Make Electronics

What Next? 253

Experiment 29: Filtering Frequencies

Crossover Networks

In a traditional audio system, each speaker cabinet contains two drivers—one
of them a small speaker called a tweeter, which reproduces high frequencies,
the other a large speaker known as a woofer, which reproduces low frequen-
cies. (Modern systems often remove the woofer and place it in a separate box
of its own that can be positioned almost anywhere, because the human ear
has difficulty sensing the direction of low-frequency sounds.)

The schematic that you just looked at and may have constructed is known as
a “crossover network,” and truly hardcore audiophiles have been known make
their own (especially for use in car systems) to go with speakers of their choice
in cabinets that they design and build themselves.

If you want to make a crossover network, you should use high-quality poly-
ester capacitors (which have no polarity, last longer than electrolytics, and are
better made) and a coil that has the right number of turns of wire and is the
right size to cut high frequencies at the appropriate point. Figure 5-44 shows
a polyester capacitor.

Figure 5-45 shows an audio crossover coil that I bought on eBay for $6. I was
curious to find out what was inside it, so I bought two of them, and took one
apart.

First I peeled away the black vinyl tape that enclosed the coil. Inside was some
typical magnet wire—copper wire thinly coated with shellac or semitransparent
plastic, as shown in Figure 5-46. I unwound the wire and counted the number
of turns. Then I measured the length of the wire, and finally used a micrometer
to measure the diameter of the wire, after which I checked online to find a con-
version from the diameter in mils (1/1,000 of an inch) to American wire gauge.

As for the spool, it was plain plastic with an air core—no iron or ferrite rod in
the center. Figure 5-47 shows the spool and the wire.

Figure 5-44. Some nonelec-
trolytic capacitors have
no polarity, such as this
high-quality polyester film
capacitor. However, they tend
to be much more expensive,
and are hard to find in values
higher than 10 μF.

Figure 5-45. What exotic

components may we find

inside this high-end audio

component that’s used with

a subwoofer to block high

frequencies?

Figure 5-46. The black tape is

removed, revealing a coil of

magnet wire.

Figure 5-47. The audio crossover coil

consists of a plastic spool and some

wire. Nothing more.