Dieter Doepfer explains:
The main difference compared to other filter or phaser
designs is that
the
variable elements used (i.e. light depending
resistors) are so-called
linear
elements. Other filter designs use so-called
non-linear elements (e.g.
the Moog
or Roland/TB303 transistor ladder, the EMS diode
ladder, the FETs in
Phasers or
the OTA's in Oberheim or Wasp filters) that lead to
distortions as the
audio
level increases (sometimes this is desired). In these
filter/phaser
designs the
incoming audio level is normally very much attenuated
(1/100 or even
1/1000)
before it is processed. At the end of the processing
circuit the audio
level is
amplified (e.g. 100 or 1000) to obtain the same level
as the input
signal. For
the vactrol type filters/phasers this is not necessary
due to the
linear
frequency/phasing controlling elements. Most people
describe the vactor
filter/phaser sounds more "smooth" or "soft" or
"mellow". The main
disadvantage
of the vactrol filters/phasers is the slow response as
the LDR (light
depending
resistors) are not very fast (up to 100 milliseconds =
1/10 second to
cover the
whole range). Short ADSR filter attacks are not
possible with these
filters. But
they are good for slowly changing filter or phaser
effects. Even VCA's
can be
made with vactrol elements.
A very well known vactrol phaser is the so-called
"compact phasing"
manufactured
in the eighties. It is famous for its soft and mellow
phasing sound

=====
synthfreak(parallel worlds)
athens-greece
1 group owner-moderator

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