[sdiy] Good 24db/oct HP-LP-BP-Notch filter Schematics ??

Andrew Simper andy at cytomic.com
Fri Mar 7 10:25:47 CET 2014

On 6 March 2014 18:22, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Richie wrote:
>> More likely option 2 if the product has a single global feedback path in
>> which you can insert an external processor.  The 4-pole SVF is a "leapfrog"
>> arrangement with multiple feedback paths if I remember correctly.
> It's horrendous - the maths is taking some time to crunch
> successfully...not helped by not having much spare time at the
> moment...
> Rane have an app note with the design of a 4-pole SVF, and some guy in
> Finland designed one for his masters thesis.  But in both cases the
> frequency and damping are fixed.  Turning them into successfully
> controllable Freq and Q SVFs is a bit more of a challenge :)

The math isn't so bad, not sure if there are any gotchas in an actual
circuit realisation, I've only done the theory and simulations thus
far. You do end up needing two different resonance gains, but they
don't have to be that exact exact because of the structure. It turns
out to be pretty much the same as a regular SVF, you need to limit the
bandpass output in the same way, it is the "dominant" damping term
that needs to be tamed for self oscillation.

Here is an audio example of a circuit model of a 4 pole SVF done in
the same style as a OSCar 2 pole SVF:


Perhaps I should apply for Masters accreditation at that Finish university ;)


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