[sdiy] Possible 2164 12db/oct bandpass filter design..

[David G Dixon]

Yes, as Neil says.  It is super easy to get a 2-BP-2 mode from a cascaded
four-pole topology.  One simply sums the outputs of the 2nd, 3rd, and 4th
stages, with gains of 1, 2, and 1.  Hence, if you are using a 30k feedback
resistor, then bring these outputs into the summer through 30k, 15k, and 30k
input resistors.  The output of this summer will be the symmetrical bandpass
mode which is 12dB per octave on both sides of the centre frequency.

You should know, however, that these bandpass modes don't necessarily act
the way you might want them to when resonance is applied.  The low-frequency
side is attenuated and the corner frequency rises to a peak, but the
high-frequency side is left unaffected.  Hence, the BP mode is only
symmetric at zero resonance.



-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Neil
Johnson
Sent: Wednesday, June 05, 2019 8:20 AM
To: Jean-Pierre Desrochers
Cc: Paul Dugan; SDIY List
Subject: Re: [sdiy] Possible 2164 12db/oct bandpass filter design..

Hi,

> Use the following schematic :
>
> https://userdisk.webry.biglobe.ne.jp/000/024/65/1/VcfSSM0602a.GIF
>
> and modify the 1st and 2nd   filter sections to be HPF and keep the 3rd
and 4rth as LPF.
>
> You will end up with a 12db/oct Bandpass filter using one SSM2164 IC.

Or refer to this paper by Emilie and use pole mixing to get a BP4 filter:
https://mutable-instruments.net/archive/documents/pole_mixing.pdf

Neil
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