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Experiment 29: Filtering Frequencies

Experiment 29: Filtering Frequencies

In this experiment, you’ll see how self-inductance and capacitance can be
used to filter audio frequencies. You’re going to build a crossover network: a
simple circuit that sends low frequencies to one place and high frequencies
to another.

You will need:

• Loudspeaker, 8Ω, 5 inches in diameter. Quantity: 1. Figure 5-35 shows a
  typical example.


• Audio amplifier, STMicroelectronics TEA2025B or similar. Quantity: 1. See
  Figure 5-36.

Figure 5-35. To hear the effects of audio
filters using coils and capacitors, you’ll
need a loudspeaker capable of reproduc-
ing lower frequencies. This 5-inch model is
the minimum required.

Figure 5-36. This single chip contains a

stereo amplifier capable of delivering a

total of 5 watts into an 8Ω speaker when

the two channels are combined.

Figure 5-37. A nonpolarized electrolytic

capacitor, also known as a bipolar capaci-

tor, looks just like an electrolytic capaci-

tor, except that it will have “NP” or “BP”

printed on it.

• Nonpolarized electrolytic capacitors (also known as bipolar). 47 μF. Quan-
  tity: 2. A sample is shown in Figure 5-37. They should have “NP” or “BP”
  printed on them to indicate “nonpolarized” or “bipolar.”


• Nonpolarized electrolytic capacitors (also known as bipolar). 100 μF.
  Quantity: 5. (Because you’ll be working with audio signals that alternate
  between positive and negative, you can’t use the usual polarized electro-
  lytic capacitors. If you want to avoid the trouble and expense of ordering
  nonpolarized capacitors, you can substitute two regular electrolytics in
  series, facing in opposite directions, with their negative sides joined in the
  middle. Just remember that when you put capacitors in series, their to-
  tal capacitance is half that of each individual component. Therefore, you
  would need two 220 μF electrolytics in series to create 110 μF of capaci-
  tance. See Figure 5-38.)


• Potentiometer, with audio taper if possible. 100K. Quantity: 1.


• Coil, for crossover network. Quantity: 1. You can search a source such
  as eBay for keywords “crossover” and “coil,” but if you can’t find one at a
  reasonable price, you can make do with a spool of 100 feet of 20-gauge
  hookup wire.


• Plastic shoebox. Quantity: 1.

220uF

100uF

220uF

Figure 5-38. You can make a nonpolar-

ized electrolytic capacitor by putting two

regular electrolytics in series. (In fact,

that’s what you’d find if you opened a real

nonpolarized capacitor.) The symbol at the

bottom is roughly equivalent to the pair of

symbols at the top; bear in mind that two

capacitors in series have a total capaci-

tance that is half that of each of them.