[sdiy] CEM VCF transconductance stage difference!

[Scott Nordlund]

I think that once resonance is applied, an offset in one of the filter stages will be relatively insignificant.  Negative feedback (resonance) pushes each pair of poles together to form a conjugate pair, detuning one pole would mean that more feedback is required to form that conjugate pair (between the detuned poles) and that it would be offset by half of the detuning amount (or at least that's what my understanding of root locus would lead me to believe).

I think the popular "TB-303 has a 3-pole filter that gives it a characteristic sound" rumor has been debunked already.  If I recall correctly it's more likely done for stability reasons (?).

This was covered in the recent analysis of transistor/diode ladder filters that was posted to this list, but I can't cite/reference it in more detail because it's not on this computer.

Anyway I think the different transconductance of one stage would be
directly equivalent to a different capacitor value.  Probably this could be
undone (to what effect?) by using a larger capacitor on that one stage.

Independent detuning of filter stages is an interesting idea, though I'd imagine an analog implementation would present its own difficulties (four matched exponential amplifiers?).  Detuning two stages by the same amount might give an interesting effect under resonance.  Detuning one stage by a large amount would make a three pole filter with no resonance, and then as resonance is increased it would make a genuine two pole resonant filter (rather than a 4-pole filter tapped at the second stage) with two negligible (?) poles in the stop band.  Neat idea.


> Date: Sat, 27 Dec 2008 02:36:45 +0000
> From: dalenkarl at yahoo.se
> Subject: Re: [sdiy] CEM VCF transconductance stage difference!
> To: synth-diy at dropmix.xs4all.nl; mverbos at earthlink.net
> CC: 
> 
>>mark verbos :
>> 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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