In trying to simplify or overstate the capabilities of their amplifiers, some manufacturers have blurred the definition of the word "crossover." Let's see if we can clarify the situation.
Part of the confusion comes from the word "crossover" itself. Things would be simpler if crossovers were called splitters. That's what crossovers do: they split signals into two (or more) parts.

A crossover is called a crossover because it can:
1. Take an input signal that changes in frequency,
2. filter the signal (with high-pass and low-pass filters),
3. so the signal moves ("crosses over") from one output to the other as its frequency changes

As the signal rises or falls in frequency, it crosses over from one output to the other as you might cross a road from one side to the other.

Therefore, by definition, a crossover must contain at least:
* a low-pass filter that lets low frequencies pass through and blocks high-frequencies and
* a high-pass filter that lets high frquencies pass through and blocks low frequencies and
* outputs for both the lows and the highs

Some amps claim to have crossovers when they only have a low-pass filter to restrict their output range to low frequencies (for driving subwoofers). Others can switch their filtering from low pass to high pass - but they are NOT crossovers because you can't use both filters at the same time.
However, calling a low-ass or high-pass filter a "crossover" is like calling a unicycle a "bicycle." It's just not right.
(One excuse for calling a high-pass or low-pass filter a "crossover" is that calling them "filters" could confuse people who think that all filters are noise filters. This has led to "one-way crossovers." which are as impossible as forks in the road that only go one way.)

Crossover Parameters
Each crossover or filter has two major parameters:
* The cut-off frequency is the frequency at which the output has dropped by 3dB (half power) from its in-band level on its way down.
* The slope is the rate of the output-level drop beyond the cut-off frequency expressed in dB per octave. An octave is a ratio of two in a frequency. A simple single-component filter has a slope of 6 dB per octave. If it is a low-pass filter with a cut-off frequency of 100Hz, it will have a flat frequency response from about 20 to 80Hz. Its output will be down by 3dB at 100Hz and another 6dB (9 total) at 200Hz, 6dB more at 400Hz (15 total), and so on.

Slopes of 12 and 18 dB per octave (which drop twice and three times as sharply) are more typical in active (electronic) crossovers. Usually, higher slopes are better.

Active And Passive
Active filters and crossovers are built with amplifying devices (transistors, ICs, or tubes) and small components. They operate at line or preamp levels of about a volt and high impedances of about 10,000 ohms. There are also a few passive high-impedance filters available that are made to insert in line with the amplifier inputs.

Passive speaker crossovers use large inductors and/or capacitors since they operate at speaker levels of up to 100 volts and low impedances of about 4 ohms. Your two-way front speakers (separate woofer and tweeter) come with passive crossovers.

- by Joel Cohen (Car Audio And Electronics - May 1996)

Post edited by: Jeff, at: 2007/08/30 01:08