Doepfer A-106-1 Xtreme Lowpass/Highpass Filter - Manual (Page 3)

DOEPFER DOEPFER DOEPFER DOEPFER System A-100 X-treme Filter A-106-1
3
o Remark for settings 1 and 2: The original MS20
circuit was not planned for notch or band pass
applications. The pseudo notch and pseudo band
pass filters should be treated as a free bonus and
have the disadvantages mentioned above. The
reason is that the low pass has a 12dB/octave
slope and the high pass has 6dB/octave. This leads
to phase relations that do not allow a "perfect" band
pass and notch simply by adding/subtracting
signals as for other filter designs (for insiders: there
remains always a 90 degree phase shift). For better
notches and band passes other A-100 filters should
be used - or two A-106-1 patched in series (band
pass) or parallel (notch) with suitable frequency
settings.
• Low pass and high pass with two different audio
signals: the two audio signals are fed to the LP input
resp. HP input and the level controls for LP and HP are
set to the desired levels. For the +/- control of the HP
input it is essential in this case if the two input signals
are phase correlated (e.g. two different outputs of the
same VCO or VCO output and a frequency divided
signal derived from this VCO) or if there is no fixed
phase correlation between the two signals (e.g. two
different VCOs). In the first case the - and + range of
the HP control leads to different filter results. In the
second case there is no difference, if the + or - range of
the HP control is used.
This design allows even some very special functions: It is
e.g. possible to adjust the controls so that the LP signal
does not distort, but the HP share does (or the other way
round) - alternatively with the same or opposite polarity
compared to the LP signal. For this the LP level has to be
set to a small value so that the signal does not distort. The
HP level control has to be set to a higher value (in the + or -
range) so that the HP share will distort.
The variety of controls allows a lot of functions which are not
available for any other filter we know.