CEM Oscillator woes

[Grant Richter]

PWM produces psychoacoustic pitch shifting, in the way the ear handles the
waveform. Are you monitoring the triangle wave output and hear the pitch
shift?

Try shorting out the resistors in series with the power rails on the Ekx
boards. That should prevent the voltage from shifting under load. John
always uses decoupling circuits and those can be a problem for VCOs.

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>From: klosmon <klosmon at earthlink.net>
>To: synth-diy at node12b53.a2000.nl
>Subject: CEM Oscillator woes
>Date: Sat, Sep 2, 2000, 4:52 PM
>

> Decided to unearth & recommission my old Curtis CEM 3340 vcos -- have two
> on Paia experimenter cards and one on a homemade circuit from POLYPHONY --
> the weird thing is that they ALL have the same problem:
> altering the pulse width has an effect on the pitch.  It is the same effect
> on all three waveforms;  measuring the control voltage going into the chip,
> I see a change of about .06 volts moving the pulse width control from one
> extreme to another.  this same effect occurs using external pwm voltages.
> I've checked all I can think of to check (on all three units);  I've
> switched power supplies.
>
> This is all news to me.  I'd conclude that it's a systemic problem with the
> CEM chips, but they work just fine in my Pro-One.
> Anybody (who's still around, and not carousing for the three-day weekend)
> ever encounter this?
>