Doepfer A-106-1 Xtreme Filter

Module A-106-1 has it's origin in our experiments to built a MS20 filter clone. The module is not exactly the same as the MS20 filter but similar. 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, look at the frequency response curves at the bottom of this decoment for details)

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, look at the frequency response curves at the bottom of this decoment for details)
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.