[sdiy] Dual/quad opamp bleed-through

[David G. Dixon]

OK, but why?  Also, any ideas about what I should do with the spare OTA?

BTW, the effect is much stronger on one VCO than on the other, and only
occurs when the offending VCO is above the audible range (at least for me,
about 15,000 kHz -- too much loud rock music!).  Also, my scope suggests
that the frequency of the offending VCO does not increase beyond the point
where the problem occurs, suggesting that the problem is not really
bleedthrough, but some kind of saturation issue.  The frequency pots are
connected directly to the rails.  I think that putting some limiting
resistors between the rails and the pots to limit frequencies to the audible
range at the top will eliminate the problem.

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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> -----Original Message-----
> From: JH. [mailto:jhaible at debitel.net]
> Sent: Saturday, January 31, 2009 2:06 PM
> To: David G. Dixon; 'sdiy'
> Subject: Re: [sdiy] Dual/quad opamp bleed-through
> 
> 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 dropmix.xs4all.nl>
> 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
> 
> "PERFECTA FINGAMUS SERVIAT NATURA"
> 
> The information in this email and in any attachments is confidential and
> intended solely for the attention and use of the named addressee(s).  It
> must not be disclosed to any person without the writer's authority.  If
> you
> are not the intended recipient, or a person responsible for delivering it
> to
> the intended recipient, you are not authorized to and must not disclose,
> copy, distribute, or retain this message or any part of it.
> 
> 
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