[sdiy] Dual/quad opamp bleed-through

[harrybissell at wowway.com]

The opamps share power supply rails inside the chip, you cannot
add enough decoupling...  They also have a lot of capacitance
between them.

That said... ~could~ you make them share... quite possibly. I would first
try to arrange the opamps so that they do not share packages on opposite
sides of the breadboard.

And check your power supply references (frequency pots etc...) these can
introduce a lot of coupling if they come right from the power supplies. I like
to make these with onboard zener diode references... preferrably 5.1V
zeners for best temperature stability.

and lose the solderless breadboard :^)

(JH's advice is best if you want them to be truly independent... but it
might be 'good enough' for you)

H^) harry




On Sat, 31 Jan 2009 17:27:57 -0800, David G. Dixon wrote
> 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
> 
> "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.
> > -----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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Harry Bissell & Nora Abdullah 4eva