Korg MS20 filters

[Haible Juergen]

	>Yesterday I also wanted to have a closer look to the Korg MS20
filters
	>(hyperreal). Oh dear, Korg-35 "modules" instead of a plain
schematic.
	>The really important parts of the filter are missing.
	<Some special Korg IC? 
	>Now I looked into the archive, later models used LM13700,
	>no sound change was observed, and could find this version:
	>
> http://www.mindspring.com/~vco/maxx2/page_015.html
	>
	>Could some guru please confirm, that this (nice) cicuit is equal to
the
	>newer MS20 models?
	>
	>J.H. ?

	Well, the guru for MS-20 filters is Paul Perry; he even builds them
	commercially. (the 13700 version).

	As for myself, I have once cloned the first version, using two
Korg-35
	modules that I bought as replacement parts from Musik Meyer.
	(Reportedly their last ones.)

	I don't know what's inside the Korg-35. It looks like a hybrid, and
my
	guess would be that they use a diode ring, similar to the MS-50
	filter.

	Building a 13700 version is high on my priority list. It's hard to
believe
	that old and new MS-20 sound the same, though that is often claimed.
	I'd say this cannot be true, just because the VCA has changed as
well.
	But maybe in the mere filter the main influence on sound is not the
	variable R or gm part in the first place. I guess the typical
behaviour
	at overdrive and / or self oscillation is because of the special
clipping
	in the feedback loop. But these are all just guesses.

	I'd love to see the internal diagram of the Korg-35.
	One thing I'd even love more to see is the internal circuit of the 
	old Keio ICs. Wasn't there someone on the list who said he had
	an unpotted filter pedal from the Minikorg aera ??
	Any news in reverse-engineering ?

	JH.