[sdiy] Dual/quad opamp bleed-through

[David G. Dixon]

Hmm.  The key here is that the 3086 has no power pins, and the individual
transistors can be well isolated from each other by grounding pin 13.  It's
the common power pins of dual/quad IC's which are the problem, I think, as
Harry Bissell pointed out earlier.  There is simply no way to bypass the
rails inside the IC.  I don't think there is much problem with two VCO's
living side by side on the same pcb.  Indeed, it may be better that way than
back to back on two separate pcb's, given the potential for inductive
effects of the latter.  I also like the symmetrical layout.

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
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Dave Manley
> Sent: Saturday, January 31, 2009 11:37 PM
> To: Synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Dual/quad opamp bleed-through
> 
> Here's another single board dual VCO - the Oberheim SEM
> 
> http://www.sequencer.de/pix/oberheim/oberheim_sem_module_m.jpg
> 
> The two VCO's are symmetrical and in the upper right of this image.
> Voltage
> summing at the top, two tempco's on top of a 3086 expo, two polystyrene
> timing caps at the bottom (just above the 'de' in "image
> www.sequencer.de".
> No common parts, except the 3086.  Looks like good use of a ground fill
> to isolate parts of the two VCOs.
> 
> -Dave
> 
> > I'm going to play devil's advocate and mention a few well known
> > synths that have multiple VCOs on the same board: Moog Mini,
> > ARP Odyssey, and KORG MS-20.  They don't share ICs, but
> > they are on the same board.
> >
> > Do any of these synths have unintentional oscillator sync problems?
> >
> > -Dave
> >
> >> The first prototype of my dual VCO module had this same problem.
> >> I then split the two VCOs onto separate boards with shared panel
> >> controls.
> >> This design still had sync problems. It was only after I placed the
> >> boards at least 3/4" apart using standoffs that the syncing was
> >> eliminated. This was with good de-coupling and hefty gauge star
> >> wiring etc.
> >> No shared ICs or even on the same board.
> >>
> >> Bruce
> >
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