AW: How the heck does the MS-50 filter WORK?

[Haible Juergen]

A few additions to the basic description:

Diodes in opamp feedback are for soft clipping.
Remember that almost *everything* is better than
an opamp recovering from being stuck at one of
its supply rails.

Note this other, inconspicuous, diode D28. It's used to
sense the ambient temperature, for compensation of
the diode ring's drop voltage.

Also note the resistors in both supply connections of the
ring driver dual opamp, IC7, and the 100 Ohm load resistors
at the opamp outputs. Voltage and current limiting for the
diode ring, without loosing the low output impedance of the
opamps.

I have to correct a little bit of my old mail (which Marjan quoted):
There is another little difference to the standard sallen and key 
filter. Between the two series resistors of the S&K, there is 
a 3rd (shunt) resistor to GND. This is formed by diodes
pin1-9 and pin1-4. It doesn't make much difference, as this
"resistor" is tracking to the other ones. It will affect corner
frequency, attenuation and resonance loop gain, however.
Why these extra diodes ? They prevent the diode ring from
being shortened by the driver opamp outputs. Ok, but one could
have used current sources instead to drive the ring (have seen
this method somewhere else, I think). But the way it is done
in the MS-50, with direct voltage control, the whole diode works
as exponential converter all by itself.

Not bad, these Korg people !!

JH.