DOEPFER / A-106-1 MS20 Type VCF / Xtreme

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Variations

This module was manufactured by German synthesizer manufacturer Doepfer and was designed based on the filter section of the MS-20.

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The A-106-1 was designed based on our experience building a clone of the KORG MS-20 filter section. The famous original MS-20 features two specially designed filters connected in series: a 12dB low-pass filter and a 6dB high-pass filter. (The MS-20's high-pass filter is sometimes described as 12dB, but this is incorrect.)

During our circuit research, we discovered that applying different audio signals to the low-pass and high-pass filters within the same circuit produced very interesting results. (This is similar to the A-101-1, but the sound is significantly different from the MS-20's special circuit.) For this reason, the A-106-1 provides audio inputs with independent level controls for the low-pass (LP) and high-pass (HP) filters. When nothing is plugged into the HP input, the signal input to the LP is also connected to the HP. When a plug is inserted into the HP input, the LP signal is decoupled, and different signals are input to each. The HP input level can be controlled in both positive and negative directions. This means that the HP signal can be added to the LP signal in either positive (+) or negative (-) polarity. This function makes it possible to create notch (+) or bandpass (+) filter characteristics. From our perspective, this method is the most flexible way to achieve the following functions.

■ Low-pass filter:

Input the audio signal to LP In, set the HP Level control to zero, and the LP Level control to the desired level.

■ High-pass filter:

Input the audio signal to LP In or HP In, set the LP Level control to zero, and the HP Level control to the desired level (for this setting only, it makes no difference whether the HP Level polarity is set in either direction).

■ Mix low-pass/high-pass with one audio signal:

Input the audio signal to LP In, and set the HP Level control and LP Level control to the desired levels.

○Setting Example 1: When the LP/HP Level controls are set to the same level with the same polarity (HP Level = +) and no distortion or slight distortion, the result is a ~ notch filter. (The notch filter generated here is different from the notch filters in other A-100 systems and is not considered perfect, hence the ""~"" prefix.)
○Setting Example 2: When the LP/HP Level controls are set to the opposite polarity (HP Level = -) and the same level with no distortion or slight distortion, the result is a ~ bandpass filter. (The bandpass filter generated here is different from the bandpass filters in other A-100 systems and is not considered perfect, hence the ""~"" prefix.)
○Note for Setting Examples 1/2: The original MS-20 circuit was not designed for use in notch/bandpass applications. The ~ notch and ~ bandpass filters should be considered merely an added bonus with the drawbacks mentioned above. The reason for these imperfect characteristics is that the lowpass and highpass curves are different, at 12dB and 6dB, respectively. This means that a ""perfect"" notch/bandpass filter cannot be created by simply adding/subtracting them, as with other filters (a 90-degree phase shift will remain). To achieve a better notch/bandpass filter, you need to use another A-100 filter, or connect two A-106-1s in series (bandpass) or parallel (notch) and adjust them to the appropriate response/cutoff frequency settings.

■ Inputting different audio signals to the lowpass/highpass:

Input two audio signals to the LP In/HP In, respectively, and set the HP Level and LP Level controls to the desired levels. The HP Level +/- control works differently when the two audio input signals are phase-aligned (such as different waveforms from the same VCO or signals divided by a divider one octave apart) versus unrelated phases (such as two different VCOs). In the former case, different filter characteristics are obtained in the +/- direction. In the latter case, there is no difference in sound between the two directions.

This design also creates an additional special function. For example, you can mix the low-pass filter signal undistorted while the high-pass filter signal is distorted (or vice versa) in in-phase or out-of-phase. To do this, set the LP Level control to a low value where the signal does not distort, and then set the HP Level control to a higher value (in both directions) to distort the signal. By combining these settings, you can create a variety of unique characteristics and sounds that cannot be achieved anywhere else.

While analyzing the original MS-20 during the development of this module, we discovered something interesting. The original MS-20 had one diode on each side of the filter's output stage, which acted as a limiter, limiting the maximum output to a range of approximately ±0.7V. By removing one or both of these diodes, we found that the filter behaved in a very interesting way. The absence of the limiter resulted in increased output level, resonance, and a unique distortion due to asymmetric amplification, resulting in sounds not possible with the original MS-20.

The A-106-1 allows this diode to be controlled with two knobs, CL+ and CL-, allowing for flexible control from the normal MS-20 to extreme operation. When the CL+/CL- values are changed to extreme values, the asymmetrical amplification occurs, causing the resonance to self-oscillate, producing a unique short pulse wave rather than a sine wave. Furthermore, certain combinations of CL+/CL- and input level produce dirty, noisy sounds. Each control is closely related to the others, allowing for infinite sonic variation, a major feature of this unit.

The resonance insert jack can also be used in a variety of ways. The filter's resonance refers to the amount of filter output fed back to the input. Inserting a VCA into this path allows for CV control of the resonance, and inserting various sound processing modules allows for unusual resonance behavior.

Width 14HP
Depth 40mm
Current Consumption +12V: 30mA
-12V: 20mA