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

[Scott Nordlund]

It's a highpass filter (1 pole OTA) and lowpass filter (4 pole diode ladder) that both track together.

Lowpass mode is just the lowpass (the passband gain seems to be compensated by the dual ganged resonance pot).

Bandpass is the highpass filter feeding into the lowpass filter (1 pole HP + 4 pole LP).

Highpass is the same as bandpass, only the highpass filter is also mixed into the output signal.  This is clever as the high harmonics that the LPF removes are added in again, thus approximating (sort of) a resonant highpass filter.  I don't know what sort of phase cancellation, etc. results from this, though.

It seems a little less exotic when considering the more common Roland filter structure of a fixed highpass before a VC lowpass, but it's still pretty interesting.

Note that the bandpass filter at the bottom of the schematic isn't voltage controlled...

I was very interested to see this as I've been looking for it for a long time.  I've found SH-5 schematics online but they don't include this page.  I wanted to find the schematics after hearing it used in some very wild (sample and hold, ring modulator, etc.) synth solos...

The Jupiter 6 also has an interesting arrangement (standard Roland VCF chips arranged as a cascaded state variable filter) to achieve its LP/BP/HP responses.



----------------------------------------
> Date: Wed, 22 Apr 2009 16:29:30 +0200
> From: fanwander at mnet-online.de
> To: Synth-diy at dropmix.xs4all.nl
> CC:
> Subject: [sdiy] How does the Filter of the Roland SH5 work?
>
> 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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