[sdiy] 36dB VCF output drop while increasing emphases

[Olivier Gillet]

It would be more accurate to say that we got our inspiration from the
same sources...

Thomas Henry, Osamu Hoshuyama and Roman Sowa had all used the 2164 as
a 1-pole LPF cell before.

As for the resonance trick, I got it at the source while looking at
IR3109 filters (it's straight from the Jupiter-8 schematics, 25A OTA :
http://fa.utfs.org/diy/roland_filters/JP8.jpg). My previous attempt
was to sum the output of the resonance VCA with the 4th stage, and let
people customize the soft-clipping they wanted by installing
back-to-back diodes there.

Actually I use none of the design choices unique to the Dr Octature
(switchable gain for each stage, 2164 for the feedback VCA). I see two
benefits in using an OTA over a VCA for the resonance control here :
differential inputs save you an op-amp, but more importantly the OTA
non-linearity is what stabilizes the filter and provides the "right"
flavour of soft-clipping at high-resonance.

On Sun, Apr 19, 2015 at 9:29 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
>
>> Am 18.04.2015 um 19:15 schrieb Chris McDowell:
>> > You might try what Olivier from Mutable did on "Ripples"
>> >
>> > http://mutable-instruments.net/static/schematics/Ripples-v01.pdf
>> >
>> > The input goes through the same OTA controlling feedback
>> for resonance.
>> > Perdy clever!
>
> I think he might have learned that from the Dr. Octature, as I do the same
> thing there, and I believe that Olivier's 4-pole is based on mine.
>
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