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 Forum index » DIY Hardware and Software
LM13700 VCAs
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frijitz



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PostPosted: Fri Dec 02, 2011 9:52 pm    Post subject: LM13700 VCAs
Subject description: using the W. Jung biasing trick
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Hello --

A "trick" for biasing the CA3080 OTA by drawing the input current through the V- power pin has been around for decades. Since the 3080 is no longer in production the question has been asked whether the same trick can be used for the LM13600/LM13700. The standard answer has been no, because the two halves of the device share the power pins.

However, what if only one side of the device is used? Or what if the two devices are connected in parallel? The answer then, it turns out, is yes. I can't remember seeing any actual circuit examples, though. So here are two examples of "standard" OTA VCAs using the power-pin biasing method.

The single-device method is basically trivial, ie, there is no change in the original design. For the double-device method two resistors have been added to ensure that the bias current is divided equally. R6 and R7 may be reduced by a factor of two, to keep the maximum bias current the same as in the standard design.

Very Happy

Ian


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diablojoy



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PostPosted: Sat Dec 03, 2011 12:10 am    Post subject: Reply with quote  Mark this post and the followings unread

hmm very interesting and good to know it works
but with the 2 sides in parallel as drawn i see no advantages
over just using one half of the chip though.
if the circuitry could be duplicated but kept seperate for the signal in / out
but still a single bias for both halves would this allow one cv control over 2 signal paths ?
in this instance perhaps a very limited dual VCA
perhaps more interesting is if more change can be made to each side
will need to play around with this a bit i think

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andrewF



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PostPosted: Sat Dec 03, 2011 3:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Ian
that puts quite a few EN circuits back on the menu.
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frijitz



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PostPosted: Sat Dec 03, 2011 5:10 am    Post subject: Reply with quote  Mark this post and the followings unread

diablojoy wrote:
hmm very interesting and good to know it works
but with the 2 sides in parallel as drawn i see no advantages
over just using one half of the chip though.

Improved S/N? The pins are opposite each other so it's simple to do. I'd be interested to see a circuit diagram what other ideas you have.

Very Happy

Ian
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EdisonRex
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PostPosted: Sat Dec 03, 2011 11:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Very timely of you to share this, Ian. Thanks. Smile
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Dan Lavin



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PostPosted: Sat Dec 03, 2011 10:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Ian! I like to keep a stock of LM13700's since they are versatile for easy VCA's and VCF's. I'll have to try these out!
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Boogdish



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PostPosted: Sat Dec 03, 2011 11:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

I hadn't heard of this trick before, is there an article somewhere that explains the benefit of drawing bias current from V-? I feel like there's something obvious that I'm missing.
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diablojoy



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PostPosted: Sun Dec 04, 2011 12:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Improved S/N? The pins are opposite each other so it's simple to do. I'd be interested to see a circuit diagram what other ideas you have.



Ian


improved signal to noise - valid point i didnt think of that possibility.
as far as other cct idea's i havent actually thought of anything specific yet
just considering i dont like to leave such a chip only half used if i can help it.
I suppose as this was brought up most recently in the simple x-fader topic
and i stated there that i would look into drawing up a board for a dual module from the elegant schematics by richardc64
i could see if a linked cross fading quad VCA could be accomplished
using 2 x lm13700. mind you this is a very unthought out proposal
just the very vaguest concept of this moment .
but it may hold some merit
2 CV's controlling 4 VCA's in pairs crossing each other
I am sure its been done before but perhaps not exactly this way

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richardc64



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PostPosted: Sun Dec 04, 2011 7:01 am    Post subject: Reply with quote  Mark this post and the followings unread

What, aside from fewer trivial components, is the benefit, if any, of the Jung method over the more conventional way, which allows the two OTAs to be independent?


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frijitz



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PostPosted: Sun Dec 04, 2011 10:16 am    Post subject: Reply with quote  Mark this post and the followings unread

diablojoy wrote:
improved signal to noise - valid point i didnt think of that possibility. as far as other cct idea's i havent actually thought of anything specific yet just considering i dont like to leave such a chip only half used if i can help it.

It could be used as a stereo VCA, I guess, if you ever needed that.

Very Happy

Ian
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frijitz



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PostPosted: Sun Dec 04, 2011 10:21 am    Post subject: Reply with quote  Mark this post and the followings unread

Boogdish wrote:
I hadn't heard of this trick before, is there an article somewhere that explains the benefit of drawing bias current from V-? I feel like there's something obvious that I'm missing.

It's just a simple (low part count) method. Myself, I prefer the 1 opamp / 1 transistor method, but if someone wants to follow one of the old designs exactly, this will work. It's disadvantage is that the 100k input resistor produces a pretty low bias current range, so the S/N isn't as good as it could be.

Very Happy

Ian
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oldcrow



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PostPosted: Wed Dec 21, 2011 10:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ian, have you tested a paralleled-cell version for SNR? Interested in seeing what improvements, if any, can be made there.

Crow
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frijitz



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PostPosted: Thu Dec 22, 2011 8:18 am    Post subject: Reply with quote  Mark this post and the followings unread

oldcrow wrote:
Ian, have you tested a paralleled-cell version for SNR? Interested in seeing what improvements, if any, can be made there.

I believe Serge in his old EN article mentioned paralleling devices for the expected sqrt2 improvement.

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Ian
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oldcrow



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PostPosted: Thu Dec 22, 2011 10:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
I believe Serge in his old EN article mentioned paralleling devices for the expected sqrt2 improvement.

Very Happy

Ian


Oh yeah, I'd forgotten all about that one. Thanks!

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