Specifications

185
Car Audio Reference Guide
Passive Crossovers
When connecting multiple speakers to an ampli-
fier, passive crossover components are often
required to filter the signal going to each speaker.
For example, if you connect a midrange and a
tweeter, you don't want bass to go to the tweeter
and you don't want highs to go to the midrange.
Similarly, if you bridge a woofer to a two channel
amp and a set of full-range speakers, you only
want bass to be heard from the woofer and only
mids and highs from the full-range speakers. This
section will describe how to calculate the appro-
priate component (capacitor or inductor) values
to build these systems.
Nomenclature
Capacitors have two ratings - a capacitance value
and a voltage. The amount of energy storage in a
capacitor is measured in Farads. Since the Farad
is a very large unit, most capacitors, especially
those used in filter circuits are measured in 1 mil-
lionth of a farad, or a microfarad. The greek letter
mu (µ) is used to describe micro:
1µF = 1/1,000,000 of a Farad and is pronounced
1 microFarad.
Common values for filter capacitors range
between 1µF and 250µF
Inductors are rated in Henries. Again, this is a very
large unit of measurement, and in filter networks,
we use 1 thousanth of a Henry or a milliHenry.
1mH = 1/1000 of a Henry and is pronounced 1
milliHenry
High-Pass filters
A high-pass filter allows only frequencies higher
than the selected crossover point to pass through
the circuit. We use non-polarized capacitors
wired in series with the load to create these filter
networks. The equation above outlines how to
calculate the correct component value. The chart
below outlines common capacitor values for
common speakers sizes in a 4 Ohm impednace.
Load impedance is ciritcally important to passive
crossover component value. If you change the
impedance, you need to recalculate the compo-
nent value.
Speaker Size Capacitor
6x9, 6x8 250 µF
6.5, 5x7 200 µF
5.25 150 µF
4, 4x6 100 µF
3.5 47µF
Tweeter 3.3µF