Mike Sims LM13700 VCA in Euro

[jbeuckm]

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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Description:	Mike Sims VCA Clone for 12V
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[jbeuckm]

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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Description:	mike sims vca layout
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[xjn]

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!

[jbeuckm]

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

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

[xjn]

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.

[LFLab]

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?

[Electric Druid]

[dougcl]

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.

[LFLab]

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).

[dougcl]

[jbeuckm]

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

[LFLab]

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