AW: VC Phaser using the LM13600

[Michael B. Irwin]

Hello,
There should be no difference in the sound of a phaser using either
leading or lagging stages.
Consider a phaser made up of 4 leading (or lagging) simple all-pass
stages in series, with an output summing stage where the phase-delayed
signal is summed with the unmodified input signal. Assume the input
signal phase is zero degrees.  A reinforcement peak will be present at
the turnover freq ( total shift=360 degrees with a 90 degree
contribution from each stage). This is the case for both leading and
lagging stages. Two notches are present. If the turnover freq is
normalized to 1 the notches are at the following freqs :

    0.414 (4 x 45 degrees) and 2.414 (4 x 135 degrees) for leading and
lagging stages.

For the lagging stage case  the 4 x 45 degree notch is at 0.414 x
turnover freq while for the leading stage case  the 4 x 45 degree notch
is at 2.414 x turnover freq. The same inverse relationship applies to
the 4 x 135 degree notches. Both cases produce identical amplitude
responses (theoretically). I don't know how they compare in practice,
but simulations using Electronics Workbench show identical results when
the response is displayed with the Bode plotter.