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An active crossover is also referred to as an electronic crossover because it requires power and ground connections in order to do its job. It is installed between your receiver or equalizer and your amplifiers. An active crossover filters and directs the frequency bands in the low level signal before they reach the amplifiers. This saves your amps from doing unnecessary work, making your system more efficient.
Active crossovers are usually adjustable, giving you more flexibility and letting you tweak the sound to meet the needs of your vehicle. They often have other features like bass boost and let you independently control the relative volume of different types of speakers.
A bandpass filter combines a high-pass filter and low-pass filter, allowing a certain band of frequencies to pass while restricting the higher and lower frequencies. Bandpass filters can be fixed in frequency and bandwidth, or variable.
Butterworth filters are high quality, ultra-flat electronic filters which exhibit minimal phase shift when compared to other conventional filters.
A capacitor, or cap, placed in series with a speaker acts as a high-pass filter, allowing only frequencies above the crossover point to pass through.
A coil, or inductor, placed in series with a speaker acts as a low-pass filter, allowing only frequencies below the crossover point to pass through.
In high-pass filters, low-pass filters, and crossovers, the crossover point is the frequency at which the level of the output signal has been reduced by 3 dB.
The rate at which the crossover attenuates the blocked frequencies. Slope is expressed as decibels per octave. A 6 dB/octave crossover reduces power by 6 dB in every octave starting at the crossover point. An octave is double the crossover point when you're going higher in frequency, and 1/2 the crossover point when you're going lower in frequency. (For example, an octave above A440 is 880 Hz, an octave below is 220 Hz.) With slopes of 12 dB and higher, you'll hear little output beyond the crossover point.
A digital crossover does all its filtering in the digital domain, and displays all your options on a menu screen. A digital crossover offers two advantages over the analog version. First, you can set your filters much more precisely, and you'll find much greater flexibility in setting up a complex system. Second, you can store different crossover settings in memory locations, and recall them with the touch of a button — that's a big plus if you're comparing settings or switching settings for a car audio competition.
A high-pass filter allows "high" frequencies to be sent to the speakers, while it blocks frequencies below the crossover point.
Inline crossovers are a hybrid design. They are considered to be passive crossovers because they are not adjustable — they are set at a particular frequency. But they work just like an active crossover — they modify a preamp level signal, plugging into your amp's preamp inputs and setting up the frequency roadblock before the amp.
The patch cord from your receiver plugs into the other side of this kind of crossover. Since they act as active crossovers, they make your system more efficient than passive crossovers. But, like passive crossovers, installation is simple and they are relatively inexpensive.
A low-pass filter removes frequencies above the crossover point, while allowing "lows" to pass on to the speakers (typically subwoofers).
A Linkwitz-Riley crossover is a professional-grade crossover, made of two Butterworth filters, with a crossover slope of 24 dB/octave.
A passive crossover is basically a capacitor or coil installed on the speaker leads between an amplifier and a speaker. It sets up a roadblock to stop certain frequencies from reaching the speakers. Passive crossovers are relatively inexpensive and easy to install. Their primary disadvantage is that they tend to make your overall system slightly inefficient because they filter out signals that have already been amplified.
A tri-way crossover allows you to power a pair of stereo speakers and one subwoofer simultaneously from the outputs of a single 2-channel amplifier. It accepts the two sets of speaker wires from your amplifier, and provides three sets of speaker wires for your speakers and sub. Typically, the speakers will receive the rated stereo power of the amp above the crossover point, while the the amp's bridged power goes to the sub.