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

[jhaible at debitel.net]

The High Pass is the classic OTA-circuit (just a single stage, 6dB/Oct filter),
as shown in the LM13700 data sheet, figure 12.

JH.

> 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
> 
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