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Low Frequency Notch Filter (8)
This is a parametric filter with 2 fixed parameters: a fixed cut of 30dB and a fixed Q of 2. You
can select the centre frequency of the low frequency notch filter between 20Hz and 1200Hz
via the rotary control. The purpose of this filter is to quickly remove a feedback frequency.
To be able to quickly find the feedback frequency the Q has been chosen to an intermediate
value. Reducing this Q would make the filter narrower (thus avoiding cutting out too much
musical signal) but it would take too much time for you to find the exact frequency. If you
want or need more accurate notch filters we would recommend you our Apex PE analogue
models who have full parametric filters who enable you to zoom in on the offensive frequency.
Another alternative is our Apex IntelliQ digital system optimiser.
When trying to remove unwanted feedback, start with the low frequency notch filter and then
move on to the high frequency notch filter. Feedback mostly consists of more than one fre-
quency, starting with the low frequency notch filter assures you to find the fundamental fre-
quency first. Since the notch filter have a cut of 30dB and a narrower Q than the graphic filters
they are much more efficient for removing feedback than the graphic section only.
20
15
10
5
dB 0
-5
-10
-15
-20
-25
-30
-35
-40
25 31 40 50 63
80
100 120 160 200 250 315 400 500 630 800 1k 1k25 1k6 2k 2k53k15 4k 5k
Notch filter with centre frequency at 140Hz, attenuation of 30dB and Q = 2
Low Frequency Notch Filter on/off Switch (9)
With this switch you can turn the low frequency notch filter on and off. Standard position is off
(LED out). Push the button if you want to switch the notch filter in the signal path, the LED
will light.
6k3
8k 10k12k5 16k 20k Hz
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