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WPI Technical Theatre Handbook: Crossovers
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Most consumer speakers, and some low-end professional speakers have passive crossovers built in to them. These crossovers are responsible for taking the single full-spectrum signal from the amplifiers and sending the appropriate portion to each driver within a speaker. This assures that the woofer is only reproducing bass frequencies, the mid-range driver is reproducing only mids, and the tweeter is reproducing highs. However, passive crossovers aren't the most efficient devices in the world, and most high-end professional systems do not use them. Instead, they use devices called active crossovers, which do much of the same thing, but they do it on line-level signals, before the amplifier stage of a sound system.

With active crossovers, each amplifier is responsible for driving speakers only within a specific range of frequencies. This division of power, so to speak, makes for a more efficient system. The system can be driven to much higher volume levels, and will generally sound much clearer and more defined. Also, the levels sent to each of the drivers can be adjusted, which makes balancing the system easier than with systems with passive crossovers.

The practice of using an active crossover scheme in a system is known by several names, which depend on how many splits are being made in the audible frequency range. If the system is simply being broken into lows and highs, it is said to be bi-amped. If it is broken into low, mid and high, it is said to be tri-amped. Logically enough, if a system is broken into low, low-mid, high-mid and high, it is a quad-amped system. If a system with a passive crossover is used, the speakers are generally referred to as two-way, three-way, or four-way, respectively. The distinction is important. A tri-amped system requires an active crossover unit, three separate amplifiers, and speakers capable of having their drivers driven by separate inputs. A three-way system only requires one amplifier and speakers with built-in passive crossovers.

Figure 6.10: An active crossover unit, manufactured by Ashly Audio, Inc. Note the large number of controls on the unit, for controlling crossover frequency, slope, and volume levels.

Active crossovers have several controls for adjusting their operating parameters. They provide a means to adjust the crossover frequencies - the frequency at which the sound will stop being reproduced by one driver and start being reproduced by the next. This transition is not instantaneous. In fact, it is modeled as two intersecting slopes. These slopes are adjustable on most active crossovers, allowing a very gradual change between two drivers to a farly steep change. Setting a crossover requires knowledge of where the individual drivers of a speaker system are designed to cross over. This information is usually contained in the manuals for the speakers, and is sometimes printed on the back of the speakers themselves. Improperly set crossovers can, at best, make a sound system sound terrible, and at worst, destroy drivers.

next up previous contents index
Next: Cabling and Connectors Up: Output Previous: Speakers   Contents   Index
Steve Richardson 2000-07-06

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