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Doepfer Musikelektonik logo

Analogue  Modular  System  A-100







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Owners Manual

Xtreme “MS20” Voltage Controlled Filter (multi mode)

The A-106.1 filter is based on the Korg MS20 design which has a 12dB/octave voltage controlled low pass filter and a 6dB/octave voltage controlled high pass filter. In addition to the original MS20 design, the A-106.1 has asymmetrical limiting controls (CL+ and CL-) which gives unusual instability to the filter. There is also an audio insert point for the resonance feedback loop. The two filters are internally connected in series for Low Pass and High Pass, but unlike the original MS20 design, bandpass and notch are available. The audio input is very sensitive and so can be easily overdriven.

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  LP and HP are set so that both levels are identical with the same polarity a (slightly imperfect) notch filter is achieved.
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 a (slightly imperfect) bandpass filter is achieved.
Remark for settings 1 and 2: The original MS20 circuit was not planned for notch or bandpass applications. Due to the 12dB  lowpass and 6dB highpass slopes, this leads to phase relations that do not allow a "perfect" bandpass and notch simply by adding/subtracting signals,  but does produce a new type of filter effect.

The variety of controls allows a lot of functions that are not available for any other filter, one of the main effects of the asymmetrical limiting is that in self-oscillation the filter does not always generate a sine wave but short pulses.   The audio inputs are very sensitive to allow extreme distortion effects, also the A-106.1 has an insert option for the resonance feedback loop, which allows processing of the resonance feedback loop.  It is a very strange and awesome filter  - this is why it is called the module "Xtreme-Filter"







Doepfer A-106.1