AW: MS50 LP-Filter

[Haible Juergen]

Martin,

good analysis of the filter !

	>Amazing, they did everything the other way arround! Just to
avoide
	>any similarity to a normal cacade? Affraid of infringing Moogs
patent?

You might look at it from another point of view. Don't start with
a cascade at all, but with a simple one-opamp / positive feedback
LPF. The one with a non-inverting amplifier, two resistors in series
from the filter input to the opamp +input, a cap from the opamp
+input to gnd, and a second cap from the resistor connection point
to the output of the opamp. You surely know what I mean. Now replace 
the two (floating) resistors with diodes for voltage control. Use a
symmetrical configuration at the diodes to avoid offset voltages.
Keep the levels down (both from the input and from the feedback 
path), to keep distortion low. Voila !

BTW, I wonder how much the difference between this filter,
and the MS-20's original (Korg-35) LPF, and the 13700-based
replacement circuit in later MS-20's really is.
As I have pointed out before, the 13700 filter in later MS-20's
is also an attempt to get voltage controlled resistors in the
one-opamp standard LPF mentioned above.

So I wonder what's inside the Korg-35 chip !  The diode ring?
ota structures? something completely different ?

I really don't know. And I haven't heard a MS-50 and MS-20 side
by side. And I haven't heard a new-style MS-20 side by side
with an old style one. This would really be interesting.

JH.