Active versus Passive Speakers
A relatively recent design concept for loudspeakers is to include an active crossover plus power amplifiers built into the speaker enclosure. Mackie was one of the first modern companies to offer this in its HR824 and
HR624 models, though studio monitors with active crossovers were produced as long ago as 1967 by Klein+Hummel. Today all-in-one active loudspeakers are very common. There are many advantages of active monitors for the typical project studio, besides a simpler hookup with fewer pieces to carry if you ever do remote recordings: Active speakers are typically bi-amped, which yields less distortion as already explained. Bi-amping also offers more ways to optimize the crossover performance because it uses active rather than passive components, as was also explained. Further, using an active crossover increases headroom within each band by segregating the bands. That is, if the amplifier that powers the bass range clips at a very loud volume, high frequencies are still reproduced cleanly unless that amplifier is also driven into distortion.
Further, the power amplifiers can be well matched to the speakers, they won’t have a fan, and the wires between each power amp and its speaker are shorter, which might improve damping. (Amplifier damping is explained in Chapter 21.) An active loudspeaker can also contain DSP circuitry to counter frequency response errors in the drivers themselves and to add any needed delays so frequencies near the crossover point emit from both drivers at the same time. This not only improves frequency response, but it can also reduce the radiating directivity problem known as lobing, which will be described shortly. But to me, the overwhelming advantage of powered monitors, as implemented in Mackie speakers anyway, is that the woofer cone’s mechanical motion is included within the
power amplifier’s electrical feedback loop. This improves low-frequency response, reduces ringing at the port’s resonant cutoff frequency, counters thermal compression in the voice coil, and reduces driver distortion.
Negative feedback will be explained more fully in Chapter 21. But briefly for now, negative feedback lets an amplifier circuit self-correct its own output signal to reduce distortion and frequency response errors. A portion of the output is fed back to the input, but with the polarity reversed. So if the amplifier’s output is not an exact voltage-multiple of its input at a given point in time, its input receives more or less of the feedback signal. The amplifier in turn sends more or less signal to its output to compensate, which forces the output to better match the input. With an active speaker, the amplifier can be designed to sense the amount of current being drawn by the driver and compare that to the amount of current that should be drawn for a given input signal. If the two do not match, which means the driver is distorting or ringing, the amplifier varies its output signal to compensate.
