA is non inverting.

I believe your 1M resistor feeding the non inverting input of the second OTA should be 1k.
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LFLab



Joined: Dec 17, 2009
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PostPosted: Wed Mar 23, 2016 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the comments Doug. The originator of the circuit, Mike Sims, also used a 1M resistor at that spot: EDN Article about this VCA

By the way Joe, the Oakley triple VCA board, which also uses Mike Sims circuit, also has a PNP transistor (he's using BC560).
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dougcl



Joined: Feb 08, 2008
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Location: Portland OR

PostPosted: Wed Mar 23, 2016 8:10 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
Thanks for the comments Doug. The originator of the circuit, Mike Sims, also used a 1M resistor at that spot: EDN Article about this VCA


Ha just logged in to fix my response but you beat me to it. Smile
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jbeuckm



Joined: Nov 30, 2008
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PostPosted: Wed Mar 23, 2016 2:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also, Mike mentions swapping inputs on the second OTA to realize a non-inverting configuration. I believe this is inverting as pictured.
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LFLab



Joined: Dec 17, 2009
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PostPosted: Thu Mar 24, 2016 12:47 am    Post subject: Reply with quote  Mark this post and the followings unread

I added a mixer on the input, to make it non-inverting.
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