Doepfer A-106-1 Xtreme Lowpass/Highpass Filter User Manual

Page 5

DOEPFER

System A-100

X-treme Filter A-106-1

During the A-106-1 development we found also that it might
be useful to add controls not available on 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. Removing one or both diodes
leads to noticeable different behaviour of the filter. We
added two rotary controls CL+ and CL- to adjust the effect
of each limiting diode (i.e. from original MS20 behaviour with
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.

Fig. 3 : A-106-1 Clipping functions

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 level of the filter can be
obtained (which is 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 to be treated 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 because it does not reach the clipping levels. Increasing
the resonance also increases the audio level and the
CL+/CL- controls may now have an effect on the output
level without changing the input level !

Same applies if the resonance control remains unchanged
while the input level increases. Now the CL+ or CL- control
will have an effect as the level reaches the clipping
thresholds. Increasing the audio level may also 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 extreme
distortion effects, much more than possible for the original
MS20.

The module is equipped with an insert option for the
resonance feedback loop. This allows to insert other A-100