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 Forum index » DIY Hardware and Software » The layout factory
Dual VCA Idea
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jbeuckm



Joined: Nov 30, 2008
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PostPosted: Tue Sep 19, 2017 12:22 pm    Post subject: Dual VCA Idea
Subject description: Floating an idea for building a stereo VCA from LM13700
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Since LM13700 is dual, it makes sense to build a dual VCA. I like to build Ray Wilson's dual VCA circuit.

But I also like to put connectors on the edge to avoid wiring. So I was thinking about making a "stereo" VCA with a dual gang pot for offset and a single CV connector connected to both CV inputs.

This could be even slicker by using TRS 3.5mm jacks for in and out. If you wanted to use it as a mono VCA, just use a mono cable and the "ring" channel input will be grounded to the sleeve. My question is: If you plugged in two hot signals but used a mono cable at the output, would that cause a problem? One of the VCA outputs would be grounded to the sleeve of the mono output connector. Ray has 1k resistors on the outputs for this type of situation, but maybe just for momentary? Say the "ring" output was at 12V and the mono connector shunted it to ground. I guess the current required from the OTA buffer would be 12mA. Would that put noise on the ground? Would it eventually burn out the buffer? The datasheet says 20mA typ. but I wonder if that means DC?
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electrotech



Joined: Apr 24, 2013
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Location: Ayrshire Scotland

PostPosted: Wed Sep 20, 2017 3:09 am    Post subject: Reply with quote  Mark this post and the followings unread

In general terms, if you are using a stereo jack socket for an output you should never use a mono plug with it, because as you say, it will short one of the outputs to ground.
This is not a good idea as it can cause excessive dissipation in the grounded output channel and if there are no series protection resistors can also damage the output stage.
Shorting together inputs is OK as long as there isn't any DC potential difference between them.
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jbeuckm



Joined: Nov 30, 2008
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PostPosted: Thu Sep 21, 2017 9:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

You may have missed that there are series resistors on the outputs. What I was asking about is the probability that the output buffer on the LM13700 will be damaged in the event that its output is at the rail and the connector is grounded. I also mentioned a figure from the datasheet.
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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Fri Sep 22, 2017 1:53 am    Post subject: Reply with quote  Mark this post and the followings unread

The data sheet is a bit vague isn't it ... Confused

It says 20 mA max output current, it also says that you can short circuit the output indefinitely, and then it says you need to limit the output current so as to not exceed the maximum package power dissipation ... let's work with the dissipation limits and see where that ends ...

Assuming 40C/W you could have roughly 1W of dissipation (or 1/2W for the SMD package) for ambient temps up to like 50 C (which would make the chip internal temp rise to 90C then) ... so let's take the lower value ... 500mW .. divide by 20mA -> 25V .. so with 20 mA you are safe when you short to ground and the supply voltage is < 25V (and it happens in one of the OTA's only in the package).

So let's say we want to allow for both outputs to be shorted to ground simultaneously ... current is best limited to 10 mA then for the SMD package ... so wit a supply voltage of 20V (say) that would be a 2k resistor.

For the DIP package that would be 1 kOhm then.

now it *may* be that the 20mA max output current is due to internal resistance in the OTA outputs (which would be my interpretation ... but I'm not seeing this explicitly mentioned in the data sheet), so it would never get higher than that ... in that case you can relax the limits above ... as in for the DIP package you could just short the outputs indefinitely without protection resistors and for the SMD package you would use say .. 1 kOhm ....

This is all a bit rough ... but all on the safe side at least.

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cslammy



Joined: Apr 27, 2018
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PostPosted: Mon Apr 30, 2018 4:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thomas henry's 3080 book has a single VCA based on the 3080 which I imagine you could adapt for a 13700 dual, and maybe the darlingtons in the 13700 could be used to improve the SN ratio a bit.

As far as shorting the outputs/adding 1K resistor in series, I'd think one would want to do that as a best practice.

Shorting the output of an OTA or op amp to ground--which you can do momentarily by connecting Male tip to female sleeve in a modular when you are patching, is never a good idea; you want to limit the current at your output stage somehow I would think.

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cslammy



Joined: Apr 27, 2018
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PostPosted: Wed May 09, 2018 1:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here is a 13600 VCA on this forum that I think you could easily make into a dual 13700

http://electro-music.com/forum/topic-41277.html

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cslammy



Joined: Apr 27, 2018
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PostPosted: Wed Jun 27, 2018 9:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Another

OTA based dual VCA

at the PAIA site. I haven't dug into it but I would think 13700 or 13600 would both work. Looks very straightforward; requires +/- 12V.

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steved



Joined: Oct 07, 2015
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PostPosted: Thu Jun 28, 2018 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

i have made VCA 1 and 2 with 13700 from here: -wld be easy to make dual

http://hem.bredband.net/bersyn/VCA/vca_shootout.htm

-steved
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cslammy



Joined: Apr 27, 2018
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PostPosted: Fri Jun 29, 2018 7:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Steve D

Great page on the specs of various synth VCAs. Very Happy I imagine you put a lot of work into this and it's a great help to the community.

Thanks!

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steved



Joined: Oct 07, 2015
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PostPosted: Thu Jul 05, 2018 5:04 am    Post subject: Reply with quote  Mark this post and the followings unread

cslammy -

I did not make the page, only made the two VCA's

I just found the page a bit ago and thought it was great!
it is Jörgen's page.. not 100% but here is the main link:
http://hem.bredband.net/bersyn/

thanks!
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cslammy



Joined: Apr 27, 2018
Posts: 66
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PostPosted: Thu Jul 05, 2018 7:49 am    Post subject: Reply with quote  Mark this post and the followings unread

steved wrote:
cslammy -

I did not make the page, only made the two VCA's

I just found the page a bit ago and thought it was great!
it is Jörgen's page.. not 100% but here is the main link:
http://hem.bredband.net/bersyn/

thanks!



OK--got it, thanks for the link.

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