Specifications

186
Car Audio Reference Guide
Low-Pass filter
Not surprisingly, a low-pass filter allows only
those frequencies below the selected crossover
point to pass un-attenuated. Low-Pass filters are
typically used with subwoofers, as you don't want
mids and highs to be reproduced.
In car audio, we use an inductor (also called a
choke) as a low-pass filter. The formula above
allows you to calculate the appropriate inductor
value. For most single 4 Ohm subwoofers, a 6.0
or 6.6mH inductor works well.
Passive Crossover Wiring - Mixed mode
Combinations of high- and low-pass filters can
be combined amongst different speakers. This
is commonly used when wiring mixed-mode
systems. The illustration below shows the wir-
ing drawing for a pair of 6x9 speakers and a
subwoofer on a single two channel amplifier. The
subwoofer would be connected to the bridged
terminals and the 6x9s would be wired in stereo.
Using the crossover information provided previ-
ously, we would use a 250uF capacitor on each
6x9 and a 6.6mH inductor on the subwoofer.
In application, wether the passive component is
installed on the positive wire or the negative, it will
have no effect on the performance of the circuit.
However, it is common to wire all speakers in the
same fashion for simplicity and to make trouble-
shooting and adjustments later on.