Ashly Electronic Amplifier none User Manual
Page 23
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Beyond
these
basic
classifications
of
filter
types,
the
audio
designer
is
concerned with the details of filter performance near and within the cutoff or
stopband. In other words, we'll assume that a filter is fairly linear within
its pass-band
(for example, the flat or "plateau" portion of the low-pass
response shown in figure 22(a), but how does it behave as frequencies approach
the cutoff frequency? And, once the filter is operating in its cutoff range,
how quickly does it attenuate those undesired frequencies? Filters are not
brick walls; they will always pass frequencies outside their pass band to some
extent.
SLOPE
The
rate
at
which
a
filter
attenuates
frequencies
outside
its
passband
is
known as the slope, and is generally expressed in decibels of change per
octave. For example, if a low-pass filter has a 12dB/octave slope, then any
frequencies outside its pass band will be reduced in volume by an additional
12dB for each octave above the cutoff frequency. If the low-pass filter has a
cutoff frequency of IkHz, then a 2kHz signal can be expected to be reduced in
volume by 12dB. Likewise, a 4kHz signal will be reduced by 24dB, and an 8kHz
signal will be 36dB down.
The
slope
of
a
filter
is
a
function
of
the
number
of
frequency-reactive
components within the filter. Avery gentle slope, such as would be found in
a 6dB/octave passive filter, might contain only one such component in the form
of a capacitor connected from the audio to ground, as shown in figure 23. A
capacitor
passes
high
frequencies
very
well
while
blocking
low
frequencies.
Therefore, any high frequency audio will be shorted to ground and not heard at
the
output.
This
type
of
filter
is
called
a
first-order
filter.
It
is
not
terribly useful for crossovers because of its gentle slope; it won't provide
much protection to high-frequency drivers when used in its high-pass version.
(a)
(b)
Figure 23 Passive RC first order low-pass filter and response.
High frequency loudspeakers will be better protected by a steeper slope, which
can be obtained with a second-order filter such as the passive LC second-order
filter shown in figure 24. Here, there are two frequency-reactive components:
a capacitor (C), which easily passes high frequencies, and an inductor (L),
which easily passes low frequencies. The slope will be 12dB/octave.
22