Doepfer A-100(~ 40 MB) - Manual (Page 59)

doepfer System A - 100 Modular Vactrol Phaser A-101-3
3
2. Basic principles
The module is made of two identical units that differ
only in the normalling of the sockets (i.e. how the
switching contacts of some sockets are internally
pre-wired). Therefore only one of the two units is
described with reference to fig. 1 on the preceding
page. The stages 1-6 belong to the first unit, 7-12 to
the second one. Some basics concerning the function
of a phaser (frequency response, comb filter) can be
found in the manual of the VC phaser A-125 too.
The vactrol control unit is made of a logarithmic
voltage-to-current converter that converts the sum
of the manual phase shift setting and the external
control voltage into a current. This current is used to
drive the 6 LEDs inside the vactrols of the phase
shift stages and the display LED. The 6 photo resi-
stors inside the vactrols that work as phase shift
controlling elements respond to the illumination inten-
sity and create different phase shifts for different
illuminations.
The (attenuated) audio input signal is fed into the first
phase shift stage (stage 1). The 6 phase shift stages
are daisy-chained. Each stage is equipped with it's
own audio output (stage out) and feedback input.
The sum of the feedback input and the output of the
preceding stage are mixed and fed into the correspon-
ding phase shift stage.
This modular concept allows the realization of dif-
ferent phasers as well as a lot of other filters too as
all inputs and outputs are free patchable. For example
phasers with 2 up to 12 stages, phasers with free
eligible simple or multiple feedback loops or parallel
working phasers are possible. On our web site a lot of
frequency response curves are available that show
which types of phasers and filters are possible with
this modular concept.
The module contains two additional circuits: a mixer
and a polarizer (one for each unit).
The mixer is used to combine the incoming original
audio signal (audio In) with the phase shifted signal
(shifted audio In) with adjustable ratio. The typical
setting for a phaser is 50:50, i.e. both the original and
the shifted signal have the same level. This creates
the typical phaser jet sound that is based on the
comb-like frequency response curve. But even diffe-
rent ratios make sense. If only the phase shifted
signal is used one obtains the so-called phase vi-
brato effect. The socket "Shifted Audio In" is normal-
led to stage output 6. Consequently a 6-stage phaser
is obtained at the socket "Mixed Audio Out" provided