[sdiy] GRP Genesi

[Michael E Caloroso]

On the ProSoloist, a ten octave range is required for the VCO charged cap
core, which is not in the feedback loop.  You can't achieve that with a
v/hz architecture.  With a v/hz system, the control voltage doubles for
every octave and you quickly reach the power rail.  Thus the CV for top
pitch is defined just below the power rail (opamps back then were not
rail-to-rail opamps).

There's two problems with a v/hz system.  One, the pitch range is limited.
For a top pitch CV set at 14VDC, a ten octave range would have a descending
CV scale of 14V, 7V, 3.5V, 1.75V, 0.875V, 0.4375V, 0.21875V, 0.109375V, uh
oh.  We have a problem and have only reached eight octaves. The CVs for the
lower pitches descend into the inherent limitation of the opamp - the
microvolt error domain.  The offset error voltages are now a significant
contributor to the pitch CV, causing tuning problems that are impossible to
trim out because the errors drift with temperature.  So a realistic
range for a v/hz system is actually six octaves.

Problem two is modulation CV.  A fixed modulation CV will not produce the
same modulation depth across the entire range of a v/hz system.  If you
scale the gain of the modulation CV with pitch you run into another
problem; as the pitch CV ascends toward maximum pitch CV, the sum of pitch
and modulation CV must not exceed the maximum or you risk opamp latchup or
phase inversion.

That's why v/hz instruments - Moog Taurus I, Taurus III, Minitaur, Sirius,
Korg MS series, etc - have a limited pitch range.

So yes it does need the linear to expo converter for a v/oct system.  The
beauty of the PS VCO is that no tempco resistor is needed, although the
exponential DAC does require a custom resistor array of precision
non-standard values.

MC

On Sun, Jul 5, 2026 at 10:01 AM Mike Bryant <mbryant at futurehorizons.com>
wrote:

> Does it really even need the linear to exp convertor ?  The feedback loop
> will force it to the correct frequency.  Portamento could just use a
> different charging function on the capacitor.
>
> ------------------------------
> *From:* Neil Johnson <neil.johnson71 at gmail.com>
> *Sent:* 05 July 2026 10:26
> *To:* Michael E Caloroso <mec.forumreader at gmail.com>
> *Cc:* Mike Bryant <mbryant at futurehorizons.com>; SYNTH DIY <
> synth-diy at synth-diy.org>
> *Subject:* Re: [sdiy] GRP Genesi
>
> Michael E Caloroso wrote:
>
> US 3,930,429
>
>
> Don did a write-up of it here:
> https://till.com/articles/arp/patents.html#US03930429
> Very weird architecture indeed!
>
> Neil
>
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