Discussion about the Korg PolySix synthesizer

OK, I have decided to make a modification of the LFO range, but it
seems like this modification was only mentioned in passing somewhere,
so I will have to be intentive here, and would appreciate any help
from seasoned PolySix modificators.
The idea is to make an alternative frequency range available by
augmenting the LFO speed capacitor with another with a smaller value.
I want to be able to take the LFO into the audio range. Any good
suggestions for an alternative cap value? Maybe 1/10th of the
original cap value is suitable? Could/should an even lower value be
used? I know from the MiniMoog that the faster the modulation
freqency, the wilder it will sound. (My best Mini modulations are
with osc3 in positions 8', and especially 4').

To make this a reversible and switchable modification, I plan to
replace the LFO speed pot with one of the type that has a push-pull
switch inside too, so that by pulling the pot the LFO goes into the
desired FM range, pressing it returns to normal range. This could be
achieved by putting the new small capacitor in series with the
existing one and using the switch to bypass the old capacitor,
leaving only the new smaller one in the line.
Any comments to this. Anything I should be aware of before I do this?

Thanks

Frank Carvalho

On Wed, 6 Feb 2002, frankcarvalho wrote:

> Any comments to this. Anything I should be aware of before I do this?

Actually just wanted to check that you already knew about adjusting the
internal pot for LFO-frequency well into the audiable spectrum...works
well enough for me anyway but I guess thats only like the same as using a
oscillator in the 2' range or something...maybe by changing the internal
pot with a higher value you would be able to expand the range of the
LFO-speed...otherwise putting a circuit with a additional resistor in
series with the internal pot would do the trick (and making that circuit
switchable off coarse)...

Just some thoughts...

regards,
Kristofer Ulfves, Uppsala, Sweden

> Actually just wanted to check that you already knew about adjusting the
> internal pot for LFO-frequency well into the audiable spectrum...

I haven't checked that yet, no. I would expect the oscillation to be
controlled by a resistor/capacitor pair
somewhere, and the capacitor looks like it would be C19. (?) The value is
22uF, and it's an electrolytic
cap it seems from the diagram. Is this the capacitor controlling LFO speed,
or am I entirely wrong?

But maybe a change of the proper resistor could do the job equally well.

> works
> well enough for me anyway but I guess thats only like the same as using a
> oscillator in the 2' range or something...

That would be 32' or 16' range in MiniMoog speak. I would like to go some
3-4 octaves higher! Quite
a radical modification. The thing is that modulation with low frequencies is
not as exciting as
modulation with higher frequencies. Up there you find the bell-sounds, etc.
Ideally all ranges should be
available (and the LFO should have some keyboard follow too - although you
would have to make
decisions about which key decides the keyboard follow). But even one fixed
well-chosen
high freq range will produce so many great sounds. Since the LFO can
modulate VCO, VCA or
VCF, all the classic fixed-freq Moog modulations would be possible. In fact,
the Mini won't even
be able to modulate VCA.
In principle all the MG sources could be brought into audio range with
interesting results. PWM
with an audio range modulation is another cool type of sound. But I still
think that the best and
easiest obtainable results will be from the LFO.
Open question: Would it damage the other circuits to go into the higher
audio range?

> maybe by changing the internal
> pot with a higher value you would be able to expand the range of the
> LFO-speed...
> otherwise putting a circuit with a additional resistor in
> series with the internal pot would do the trick (and making that circuit
> switchable off coarse)...

Yes, but shouldn't it be lower values rather than higher to increase
frequency?
It would be the cleanest modification to have a resistor on the switchable
pot indeed, as that
would require very little messing with the board itself.

Do you think I would fry the circuits with some cautious experimentation. I
mean, were talking about
a timer here, and not an op-amp, so I would expect it to be fairly robust
and built for many frequency ranges. Just as long as I am sure I'm replacing
the correct components. And that would be C19 and,
eh... R?? Which one is the internal pot? (I do have the diagrams with me,
but I want to be sure).

Thanks

Frank Carvalho


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