How the heck does the MS-50 filter WORK?

[Sean Costello]

Hi everyone:

Another embarassing post where I reveal how little I know about the
schematics on my own site.  Anyway...

How the heck does the MS-50 filter work?  I'm looking at the schematic
(which you can find at my site - http://www.seanet.com/~costello) and
realizing I don't know what is going on with it at all.  I can figure
out some of it - it's a Sallen-and-Key filter (to be precise, it looks
like a variant of the "equal component value Sallen Key filter" in Don
Lancaster's book, "Active Filter Cookbook").  However, what is the diode
ring doing?  And why the two op-amps providing the control voltage?  I
would really be thankful if some DIY brainiac out there could do an
analysis of the filter, that I could put up on the web site if the
author approves (along the lines of "diodes x y and z act as the first
resistor following the first capacitor, which is this cap").  

Another question:  What do the diodes in the feedback loop do?  I mean,
obviously they provide a clipping type of limiting, but what does that
do for the circuit?  They are in the MS-20 too; what do they do there?

And finally: How do I adapt the circuit for +9 volts? ;)  

Thanks,

Sean Costello

P.S.  My +9 volt obsession is simply out of wanting a compact pedal that
does this stuff.  I'd like to have something that fits in a Budd box to
use with my guitar, or to fit on a tabletop for my TR-606.