35 - the next step

[jhaible]

Hi again,

I have further improoved the MS-20 style filter.
Limiting the CV did indeed get rid of the most annoying part of
thumping. And limiting turned out to be not difficult.

(1) Limiting the upper end

A heavy level shift occured at the opamp input when the filter was 
modulated way above the audio range. It makes sense to simply
limit the range, while the exact limit is not important. 
30kHz or 50kHz - just something way above 20k and below the
point where the offset becomes a problem.
One single diode from the VCR transistos' bases to GND does
the job. (D1 in attatched circuit)

(2) Limiting the lower end

Hey, there may be many ways to limit the CV on the low side, but
I chose a circuit that might (might ! ) look like some of the Korg-35's
external components. You guessed it, the 15k / 220 divider. No,
I have no idea if it really serves this purpose in the original circuit,
but I just couldn't resist to make my version resemble the original. 
(I guess I'm still more an artist in my heart than a scientist ... (;->)  )
I wouldn't expect to have an exact 15k / 220 divider, of course.
This depends upon the chosen value of the emitter followers operating
current, amongst other parameters. I found that 15k fixed + 25k trimmer
provided a wide range of limit frequency to choose. 
Does anybody know if the original has a lower limit, and where exactly
it is ? (early MS-20 owners, please ... )

Find the new circuit attatched to this mail. I'd say in this version
it is really useful as a synth's main VCF, finally.

JH.

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