[sdiy] [AH] SEM LFO square wave mod...

[James Coplin]

I'll check out pin 6 on A25.  Kevin L. had told me that applying negative
voltage to the CA3080 pin 5 would increase the amplitude of the sine wave
which would clip into a square.

James R. Coplin

> -----Original Message-----
> From: Tim Parkhurst [mailto:tim.parkhurst at gmail.com]
> Sent: Friday, May 27, 2011 6:05 PM
> To: James Coplin
> Cc: synth-diy at dropmix.xs4all.nl; analogue at hyperreal.org
> Subject: Re: [AH] SEM LFO square wave mod...
>
> The 3080 in the original SEM is used as a tri to sine shaper. Not
> quite sure what could be accomplished by tweaking the supply voltage
> to that chip. You should be able to tap a square wave off pin 6 of
> A25, and just install a switch to select sine or square. Check the
> schematics at:
> http://www.synthfool.com/docs/Oberheim/Oberheim_SEM1A/SEM_1A_Schematics
> .pdf
>
> Remember that these schematics for the original SEM. The reissue
> appears to use 13700 OTAs instead. I don't know if there is one used
> as a waveshaper for the LFO, but a little careful poking around with a
> scope should point you to the square wave already available in the LFO
> circuit.
>
>
> Tim (hip to be square wave) Servo
> --
> "Sire, the church of God is an anvil that has worn out many hammers."
> - H.L. Hastings
>
>
> On Fri, May 27, 2011 at 1:57 PM, James Coplin <james at ticalun.net> wrote:
> > I've been told that you can modify the SEM LFO waveshape by applying
> > negative voltage to pin 5 of A23 (a CA3080).  However, I'm not sure
> how to
> > implement this exactly.  Looking at the schematic that pin is a 15v
> supply
> > for the chip.  I know how to invert the VC to get a negative voltage
> using
> > a transistor but what is the right way to sum it in with the +15v
> supply?
> > Anyone done this and know what the max "safe" negative voltage to
> blend in
> > is or am I not looking/seeing something right? Thanks as always!
> >
> > James R. Coplin
> >