[sdiy] SVF resonance overdrive problem

[Andrew Simper]

That schematic doesn't have a diode limiting section on the feedback
resonance. I suggest using a circuit more like the SEM one I posted before.

I've got nonlinear asymetric digital models of a few different filter
topololigies up and working including the SEM, but not a full model of the
CA3080, only a slightly asymetric Tanh one. Thanks to Antti for the help
with getting me started down the road to proper analog modeling - he
actually suggested I join this list as well.

I tested out feeding the input signal into the feedback section for the SEM
SVF - basically by adding an extra op amp buffer post A14 buffer to sum the
bandpass output from A14 and the input signal into the resonance feedback
path. Without resonance (equal to a damping of 2 in digital land) you get
the input signal level x3 all up (x1 from input and x2 from resonance path),
with some extra drive from the diode shaper. When the resonance is increased
to just on self oscillation (equal to a damping of 0 in digital land) you
just get the input signal level x1. So this isn't quite the same attenuation
as a moog, which has a passband of x5 at low resonance, x1 at full - well
actually the other way around with x1/5 at full and x1 at none, but it
definitely works well, and gives a less "rip your head off with full
resonance" sound.

Andrew

-----Original Message-----
From: Karl Ekdahl [mailto:elektrodwarf at yahoo.se] 
Sent: 12 December 2006 13:55
To: Andrew Simper; synth-diy at dropmix.xs4all.nl
Subject: SV: RE: [sdiy] SVF resonance overdrive problem

Hi

Unfortunley i do not have a schematic, it's based on this circuit
http://www.synthdiy.com/show/getfile.asp?get=23

I've used LM13700s with the + input grounded, leaving buffers and
linearization diode inputs unused. Could that be a problem?

Karl

--- Andrew Simper <andy-list at vellocet.com> skrev:

> Which circuit are you basing your SVF on? Can you please post a link 
> to the schematic here? Have you checked the SEM SVF schematic 
> (http://www.synthfool.com/schematics/sem.gif)?
> Forget about the ARP one if
> you want lower distortion - it actually drives more and you loose 
> resonance more quickly, I didn't read you're email properly last night 
> when I was tired. The regular SVF like the SEM keep strong resonance 
> no matter what input level you have.
> 
> Andrew
> 
> -----Original Message-----
> From: Karl Ekdahl [mailto:elektrodwarf at yahoo.se]
> Sent: 11 December 2006 19:44
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] SVF resonance overdrive problem
> 
> Hi list,
> 
> i've built a OTA-based SVF whose resonance distorts heavily if i have 
> to high input-volume. The problem is that "to high" is something like 
> >20mVpp so that won't do. Can anyone give me a hint on what's wrong, 
> is this a common problem with SVFs?
> 
> Karl
> 
> 
> 	
> 	
> 		
>
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