A-106-6

16-Fold Xpander VCF

Module A-106-6 is a multimode filter that is based on the filter circuit of the Oberheim Xpander. The module features 15 different filter types (those filters of the A-107 that were available in the Xpander) with 8 filters available simultaneously. The toggle switch Filter Group is used to switch between 2 filter groups.
Threse filter types are available:

1L (6 dB low pass)

2L (12 dB low pass)

3L (18 dB low pass)

4L (24 dB low pass)

1H (6 dB high pass)

2H (12 dB high pass)

3H (18 dB high pass)

2B (6 dB band pass)

4B (12 dB bandpass)

2N (notch)

3A (allpass)

2H1L (asymmetrical band pass made of a 12 dB high pass and a 6 dB low pass)

3H1L (asymmetrical band pass made of a 18 dB high pass and a 6 dB low pass)

2N1L (combination of notch and 6 dB low pass)

3A1L (combination of allpass and 6 dB low pass)

The module features voltage controlled resonance. For filter group 2 (2L, 4L, 2B ...) even self oscillation is possible.

All standard VCF controls are available: manual filter frequency control Frq, one control voltage input with attenuator (FCV2)and one without attenuator (FCV1, ~ 1 V/octave). In addition voltage controlled resonance with manual control (Q) and a CV input with attenuator (QCV) are available.

The circuit is based on a 24dB lowpass filter. The outputs of the four internal filter stages (i.e. the 6, 12, 18 and 24dB outputs) are mixed together with different levels and polarities to obtain 15 different filters. Because of this special circuit the outputs have slightly different levels and noise floor. This is caused by the different internal amplifications and numbers of stages that are required to generate the filter in question. If e.g. a filter is derived by one stage only (e.g. the 6 dB, 12dB, 18dB and 24dB low pass) the noise floor is smalles compared to a filter that is derived by a combination of all four filter stages.

http://www.doepfer.de/a1066.htm