Doepfer A-106-1 Xtreme Lowpass/Highpass Filter - Manual (Page 2)

X-treme Filter A-106-1 System A-100 DOEPFER DOEPFER DOEPFER DOEPFER
2
2. Basic Principles
As mentioned in the introduction, module A-106-1 has it's
origin in our experiments to built a MS20 filter clone. The
famous original MS20 included two filters: a 12 dB low pass
and a 6dB high pass filter connected in series, both with a
very special design.
Remark: The MS20 high pass if very often described as
12dB high pass, but this is not true.
During our research we found a way to use the same circuit
simultaneously as low pass and high pass for 2 different
audio signals (a bit similar to the A-101-1 Steiner Vactrol
filter, which also has different audio inputs available, but
with the special MS20 circuit). For this two separate audio
inputs for low pass (LP) and high pass (HP) with separate
level controls are available. The sockets are normalled, i.e.
the signal applied to the LP input is available for the HP
input too, provided that no plug is inserted into the HP input
socket. The level control of the HP input is realized as a
polarizer. This means that the signal can be added with the
same polarity (+ range) or opposite polarity (- range)
compared to the LP input. This feature enables pseudo
notch (+) and pseudo band pass (-) filter functions too. From
our point of view this is the most flexible solution as it
enables these functions:
• Low pass: the audio signal is fed to the LP input, HP
level control is set to zero, LP level control is set to the
desired level
• High pass: the audio signal is fed to the LP or HP input,
LP level control is set to zero, HP level control is set to
the desired level (in this special case it does not matter
if positive or negative amplification is chosen with the
polarizer control)
• Low pass / high pass mix with one audio signal: the
audio signal is fed to the LP input, LP and HP level
controls are set to the desired levels.
o special setting 1: if the level controls for LP and HP
are set in a way that both levels are identical with
the same polarity (i.e. + range of the HP level
control) and no or little distortion only one obtains a
pseudo notch filter (pseudo means that the notch is
far away from being perfect, the attenuation in the
pass band is not as good as for other filters of the
A-100 system, please refer to the frequency
response curves for details)
o special setting 2: if the level controls for LP and HP
are set in a way that both levels are identical with
the opposite polarity (i.e. - range of the HP level
control) and no or little distortion only, one obtains a
pseudo band pass filter (pseudo means that even
the band pass is far away from being perfect, there
is a significant feedthrough of frequencies below
and above the center frequency, please refer to the
frequency response curves for details)