Typical audio systems use an unpowered filter within the loudspeaker to feed the treble to the tweeter and the bass to the woofer. This unpowered filter is usually located in the loudspeaker cabinet, so it is not seen as a separate component.
Principles of Loudspeaker Design - Why do Loudspeakers need Multiple Drive Units
Most loudspeakers used in hi-fi systems use more than one loudspeaker element in the cabinet, dividing up audio frequency range into two or three ways. The advantage of this is that it is very hard to make a single loudspeaker element handle all ten octaves of the frequency range, so having a drive unit (the woofer) handling the bass and a separate drive unit (the tweeter) reproducing the treble makes the design of the individual drive units easier.
Disadvantages of Using A Passive Loudspeaker Crossover Filter
From an engineering point of view, doing the filtering in this crossover filter in the speaker at high power levels and low impedances is not a particularly great idea. It means compromise in the design of the filter, and has numerous disadvantages
- restricted choice of high-power components
- high-power parts are often less linear than low power components
- passive filters tend to require inductors, which are either lossy or non-linear at the component values needed for crossover filters
- they often need non-polarised electrolytic capacitors, which are also lossy or non-linear
- matching the different sensitivities of the drive units means attenuating the signal to the more sensitive drive unit.
Advantages of Active Loudspeaker Crossover Filters
Given all these disadvantages of filtering at high levels after the power amplifier, it would be reasonable to ask where else could the filtering be done - and the answer is just before the power amplifier. This way the terminals of the power amplifier can be directly connected to the loudspeaker drive units. For a two-way loudspeaker two power amplifiers are now needed, since the two drive units can no longer be shared across one power amplifier.
There is still the need to filter the bass and treble, and this is now done at a much lower signal level in the active crossover filter, which is inserted after the preamplifier and before the power amplifiers. The lower signal levels and free choice of impedances means that this filter can be engineered more precisely, though it still needs to be designed knowing the loudspeaker characteristics, because the split between low and high frequencies is not the same across all loudspeakers, as it must work with the characteristics of the individual drive units.
Disadvantages of Active Hi-fi Systems
That directly leads to the one main disadvantage of an active system - an increase in cost. Not only is an extra power amplifier needed, or two extra if a three-way loudspeaker system is being activated, but there is also the need for the active crossover, and for this to be specified to match the specific loudspeakers used. This restricts the choice of loudspeakers to ones which the active crossover manufacturer supports.
Thus the disadvantages of an active system are
- extra cost - power amplifiers, active crossover and extra speaker cables
- restricted choice of loudspeakers to those supported by active crossover manufacturer
- extra loudspeaker cables, making it harder to achieve a discreet installation
- the extra space taken up by the extra electronics
How does an Active System Sound Compared to the Equivalent Passive System?
This, of course, depends on the individual items used. In the case of the Naim Ovator S-600, for instance, the difference between the active Ovator configuration and the passive version was that stereo imaging was clearer, audio clarity is preserved better on loud complex passages like a crescendo, and low frequencies were controlled better. The decision as to whether the extra cost is justified is up to the listener and the capacity of their wallet in the case of a hi-fi system.
In the case of sound reinforcement, particularly outdoors and at very high powers, active operation is more common than in hi-fi systems as the extremely high power levels mean that the losses in a passive crossover become unacceptable. Professional audio monitoring systems are often active, again because the clarity and high sound levels needed.
Most people's hi-fi systems aren't driven as hard, and economic constraints tend to favour passive operation except for the highest level audiophile reference systems. Usually a top-quality amplifier driving a passive loudspeaker gives better results than an active system using amplifiers lower down the range. However, the proof is in the listening.
Richard Mudhar - Originally from London, now enjoying a less hectic pace of life in Suffolk, Richard Mudhar is a keen wildlife sound recordist and ...
Join the Conversation