[sdiy] Nonlinearities in IR3109 filters

[Scott Gravenhorst]

"Sean Costello" <seancostello2003 at comcast.net> wrote:
>Hi all:
>
>Scanning through some old posts, and I saw someone discussing a digital
>emulation of the SH-101 filter. The discussion pointed out something I have
>never seen before: the SH-101 has back-to-back diodes going to ground in the
>feedback path of the filter.
>
>Is this the reason that the SH-101 has a fairly nonlinear filter? I have
>always thought that the sound of the filter was due to the IR3109 chip, but
>I always wondered why my Juno-60 filters (which use the IR3109) sounded weak
>next to the SH-101. Do any of the other Roland synths have clipping in the
>feedback path of an IR-3109? How about clipping in the feedback path of
>filters using other OTAs?
>
>Any observations welcome.


This may relate to what you posted about: I built a modified version of the FatMan's VCF,
which is a pretty vanilla SVF taken right out of the datasheet.  I purposely added a
method to introduce different kinds of distortion precisely in the feedback path.  I
found interesting results, rather odd things, happen under the right conditions.  It
turns out that this is somewhat chaotic and as such, the sound one gets depends heavily
on things like input level.  The distortion type that seemed to work the best was reverse
parallel diodes to clip the feedback signal.  I added some amplification and gain control
to the feedback path so that the feedback signal is nicely buffered before hitting the
distortion components.






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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | Xilinx Spartan 3E
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