[sdiy] How does the Filter of the Roland SH5 work?

[Florian Anwander]

Hello,

somhow I am stupid.

Could please some explain me, how the filter of the SH-5 works.
See: http://fa.utfs.org/diy/roland_filters/SH5.jpg
I can recognize the low pass ladder in the right half, but what kind of 
filter is the hipass in the left half?



Also it is not really clear how the switching between the filtermodes is 
done:
Lowpass is clear:the input signal is sent to the ladder and the 
non-inverting input of IC506 (mixing OpAmp) is set to ground.
Highpass is input signal is sent to the highpass (or to what I assume 
that the hipass is?). But then ... the non-inverting input of IC506 is 
connected to the output of the highpass but the lowpass still will get 
some signal (and resonance also seems to be done by the lowpass).
For Bandpass there is no difference to the Highpass setting, but IC506 
is only a Buffer (non-inverting input is no longer connected to the mode 
selector switch).
Mostly I think, if I would understand the hipass circuit I'd understand 
the switching too.

Florian