AW: AW: Phasers - if you don't have enough already

[Haible Juergen]

	> This would be the perfect application for a SVF then. Control
	> poles/zeros with fc and q.

SVF are perfect building blocks if you want to construct "any"
filter. In fact you *need* 2nd order blocks to produce complex
poles.
What I have in mind (well, it's actually built, with 13700's, since a
few years), is using *real* poles and pole / zero pairs only, plus
one overall feedback loop. Not as universal as the SVF blocks,
but producing very "musical" results with minimum number of
parameters / front panel knobs. The common phaser circuits,
and the Moog filter are the two opposite end points of this
topology, of course. Now make each stage not fixed to
all pass or low pass, but switchable. And don't fix the poles
(of the open loop) to constant values, but introduce a pole
spread parameter.
There's *plenty* of useful configurations in between Phaser and
Polygon Lowpass Filter. Variable slope LP (Diode ladder ...),
and spread pole Phasers are possible, but the mix of LP
and AP stages are most rewarding.
My current module has two disadvantages: rather noisy,
and resonance tends to break into loud screams (critical loop
gain being different for different pole locations, and for 
mixed AP / LP configurations.
The first one is the reason why I look for high quality VCAs.
The resonance loop should get a Vactrol limiter, for example.

About the overall concept (which is two Electronotes ideas
combined), there should be a lot in the archives, from the time
when I build this module.

JH.