[sdiy] filters with independent resonance 'location'

[Don Tillman]

      From: Jeffrey Pontius [mailto:jpont at stat.ksu.edu]
      Sent: Tuesday, September 25, 2001 8:56 AM

      In general, a typical lowpass or highpass filter has the resonance set
      around the cutoff frequency.  Are there filters available, and if not
      available could one be reasonable constructed, that would allow the
      resonance 'location' to be set independently of the cutoff frequency?

   From: John Potter <JPotter at IT.RJF.com>
   Date: Tue, 25 Sep 2001 11:12:59 -0400

   Interesting - Yeah, you do always see resonant peaks at the cutoff freq.
   (ie. http://www.doepfer.de/a100_man/a120_d2.gif).  

That image appears to be a rough sketch of what a resonant peak might
look like and not an actual filter response plot.

I should point out that in a standard 2-pole resonant low pass filter
the peak occurs at a frequency *below* the resonant frequency.  At
high Q's the peak is asymptotically close to the resonant frequency,
but at lower Q's (around 2.0 or so) the peak is significantly lower
than the resonant frequency.  And at much lower Q's there is no peak.
Any good textbook on filters should include an accurate plot that
shows this effect.

This is one of the reasons Bob Moog choose a four-pole feedback design
for his VCF -- he thought it was musically important that the peak
frequency not change with Q.

('Just a point of interest, it's probably not what Jeffrey is asking
for.)

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com