[sdiy] Dual/quad opamp bleed-through

David G. Dixon dixon at interchange.ubc.ca
Sun Feb 1 08:16:49 CET 2009

Well, as it happens, I messed up, and the interference is occurring over the
audible range after all, so I guess I believe all the advice I've been

Also, thanks for the tip about the tri-2-sin converter for the idle half of
the LM13700; I had thought of that.  It's just that the two-BJT diff-amp
solution in the VCO-1 is so elegant, works so well, and is so much cheaper
to implement, that I was hoping to find another good use for an OTA within
the context of a VCO.  However, if I use the other half as a sine shaper,
then this frees up an opamp and gives the circuit a smaller footprint, so
I'll give it a try.  I'll just co-opt the sine shaper in Ray Wilson's VCO
circuit (thanks, Ray!).

David G. Dixon
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Tel 1-604-822-3679
Fax 1-604-822-3619
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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 8:54 PM
> Cc: Synth-diy at synth-diy.org
> Subject: Re: [sdiy] Dual/quad opamp bleed-through
> 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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