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The crossover


The function of the crossover is to separate low, mid and high frequencies
according to the number of drive units in the loudspeaker. A crossover can be
passive or active. A passive crossover is generally internal to
the cabinet and consists of a network of capacitors, inductors and resistors.
Having no active components, it doesn't need to be powered. An active crossover
on the other hand does contain transistors or ICs and requires mains power.
It sits between the output of the mixing console and a number of power amplifiers
– one for each division of the frequency band. A system with a three-band active
crossover would require three power amplifiers.

Crossovers have two principal parameter sets: the cut off frequencies of the
bands, and the slopes of the filters. It is impractical, and actually
undesirable, to have a filter that allows frequencies up to, say, 4 kHz to pass
and then cut off everything above that completely. So frequencies beyond the
cutoff frequency (where the response has dropped by 3 dB from normal)
are rolled off at a rate of 6, 12, 18 or 24 dB per octave. In other words, in
the band of frequencies where the slope has kicked in, as the frequency doubles
the response drops by that number of decibels. The slopes mentioned are actually
the easy ones to design. A filter with a slope of, say, 9 dB per octave would
be much more complex.

As it happens, a slope of 6 dB per octave is useless. High frequencies would
be sent to the woofer at sufficient level that there would be audible distortion
due to break up. Low frequencies would be sent to the tweeter that could damage
it. 12 dB/octave is workable, but most systems these days use 18 dB/octave or
24 dB/octave. There are issues with the phase
response of crossover filters
that vary according to slope, but this is
an advanced topic that few working sound engineers would contemplate to any
great extent.

Passive crossovers have a number of advantages:

  • Inexpensive
  • Convenient
  • Usually matched by the loudspeaker manufacturer to the requirements of the
    drive units

And the disadvantages:

  • Not practical to produce a 24 dB/octave slope
  • Can waste power
  • Not always accurate & component values can change over time

Likewise, active crossovers have advantages:

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  • Accurate
  • Cutoff frequency and slope can be varied
  • Power amplifier connects directly to drive unit – no wastage of power &
    better control over diaphragm motion

  • Limiters can be built into each band to help avoid blowing drive units

And the disadvantages:

  • Expensive
  • It is possible to connect the crossover incorrectly and send LF to the HF
    driver and vice versa.

  • A third-party unit would not compensate for any deficiencies in the driver

Some loudspeaker systems come as a package with a dedicated loudspeaker
control unit
. The control unit consists of three components:

  • Crossover
  • Equalizer to correct the response or each drive unit
  • Sensing of voltage (and sometimes) current to ensure that each drive unit
    is maximally protected

David Mellor

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Producing Lauren Balthrop

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David Mellor