[sdiy] CEM VCF transconductance stage difference!

[karl dalen]

>Scott Nordlund <gsn10 at hotmail.com>
> 27 december 2008 09.11
> 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 (?).

>From where did you get the TB303 are only
3pole and that is the reason for its sound?

Tim has a rather exelent transistor-diode ladder analysis.
http://www.timstinchcombe.co.uk/synth/diode_18_24/diode.html
 
> 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.

It still dosent explain why Doug had a different gm on the last stage.
If it hadnt some effect he could had stayed with the 3320 design.
Dough must had some kind of reason for his design change?
 
> 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?). 

No problem, take a Lm3046 or a CA3083. JH 2044 cloe do this.

Reg
Kd


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