[sdiy] Dual/quad opamp bleed-through

Tom Wiltshire tom at electricdruid.net
Sun Feb 1 00:42:38 CET 2009


Fairly definitive result there. The guru has spoken!

T.

On 31 Jan 2009, at 22:06, JH. wrote:

> Never share ICs between VCOs.
> Don't even share copper traces.
>
> JH.
>
>
> ----- Original Message -----
> From: "David G. Dixon" <dixon at interchange.ubc.ca>
> To: "'sdiy'" <Synth-diy at synth-diy.org>
> Sent: Saturday, January 31, 2009 10:37 PM
> Subject: [sdiy] Dual/quad opamp bleed-through
>
>
> Here's another one for the gurus:
>
> I've just finished breadboarding the triangle-core part of my  
> "Everything
> Dual VCO" (which is essentially two of Thomas Henry's "VCO-1"  
> designs with a
> bunch of added bells and whistles), and it works perfectly, except  
> for one
> aspect:
>
> One VCO affects the other if its frequency is turned up.
>
> I've laid out the circuit to give two identical VCO circuits on  
> either side
> of the breadboard, with the opamps on opposite sides of quad and  
> dual TL072s
> (and a LM13700 dual OTA amplifying the expo currents for the triangle
> integrators) serving identical functions for the two VCOs.  In  
> other words,
> the two VCOs are mirror images of each other, sharing dual and quad  
> opamp
> and OTA chips.
>
> So, is my problem:
>
> a) bleedthrough between separate opamps on the same chip at different
> frequencies,
>
> b) noisy power rails (I've used eight 0.1-uF ceramic caps and two  
> 10-uF
> electrolytic caps on the rails, so this shouldn't be the problem),
>
> c) an artifact of using the solderless breadboard, or
>
> d) something else that I haven't thought of?
>
> Also, if a), do you think it's one of the TL07x's (which I can deal  
> with
> easily) or the LM13700 (which I can't deal with, since this is the  
> entire
> basis for building a dual VCO in the first place!)?
>
> Any help would be most appreciated!
>
> David G. Dixon
> Professor
> Department of Materials Engineering
> University of British Columbia
> 309-6350 Stores Road
> Vancouver, B.C. V6T 1Z4
> Canada
>
> Tel 1-604-822-3679
> Fax 1-604-822-3619
>
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