Doepfer A-101-3 User Manual
doepfer
System A - 100
Modular Vactrol Phaser
A-101-3
1. Introduction
Module A-101-3 is a 12 stage phase shifter with vactrols as phase shifting elements. A vactrol is a combina-
tion of a light depending resistor (LDR) and LED both
put into a light-proof case. They are known for their
smooth sound behaviour as vactrols are distortion-free
linear elements in contrast to any semiconductor (like
transistors or OTA in other designs). For more general
details about vactrols please look at the vactrol basics
page on our web site www.doepfer.com.
In contrast to other phaser designs the A-101-3 is much
more flexible and offering a lot of new features not
available from other phasers on the market. The A-1013 offers access to each of the 12 input and output
stages leading to a lot of new filters that cannot be
obtained in other ways. Especially the free patchable
(yes, not only one feedback loop is
feedback loops
possible) between each of the 12 stages, the separate
phase shift control for the stages 1-6 and 7-12, and
the 2 polarizers intended to control the feedback loops
lead to completely new filter types (a polarizer is a
circuit that is able to generate positive and negative
amplifications in the range -1...0...+1 with -1 = inversion,
0 = full attenuation, +1 = unchanged signal, for details
concerning the polarizer function please look at the
A-133 VC Polarizer or A-138c Polarizing Mixer module).
On our web site are lot of frequency response curves
availabe that show which filter types can be realized.
Internally the module is made of 2 independent 6 stage
phase shifters (1-6 reps. 7-12) with separate audio
inputs (with attenuators), audio outputs (with mix con-
trol), and phase shift control units. The phase shift
control units feature both manual and voltage controlled
phase shifting (e.g. from a LFO, ADSR, Random Voltage, Theremin CV, Foot Controller CV ...). For each
sub-module a phase shift display (LED) is available.
The LED shows the illumination state of the 6 vactrols of
the sub-module in question as it is connected in series
with the internal vactrol LEDs.
Each of the 12 phase shift stages is equipped with an
audio output socket and feedback input socket to
obtain full flexibility to create a multitude of different
filters. The audio input signal and the output signals of
stage 6 resp. stage 12 are mixed with 2 manual controls
to obtain effects at two audio outputs (for normal
phase shifting effect this is 50% input signal and 50%
phase shifted signal).
The two submodules are internally connected via nor-
malled sockets so that two 6 stage phase shifters can
be obtained without external patches.
1
A-101-3
Modular Vactrol Phaser
System A - 100
doepfer
Fig.1: A-101-3 Overall view
2
doepfer
System A - 100
Modular Vactrol Phaser
A-101-3
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 intensity 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 corresponding 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 different 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 normalled to stage output 6. Consequently a 6-stage phaser
is obtained at the socket "Mixed Audio Out" provided
3
Loading...