Full blown crossovers

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One of my readers asked a great question. "You've described a simple low pass/high pass filter for a speaker's crossover, but isn't a real full blown crossover a lot more complex?"

A "full blown" crossover of a 2-way is usually both a high pass and low pass filter together, just as I described in my post. There are other variations on the simple crossover: one is to add a second high pass filter (read capacitor) to the woofer so it doesn't go down too low. Some crossovers increase the slope of their cutoff, from a simple 6dB/octave to perhaps something steeper like a 12 or even 24dB/octave, but in general they use a 2-pole filter for the woofer (inductor and capacitor) and a 1-pole filter for the tweeter (nothing more than a single capacitor).

To make a low pass filter for a woofer, you add an inductor (coil of wire), in series with the woofer. To make it a 2-pole filter, you add a capacitor at the junction of the inductor and the woofer to ground. 2 parts. If you want to make a sloppier version of this classic 2-pole filter, place a resistor in series with the capacitor. This gives you a "stop resistor" as it's known, but essentially it just slightly modifies the slope to match the woofer. Wilson wattA classic example is Dave Wilson's original Watt 2-way loudspeaker, one of the better sounding 2-ways ever built. It has just such an arrangement of an inductor in series with the woofer and a cap and resistor across the woofer terminals. The tweeter's just a capacitor. Here's the thing: the skill of designing a crossover is in matching the drivers together to create a single sound. Imagine the difficulty of making one, coherent sound, right at the point where the two drivers crossover. The woofer is sooooo different in construction than the tweeter, that at the point where both drivers are playing together, it rarely sounds the same as when they are playing alone. That's a very treacherous point in musical reproduction and it takes extraordinary skill to design a speaker to sound "seamless" at the meeting place for both drivers. The networks that divide the sound up may seem simple. Choosing the right drivers and blending them together is anything BUT simple.
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Paul McGowan

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