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A-105 / A-122
A-121 multi-mode filter's 12 dB) with voltage control-
led resonance.
To create a bandpass filter, put both modules in
series (see Fig. 3). The band-width is governed by the
difference between the cut-off frequencies of the two
filters f
(A-122) and f
L
H
is half way between the two: f
A-122
Freq.
Fig. 3: 24 dB band pass filter (resonance = 0)
A notch filter is created by putting both modules in
parallel, and controlling their outputs with an A-138
mixer (see Fig. 4). The band-width and middle fre-
quency are determined by the same factors as in the
bandpass.
When modulating these 'construction kit' filter types,
certain rules apply:-
6
System A - 100
(A-123); the middle frequency
= (f
+ f
) / 2.
M
L
H
A -123
Freq.
f L
Freq.
Freq.
f
Fig. 4: 24 dB notch filter (resonance = 0)
M
• To maintain the exact bandwidth, the cut-off
frequencies of both filters must be modulated by
the same amount.
• If you modulate the cut-off frequency of just one of
the filters, or both of them by different amounts, or
different modulators, the bandwidth and middle
frequency will themselves be modulated.
• At a resonance setting of greater than zero, or
when the resonance is modulated, the middle fre-
quency will be skewed. With different resonance
settings or modulation of each filter, this will have
the same result.
Freq.
f
L
A-122
A-138
A-123
F re q .
f H
doepfer
Freq.
f
M