Doepfer - A-106-1: Xtreme Lowpass / Highpass Filter

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Manufacturer Description
:

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 lowpass
and a 6db high pass filter connected in series both with a very special
design (the ms20 highpass if very often described as 12db high pass, but
this is not true). during our researches we found a way to use the same
circuit simultaneously as lowpass and highpass for 2 different audio
signals (a bit similar to the a-101-1 steiner vactrol filter that has
even different audio inputs available, but with the special ms20
circuit). for this two separate audio inputs for lowpass (lp) and
highpass (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 polarized
input. 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 notch (+) and bandpass (-) filter functions too. from
our point of view this is the most flexible solution that enables e.g.
these functions:


* lowpass: 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

* highpass: 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)

* lowpass/highpass 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.

* 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
notch filter (the "~" indicates that the notch is far from beeing
perfect, the attenuation in the passband is not as good as for other
filters of the a-100 system)

* 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 bandpass filter (the "~" indicates that even the bandpass is
far from beeing perfect, there is a significant feedthrough of
frequencies below and above the center frequency.

* remark for settings 1 and 2: the original ms20 circuit was not planned
for notch or bandpass applications. the ~notch and ~bandpass filters
should be treated as a free bonus and have the disadvantages mentioned
above. the reason is that the lowpass has a 12db/octave slope and the
highpass 6db/octave. this leads to phase relations that do not allow a
"perfect" bandpass 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 bandpasses other a-100 filters
should be used - or two a-106-1 patched in series (bandpass) or parallel
(notch) with suitable frequency settings.

* lowpass and highpass with two different audio signals: the two audio
signals are fed to the lp input resp. hp input, 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 there is no fixed
phase correlation between the two signals (e.g. two different vcos). in
the first case the 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 that are not available for any other filter we know.



during the a-106-1 development we found also that it might be useful to
add controls not available in the original ms20 filters. in the original
circuit the filter output level is limited to about +/- 0.7v by two
antiparallel diodes across the output/resonance amplifier. by chance we
discovered that removing one or both diodes leads to noticeable
different behaviour of the filter. moreover we added two rotary controls
cl+ and cl- to adjust the effect of each limiting diode (from original
ms20 behaviour, i.e. fully active limiting diodes, to no limiting
effect). the independent control for each diode allows asymmetrical
limiting/amplification that causes a completely new and sometimes very
strange behaviour. one of the main effects of the asymmetrical limiting
is that in self-oscillation the filter does not generate a sine wave but
short pulses if only one of the limiting diodes is activated. another
effect is that a higher output of the filter can be obtained (limited to
about +/- 0.7v for the original ms20 circuit). in addition dirty noise
effects appear at certain combinations of the control settings for
resonance, cl+, cl- and input level. the controls cl+, cl-, resonance,
lp level and hp level have be looked at always in a common context: if
the input levels are small the cl+ and cl- controls will have no effect
as the signal does not distort at all. increasing the resonance also
increases the audio level and the cl+/cl- controls may now have an
effect without changing the input level! same applies if the resonance
control remains unchanged but the input level is increases. now the cl+
or cl- control will have an effect as the level reaches the clipping
thresholds. increasing the audio level does also may suppress the
resonance if distortion becomes extreme. the "teamwork" of the five
controls is very complex and has to be learned by doing and hearing



the audio inputs are very sensitive to allow even extrem distortion effects, much more than possible for the original ms20.



we also expanded the module by an insert option for the resonance
feedback loop. this allows to insert other a-100 modules into the
resonance circuit. the standard application is to insert a vca for
voltage controlled resonance. but even other modules - e.g. waveshaper,
divider, phaser, distortion, pll, wave multiplier, spring reverb, ring
modulator, frequency shifter ony any other audio processing module - can
be inserted to obtain sounds you have never heard before.



on top of this the module is equipped with two frequency cv inputs. one
is carried out as a polarizer. this means that effect of the external cv
(e.g. envelope from an adsr a-140 or a-141) to the filter frequency is
positive (+ range) or negative (- range). especially when the filter is
moved from lp to hp it might be useful to invert the polarity of the
envelope cv. we also have to mention that the frequency response if far
from 1v/oct but rather non-linear.



to obtain the filter section of the original ms20 two a-106-1 have to be
patched in series (one in lp mode, the other in hp mode, both with cl+
and cl- set to zero).



it has to be pointed out that the a-106-1 is far away from beeing a
"perfect" filter in an academic sense: the control scale is non-linear.
with self-oscillation all sorts of waveforms except sine are generated.
with high distortion and resonance settings a lot of
roaring/rattling/noise or other unpredictible sounds may appear. high
distortion/level may "kill" the resonance at certain settings. the
filter has a significant control voltage feedthrough. the "bandpass" is
not a real band pass as a considerable share of all frequencies passes
through. the notch filter does attenuate only about 50% at the center
frequency - and many more. but the a-106-1 has a lot of character - much
more than any other filter of the a-100. it is a very strange and
awesome filter - somehow exactly the contrast to the a-108, which is a
very smooth, warm and predictable filter. the a-106-1 is definitely not
the right choice for "moogish" and "civilized" sounds but for extreme,
exceptional and experimental sounds - this is why we call the module
"x-filter" to avoid troubles with the korg company who is the owner of
the term "ms20". if you want to know more technical details please look
at the a-101-1 technical details. in this document the basics of the
a-101-1 (steiner) and a-106-1 (ms20) filters are described.



included for the hp input control can be chosen if the
polarizer is active or if a normal attenuator is available. for some
applications it might be useful to have the level controls of lp and lp
work in the same way (e.g. to obtain the same level for both inputs).
with a jumper on the pc board the type of hp level control can be
chosen. the cl+ and cl- parameters are prepared to be controlled by
external vactrols. two pin headers are used to establish a connection to
the universal vactrol module that is still under development. this will
allow voltage control of the cl+ and cl- parameter.



application ideas:

* using the vc panning module a-134 for voltage control of filter type
(audio input -> audio in a-134, left output a-134 -> lp input of
a-106-1, right output a-134 -> hp input a-106-1, cv of a-134 controls
a-106-1 filter type)

* voltage controlled polarizer a-133 instead of manual control of cv2 or hp level

* different external audio processing modules for the resonance feedback loop.



this module has a maximum current draw of 30ma. it requires 14 te/hp worth of space to fit in a eurorack frame.

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