Variable Slope Filter / Phaser

[Haible Juergen]

	>   Haven't tried staggered poles yet, but it's easy to try it
on the
	>   weekend, as I have a variable slope filter/phaser, so I can
	>   simulate staggered capacitor values with one control
voltage.
	>
	>Woah... tell us more about this!

That's an old one, one of the first modules for JH-3.

	>I would guess it's a series of OTA phase shift stages where a
control
	>voltage difference can be split accross all of them.

Six stages of lowpass / allpass circuit built around LM13700's.
(The caps are switchable between input and GND in three pairs,
with three 2P2T switches, so you can get a mix of LP and phaser,
with the extremes 6pole LPF and 6stage phaser.)
Each filter stage is controlled by it's own (very simple) expo
converter, and there are two paths of CV routing (each with a
manual control, and two CV inputs). One CV path goes to the 
bases of the 6 expo transistors with equal resistor values,
the other one uses different resistor values for each stage.
So the first CV path will shift all poles with the same amount, and
the second CV path will detune the poles.

	>How would you describe the sound a fixed frequency phase
shifter with
	>the slope varied with an LFO?

Honestly, not very exciting. I have used it from time to time as
an additional little bit of modulation, but it's not a very impressive
effect by itself. It's more effective in LPF mode (and it will alter
the amount of resonance as well, in LPF mode).

The very first phaser I built used staggered capacitors. (It
was inspired by some elektor article, and back then I had no 
idea that most commercial phasers use identical poles for all 
stages.) I still use this as some unobstrusive kind of stereo
spreader. It's much less up-front than an "ordinary" phaser.
Both options have their own use. It's just not that interesting
to modulate between these extremes.

JH.