[sdiy] 4 Pole state-variable filters

[Theo]

I toyed with 4pole statevariable filters a while ago.
The filter sounded good, but you need to tweak the feedback level
for the HP function relative to the resonance and cutoff setting.
Someone from this list send me a circuit from the EN later on.
Basically this was the same circuit as I had come up with,
and it had the same feedback level problem.
(The EN circuit used OTAs, mine used diode bridges)
Bit of a disappointment as I expected a genius solution for the
problematic feedback behavior from the mythical EN.
A possible solution would be to make the resonance voltage controlled
and use the resonance and cutoff CV to alter the HP feedback level.
(HP feedback level is probably not the proper name,
but hope you understand anyway)

Cascading two 12dB filters gave better results,
so the 24dB statevariable was dropped.
At this point I experiment with two cascaded filters,
but the resonance loop goes from the BP output of the second filter
to the input of the first filter.

I also got good results by cascading two Korg 770 style filters.
Resonance loop goes from the output of the second filter to the
BP inputs of _both_ filters.
Downside is that there are no simultaneous HP, BP, LP outputs.
Filter response of this filter is switchable between:
6dB HP, 12dB HP, 12dB BP, 24dB BP, 12dB LP and 24dB LP.
The 6dB HP and 12dB BP/LP responses are taken from the output of
the first filter.
However the second filter stays active and is part in the resonance loop.
The next step will be to add a knob to detune the filters from each other.


Theo





From: Grant Richter <grichter at asapnet.net>

> Generally designs are cascaded sections like the current PAIA filter and I
> seem to recall Digisound had a design for a cascaded dual SV.
>
> There was mention of a more advanced (second order?) topology in
> Electronotes but only in block diagram, not a schematic.
>
> > From: The Proteus <proteus at ugwarehouse.org>
> > Date: Sat, 9 Feb 2002 18:06:45 -0800 (PST)
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] 4 Pole state-variable filters
> >
> > Hello all,
> >
> > Are there any good references in text or online on 4-Pole
> > state-variable filters? I'm finally getting back into design, and would
> > like to construct a filterbox that has HP, LP, and BP outputs, and would
> > hate to build up discrete sections for each filter. I've got both the
> > Musical Engineers' Handbook and the Electronotes PCC at my disposal, as
> > well as Chamberlin. Any help would be greatly appreciated.
> >
> > Regards,
> >
> > Proteus
> >
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> >
> >
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