Splitting the Difference

Mar 14, 2008 1:58 PM, By Mike Sokol

           

A SOUND KNOWLEDGE OF CROSSOVERS IS KEY TO UNDERSTANDING YOUR P.A.

Figure 1: A graph showing a typical crossover point for a two-way speaker system. The input to the low- and high-frequency drivers falls off on either side of the crossover frequency.

Let's Split

Crossovers are divided into two basic categories: passive and active. Both types divide the entire frequency spectrum into two or more bands, and the boundaries between the bands are the cutoff frequencies or crossover points (see Fig. 1). Ideally, the split would be absolute, with all the high frequencies going to the horn and all the low frequencies going to the woofer, but in practice this is not easily accomplished, particularly with passive designs.

The actual rate at which unwanted frequencies fall off beyond the crossover point is referred to as the slope because of the way it appears on a graph. The slope varies according to the number and type of components used, but the most common crossover slopes are 6, 12, 18, and 24 dB per octave.

Figure 2: Passive crossover networks are placed between the power amplifiers and the drivers and are typically mounted within the loudspeaker enclosure.

As the name implies, passive crossovers are circuits that have no active (powered) electronic components. They are usually built into the speaker cabinet and fed driver-level signals directly from the amplifier (see Fig. 2).

The steeper the slope, the better able the crossover is to keep unwanted frequencies away from a particular driver. However, steeper slopes require more components, a setup that wastes a lot of electrical energy, even at the frequencies the crossover is supposed to be passing. In addition, you need a separate crossover for every speaker cabinet in your system. To top it off, it's not possible to change the crossover frequency without soldering in new components.

Figure 3: A two-way active crossover placed between a mixer and two power amplifiers. The crossover splits the mono mixer signal into low and high frequencies that feed two discrete amplifier channels used to power the low- and high-frequency drivers independently.

Activate Me

The disadvantages of passive crossovers soon became apparent to those using them in professional sound-reinforcement applications, and somebody came up with the idea of using active circuits instead. Rather than being placed between the amplifier and speaker, active crossovers go between the mixer and the amplifier (see Fig. 3). That arrangement requires separate amplifier channels for each driver; for example, to drive a two-way speaker system, you need two amps, a scheme called biamping. Carrying around extra power amps may seem like a hassle, but active crossovers and biamping have lots of advantages over passive crossovers.

The amount of power an active crossover is required to handle is minuscule compared with the hundreds of watts pumped into a passive crossover; as a result, it can have an adjustable design. Instead of working at a fixed frequency of, for instance, 500 Hz, it can include a control that sweeps from 100 to 5,000 Hz, allowing it to be tuned for the frequency preferences of a given driver.

In addition, active crossovers don't waste power the way their passive counterparts do; attenuation slopes of 18 or 24 dB per octave are therefore common. A greater slope removes the offending frequencies more completely from each driver, resulting in a cleaner signal with less damage. Another benefit to active crossovers is that each driver gets exactly the frequencies it can produce most efficiently, so it can be driven harder without sounding bad or having to dissipate a lot of heat — the nemesis of all drivers.

In addition to offering greater efficiency and controllability, active crossover systems are generally less expensive than passive systems, because a single active crossover can easily drive dozens of amplifiers without any problems. For larger sound-reinforcement systems, it's simply a matter of adding as many amplifiers and speakers as required. Also, more sophisticated crossovers have limiters, delays, and other processors built into them, providing additional control over the sound (see the sidebar “Delay Tactics”).

Most active crossovers offer a wide range of settings to accommodate the needs of different speaker systems. How do you know which frequencies a particular driver can accept without damage? Begin by examining the literature that came with the speakers; nearly all manufacturers provide recommended crossover frequencies and slopes for their products. If you don't have documentation, try contacting the manufacturer directly.

An additional consideration is the way in which crossovers connect to speaker systems. Some speakers have individual inputs for each driver (usually on ¼-inch or XLR connectors or on five-way binding posts that accept dual and single banana plugs, spade lugs, or bare wires). However, Neutrik NL4 Speakon connectors are becoming increasingly common. NL4s are able to switch between passive and active routing; in the active setting, the low frequencies go to the Low terminals and the high frequencies go to the High terminals.