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Since these things are not Active 3-Way Crossover Active 2-Way Crossover
Passive 3-Way Crossover Passive 2-Way Crossover possible, the passive
network must be (at best), a simplified and compromised solution
to a very complex problem. Consequently, the crossover’s behavior
changes with frequency — not something you want for
a good sounding system.
One last thing to make matters worse. There is something called
back-emf (back-electromotive force: literally,
backvoltage) which further contributes to poor sounding
speaker systems. This is the phenomena where, after the signal stops,
the speaker cone continues moving, causing the voice coil to move
through the magnetic field (now acting like a microphone), creating
a new voltage that tries to drive the cable back to the
amplifier’s output!
If the speaker is allowed to do this, the cone flops around like
a dying fish. It does not sound good! The only way to stop
back-emf is to make the loudspeaker “see” a dead short,
i.e., zero ohms looking backward, or as close to it as possible
— something that’s not gonna happen with a
passive network slung between it and the power amp.
All this, and not to mention that inductors saturate at high signal
levels causing distortion — another reason you can’t
get enough loudness. Or the additional weight and bulk caused by
the large inductors required for good low frequency response. Or
that it is almost impossible to get high-quality steep slopes passively,
so the response suffers. Or that inductors are way too good at picking
up local radio, TV, emergency, and cellular broadcasts, and joyfully
mixing them into your audio. Such is life with passive speaker systems.
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Active
Active crossover networks require a power supply to operate
and usually come packaged in single-space, rackmount units. (Although
of late, powered loudspeakers with built-in active
crossovers and power amplifiers are becoming increasingly popular.)
Looking at the accompanying diagram shows how active crossovers
differ from their passive cousins. For a 2-way system instead of
one power amp, you now have two, but they can be smaller for
the same loudness level.
How much smaller depends on the sensitivity rating
of the drivers (more on this later). Likewise a 3-way system requires
three power amps. You also see and hear the terms bi-amped,
and tri-amped applied to 2- and 3-way systems.
Active crossovers cure many ills of the passive systems. Since the
crossover filters themselves are safely tucked away inside their
own box, away from the driving and loading impedance problems plaguing
passive units, they can be made to operate in an almost mathematically
perfect manner. Extremely steep, smooth and well-behaved crossover
slopes
are easily achieved by active circuitry.
There are no amplifier power loss problems, since active circuits
operate from their own low voltage power supplies. And with the
inefficiencies of the passive network removed, the power amps more
easily achieve the loudness levels required.
Loudspeaker jitters and tremors caused by inadequately damped back-emf
all but disappear once the passive network is removed. What remains
is the amplifier’s inherent output impedance and that of the
connecting wire. Here’s where the term damping factor
comes up.
[Note that the word is damping, not damp-ning
as is so often heard; impress your friends.] Damping is a measure
of a system’s ability to control the motion of the loudspeaker
cone after the signal disappears.
No more dying fish.
Siegfried & Russ
Active crossovers go by many names. First, they are either 2-way
or 3-way (or even 4-way and 5-way). Then there is the slope rate
and order: 24 dB/octave (4th-order), or 18 dB/ octave (3rd-order),
and so on. And finally there is a name for the kind of
design.
The two most common being Linkwitz- Riley and Butterworth,
named after Siegfried Linkwitz and Russ Riley who first proposed
this application, and Stanley Butterworth who first described the
response in 1930. Up until the mid ‘80s, the 3rd-order (18
dB/octave) Butterworth design dominated, but still had some problems.
Since then, the development (pioneered by Rane and Sundholm) of
the 4th-order (24 dB/octave) Linkwitz-Riley design solved these
problems, and today is the norm.
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