Coils and caps

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Yesterday we talked of the original AR-1 two-way loudspeaker with its tweeter and woofer. The tweeter is fed only higher frequencies while the woofer, everything else. An electronic circuit called a crossover divides the music into separate outputs to feed the drivers. Here's a schematic for the AR-1: AR-1-schematic-1 You'll note the drawing has two sections. The upper is the actual schematic with its parts and values. Below that, labeled "simplified diagram", is the simplified circuit. Look at the simplified drawing. Note the woofer section first. I have labeled the curly wire separating the woofer from the input as "woofer coil". Now, look above for the same terminology. The lower diagram helps explain the more complex upper drawing. So today, let's discuss what's happening with this curly wire we refer to as a coil. What we want to do with the woofer is produce all the low frequencies, but as the music goes higher in pitch, we'd like reduce its volume and let the tweeter takeover. This is a perfect task for a coil of wire. Coils are exactly what they sound like. A literal coil of wire. Here's a picture of a woofer coil. Woofer-coil The red of this wire is a type of varnish (paint) that acts as an insulator so the wires are electrically isolated from each other. In the middle you see layers of steel, which help to make this more efficient. Place this coil between the output of the power amplifier and one terminal of the woofer, and highs will be blocked, low frequencies pass without restriction. Coils become magnets when presented with AC. At low frequencies, or DC, they act as a wire. As frequencies go up, a magnetic field is generated. The energy to generate the field no longer travels to the other end of the wire (where the woofer is). Instead, the amplifier power is diverted from the woofer, and retasked to make a magnet. Thus, the woofer makes less sound as the frequency goes up, the coil makes a larger magnetic field. One is traded for the other.
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Paul McGowan

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