Ashly Electronic Amplifier none - Manual (Page 24)

Passive LC 2nd order low-
pass filter and response.
RESPOIJSE SHAPE
The performance of the filter in the immediate vicinity of the cutoff
frequency is important to consider, as it will affect the way a filter sounds
and the way in which its response combines with another filter in a multiple-
filter
system. Some filters begin to attenuate well before the cutoff
frequency, while some remain essentially flat up to cutoff before abruptly
beginning to attenuate. Some filters even exhibit an increase in amplitude as
they approach the cutoff ooint, and then quickly reverse themselves and begin
to attenuate.
These various response characteristics each has an application, and some of
the most common ones have been given names, including Butterworth, Bessel,
Chebyshev, Cauer, and Elliptical. The Butterworth response shape is very
popular in conventional crossovers. Also called a "maximally flat" response,
it stays quite flat within the pass-band and then falls off with a very linear
slope. In actuality, it does not remain perfectly flat right up to the cutoff
frequency, but begins to roll off a little earlier, so that the response is
down 3dB at the cutoff frequency. A response plot of a low-pass 12dB/octave
Butterworth filter is shown in figure 25.
Figure 25 Butterworth response.
23