[sdiy] CEM VCF transconductance stage difference!

[karl dalen]

>mark verbos <mverbos at earthlink.net>:
> Jürgen from Jomox once gave me a demo of his Sunsyn, while
> he was finishing up its design. The 4 poles on it's
> filter can be switched from hipass to lopass to notch and
> each one has an offset control. With just one pole offset
> from the others he got the resonance to behave what he
> described as "like a 303." It was pronounced
> resonance, but it did not self oscillate.

It skews the phase releashionship so the filter has problem to oscillate.
But offseting the pole position in Fc hardly give TB303 character.

>I have always  assumed that this is the point of having a different
>cap on one pole. Is this incorrect?
> Mark

I dont think so, Jurgen just exploited the ability to have each pole
switchable/tunable, similar to the MIF minus pole tuning. Sunsyn manual
speaks about 4 pole and 2pole with 4 pole resonance, that would indicate
all transductors are in series as the graphic in his manual also are
drawn. Poles slightely of line gives a muddier sound.

Regarding to CEM, it seams that the 3320 are of classic type, same
transconductance in all but thereafter he changed the designs!

Reg
KD
 
> On Dec 26, 2008, at 2:36 PM, karl dalen wrote:
> 
> > Did anyone figured out why Doug Curtis used such a
> (gm) difference between
> > the last transductor and the preceding 3 in his
> filters? Factors of 50,
> > 75 and 100 times can be found in his various designs.
> > 
> > Reg
> > KD
> > 
> > 
> >     
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