[sdiy] Pequliar 2pole VCF

[Karl Dalen]

 Hi!

Filters!

I have designed a 2 ota cell (2pole,lp,hp,bp,np) that have some 
interesting habbits.Due to its design and the feedback paths it
have constant Q over the entire BW, and when it goes into oscillation
it is very close to perfect sinus wave , secondly the behavior of
Q control are set so (definable)it acts  a gain control (labeled 0-200)
on the VCF. So between 0 and 100 it sets the amount of resonance,
between 100-200 it sets the amplitude of the resonance wave. Then the
feedback routing, if i send a straight square wave into the filter and
look at the LP output with a scope and now increase the Q, the first
feedback  routing gives a resonance behavior where the ringing has a
very nice bell shape, and in the second routing the ringing are 
completely linear (flat) shape!? I.e. as soon as the square changes 
state the ringing starts perfectly linear in line with the input 
square wave, no over or under shot! (imagine a square wave, now remove
the horizontal line, and add in a sinus wave instead).

I have found this quite interesting, the bell shaped resonance gives
a sound (as expected) that has more of that "wine-glass- ping" to it!!
Wetness, if you like.The flat response don't have the same ping to it
but its behaves better when FM the filter from a VCO.

Have you seen this behavior before in other filter designs?
I guess that the bell shaped Q mode have more group delay then
the flat linear mode but should 'nt any filter type have a certain delay
no matter design topology or can certain feedback designs compensate
for RC delay, or are the linear flat mode just an "inphase-over- BW"
or "resonance-phase-compensated" type of filter?

Another peculiar thing i found out by random was that i had used
TL072 both as input, output and pole buffers and none goes into 
saturation or phase reverse as some other well know SVF designs do!!
When i saw this i tried with different types LF353,LM2904,MC33072/74
AD746, CA3240 to try them go bananas but all behaved just fine!

Thanks
KD

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