[sdiy] Filter problems

[Richie Burnett]

Tom, put two antiparallel diodes in the feedback path of the damping signal 
of the SVF that is taken from the output of the first integrator, like shown 
in this schematic here from...  erm... your website Tom! :-)

http://www.electricdruid.com/images/Jupiter6Filter.png

The antiparallel diodes need to go in the "forward" path of the damping 
signal as shown, not to ground like you would do to implement clamping in a 
typical 4-pole cascade with global feedback.  Because the SVF "resonance 
control" is a damping control, you actually need to implement "anti 
clipping" (or a dead-band) in the damping feedback signal to stabilise the 
self-oscillation.  Instead of limiting the feedback amplitude when the 
signal gets too large in amplitude, you actually need to feed back *more* 
damping signal when the amplitude starts to get too high.  The anti-parallel 
diodes wired like that in the damping feedback path don't conduct for small 
signals, but once the BP output signal from the SVF gets sufficiently large 
to forward bias them, they will begin to conduct and increase the damping in 
the filter in an attempt to tame the self oscillation.

-Richie,


-----Original Message----- 
From: Tom Wiltshire
Sent: Sunday, January 28, 2018 2:50 PM
To: Neil Johnson
Cc: SYNTH DIY
Subject: Re: [sdiy] Filter problems

You’d recommend putting them in the feedback path instead, Neil?

It does make more sense in many ways (since what you’re trying to do is 
limit the resonance). But the problem is that VC-resonance control is really 
a “damping” control, so it’s not as simple as just moving them to the Res 
VCA’s op-amp. Would be nice if it was that simple.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 27 Jan 2018, at 23:34, Neil Johnson <neil.johnson71 at gmail.com> wrote:
>
> Also I wouldn't recommend putting the zeners across the integrator 
> capacitor.
>
> Neil


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