What Next?249Experiment 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: 1.220uF100uF220uFFigure 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.