[Re: [Switchable ladder filter]]

[Harry Bissell]

Sean:  The resonant feedback point in the LPF is a lot less interesting than
you think, because the stages after the resonant loop are still LPF stages,
and just kill any of the "interest" generated by the feedback. The scope
traces and bode plots are totally boring. I admit that the picture is totally
different in the Allpass "phaser" case.

I didn't like moving the output tap together with the resonant tap, because
the 2 pole filter "ring" was at a completely different frequency (much higher)
for the same CV in. Cap values that gave a good 4 pole range were way too
small in the 2 pole range...

Having the feedback at the 4 pole position, and the output tap at the 2 pole
gives an output that has the same resonant "ring" as the 4 pole, but with
greater "ring" amplitude relative to the signal (well you'd figure.. its a 2
pole response...) and falls off at 12db/oct.

I wouldn't bother with 6 and 18db roll-offs... seems to subtile to me to be
worth the circuitry....   :-) Harry


Sean Costello <costello at seanet.com> wrote:

Harry Bissell wrote:

> The other point is where you get the resonant feedback from. I got the best
> (simulated) results from taking the feedback from the last stage at all
times,
> and tapping the filter with a differential amplifier at an earlier stage....

It would seem that you would get more powerful control over the sound if
you could choose where the tap point for the feedback is, as well as the
output taps. While testing out my new phase shifting opcode for Csound
(allpassn - lousy name, but what would be a better name for n number of
single-order allpass stages? Suggestions welcome), I experimented with
different tap points for the output and the feedback. Invariably, the
feedback tap dominated the sound. If there were 8 stages before the
feedback, but the output was tapped after stage 4, the sound was far
closer to a standard 8 stage phaser. An 8 stage phaser with the feedback
tap taken after the 2nd stage (like the Schulte Compact Phaser 'A')
sounded very nice, with some swirly complexity, but none of the
overbearing "fruitiness" that an 8-stage phaser can display.  I don't
know if this is true with the Moog filter, but I have a feeling that it
would be.

Sean Costello


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