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| July 15, 2003 Sound and Music: Active Loudspeakers and More This series so far has tended to focus on the broad mainstream of hi-fi loudspeakers and their core ingredients. But the fascinating thing about speakers is their immense diversity -- for every rule-follower, there’s likely to be an exception.The last chapter looked at the crossover network. Nearly all speakers include some sort of simple electrical network, usually built into the speaker (though sometimes outboard). This operates between the power amplifier and the drive units, splitting the full-range signal from the amp and passing appropriate frequency bands to the relevant drive units. Two types of speaker don’t have crossover networks, and both are relatively uncommon. One uses just a single full-range drive unit (see "Sound and Music: Loudspeaker Drive Units"). The other is the so-called "active" loudspeaker system, the name referring to a speaker with an active (i.e., powered) rather than the usual "passive" (i.e., unpowered) crossover network. A passive network is fed by the relatively strong signals that emerge from the power amp, and must therefore cope with the substantial voltages and currents that actually power the drive units. An active network, however, sits between the preamp and the power amp -- it’s effectively an extension of the preamp. It therefore deals only with signals of relatively low voltage and current, thus avoiding one major factor that affects passive networks. It also permits much finer control over the filtering and equalization characteristics, sometimes incorporating adjustments to suit different environmental conditions. The inevitable consequence of active loudspeaker drive is that a separate power amplifier must be used for every drive unit. This often involves a considerable cost penalty, but also offers significant potential performance advantages because of the superior damping and control that the power amp can exert over the drive unit -- in an active design, the two are much more tightly coupled. The "halfway house" between passive and active drive is called biamping or multi-amping. This is feasible when the speaker concerned has more than one pair of input terminals, and involves removing any links between the terminals and driving each terminal pair with its own power amplifier. Active (low-level) filtering is not needed here, as the crossover network remains in circuit within the speaker system. That removes the close coupling between power amp and driver that is found with full active drive, but a biamped two-way speaker will typically sound better than the same speaker driven by a single amp -- an advantage of reducing the work that any one amplifier will have to do. Although the sound quality of amplifiers has long been a vital part of serious hi-fi experience and perception, the audiophile community has long been reluctant to embrace active drive. That’s probably because enthusiasts and dealers have always considered amplifiers and speakers as separate entities usually manufactured by different companies, each specializing in only one type of component. Active loudspeakers necessarily require combining multiple areas of expertise. Furthermore, a dilemma invariably arises: Is it better to devote the whole amplification budget to a single, costly combination of preamp and power amp, or opt for an active multi-amp approach using more but less costly amps? There’s no easy answer -- what you might gain by buying active speakers you could lose on the power-amp side. Some hi-fi manufacturers have enjoyed significant success with active speakers. Four brands that immediately spring to mind are Bang & Olufsen, from Denmark, and three leading British brands: Linn, Meridian, and Naim Audio. But the pro audio sector is probably where active drive has been most widely adopted. Many studio monitors are produced only in active form, sometimes with the filters and power amps built into the speakers (such as in the smaller Genelec models), and sometimes with separate racks of electronics (such as with the large PMC-Bryston combos). Active speakers can be packaged in two different ways, each of which has its advantages and disadvantages. Meridian and B&O, for example, invariably build the active filters and power amps into the loudspeakers themselves, which keeps things compact and avoids the need to run several sets of heavy-gauge speaker cable. Both of these brands have models that can be fed with digital as well as analog signal inputs, and that carry out filtering and EQ in the digital domain. The down sides are that the amp-speaker package is a fixed entity with no scope for future upgrading, and the need to provide mains power to the speakers, which is not always convenient.
Perhaps the most advanced and interesting active loudspeaker is B&O’s brand-new BeoLab 5, one of the most elaborate and complex speakers ever made. This four-way design is stuffed with digital processing power and packed with no less than 2500W of built-in amplification: one 1000W power amp for each of the low-bass (15") and upper-bass (6.5") drivers, plus 250W each for the midrange (3") and treble (0.75") domes. There are two reasons for the BeoLab 5’s massive bass power reserves. One is to equalize the bass drivers with considerable bass boost so they can operate properly in their small enclosures. The other is for a very clever and complex system that measures the bass interaction of speaker and room, then computes and inserts an additional layer of digital equalization to create a flat net response. Add in the acoustic lenses that control the directivity of the mid- and high-frequency domes, and you end up with a loudspeaker that should give very consistent results, regardless of the characteristics of the room it’s used in or where it’s placed therein. That represents a revolution in speaker design that could not have happened without active drive. ...Paul Messenger
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Linn and Naim, in contrast, keep their
amplification and active filtering separate from the speakers, largely for the extra
flexibility this affords. Both brands offer convertible, passive/active loudspeakers -- a
customer can start out by purchasing a passive system driven from a single stereo amp,
then add power amps and active filtering to upgrade the sound later, when funds permit.
There may be an additional advantage in keeping the electronics well away from the heavy
vibrations created by the speaker drive units.