AW: [AW: tapped 4pole filter]

[Haible Juergen]

	>Harry Bissell: When I get a sketch drawn up of what I did to the
3320 LPF I'll
	>post it here (for bouquets or brickbats...) As I said earlier, the
idea came
	>from the moog ladder. Tapping the ladder with a additional diff-amp
at the 2nd
	>ladder rung yields a "12db" response, but the cutoff frequency and
resonance
	>stay like the 24db output. Changing the resonant feedback to the
2nd tap
	>doesn't work well, and if you do the 24db output becomes totally
useless
	>(think a 2 pole filter with resonance in series with a 2 pole
filter without
	>resonance... The second output destroys the resonant peak) When I
saw that the
	>3320 LPF was a "constant peak" type filter (like the moog ladder) I
decided to
	>try this idea. You don't need to use lots of analog switches. I'm
going to try
	>a rotary switch to tap each stage in line (6,12,18,24db...)  Five
pole
	>filters, anyone ??? Why not...   :-)  If you have pspice or
Electronic
	>Workbench, try some sims of this idea. You will like it.   :-)
Harry

Personally I like the sound of the original SVF 2-pole better than a tapped
4pole.
But I have a tapped 4pole in my OB-8 multimode VCF modification, too:
The HPF output (in SVF configuration) becomes a 6dB / 8ve filter with
resonance when switched to 4-pole. (I.e. a 4-pole polygon filter tapped
after
the 1st stage.) This is a nice addition, especially for "bright" sounds, and
it comes for free with the 2p HPF modification.

JH.