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The benefits of Active over Passive loudspeaker systems
The performance benefits of active over passive loudspeakers is
substantial. Even a system, which incorporates the best available
stand-alone power amplifier, will never achieve the performance
of a similar active system. There are very good engineering
reasons why this is true and the following brief will introduce
some of the issues.
1. The magnitude of the frequency response of both active
and passive loudspeakers can be controlled, with good design, to
be within 1dB of one another. However, the phase component of
the frequency response will always be better in an active system.
The active filters produce better filter roll-off characteristics at
crossover. Combine this with the inclusion of a variable all-pass
filter at each crossover point to correct the phase response of
the drive units through the crossover regions and the result is
a loudspeaker with much better group delay characteristics.
The benefit to the listener will be improved polar response and
therefore radiated power response. Such an active loudspeaker
will, therefore, have a large stable sound field with stable imaging
and source location not possible with a passive loudspeaker.
2. A passive crossover will only operate correctly into the
load impedance of a particular loudspeaker drive unit. However,
the impedance of a loudspeaker drive unit will change with the
amount of power input. This is because loudspeakers are very
inefficient and most of the input power is dissipated as heat in
the voice coil. As a result the temperature of the voice coil will
rise and, because copper has a positive temperature coefficient
of resistance, the impedance of the loudspeaker drive unit will
rise. The result will be frequency response errors as the filters
move from their designed response with increased input power.
This effect does not occur in active loudspeakers where
the filter response is maintained independent of input power to the
loudspeaker.
3. Because the amplifiers in an active loudspeaker system
are only required to operate over reduced frequency bands the
intermodulation distortion products present in a passive system
will be dramatically reduced, by typically 20dB, in an
active system.
4. In an active system the absence of a passive crossover and
long cable runs together with a known amplifier damping factor
prevents the modification of the loudspeaker drive unit “Q”
ensuring better controlled low frequency performance.
5. For a given amount of amplifier power an active loudspeaker
can be expected to produce approximately 6dB more level than
the equivalent passive system. Furthermore, powers may be more
optimally specified in an active system. A tweeter,for example,
requires much less power than a woofer to produce a balanced
system performance.
