In
an active loudspeaker system the crossover network is connected,
in the signal path, before the power amplifiers. An active
crossover network feeds separate signals to the bass, mid, and
treble power amplifiers. The power amplifiers in turn power the
corresponding loudspeaker drivers for the bass, mid, and treble
ranges.
This method has
several advantages, especially in large PA systems, compared to
passive dividing where the crossover network is connected between
the power amplifiers and the loudspeakers. You get for example
lower distortion, in particular distortion that is caused by loud
bass tones which intermodulate with the mid, and treble ranges. An
active system also eliminates the power losses, associated with
passive crossover networks.
LAB.GRUPPEN's ACN C crossovers are designed around a four-pole
state variable filters, which are derived from the theory of
analog computers. The most common way to alter crossover
frequencies in such a circuit is to turn a multiple ganged
potentiometer. However most potentiometers have bad tolerances (20
% or worse), and these tolerances affect the accuracy of the
crossover frequencies and the flatness of the summed output.
A better way of
altering the crossover frequency is applied in the ACN 2/3C;
namely the use of "Digital Hexcode" switches. This
allows the use of close tolerance components, which result in a
crossover frequency accuracy better than 1 %.
By careful selection
of these components, an optimum density of available crossover
frequencies has been created, especially in the region between 800
Hz and 2 kHz. This has made it possible to achieve a continuity of
phase and of group delay time together with the speaker system.
This leads in turn to an improved intelligibility in the vocal
region.
12, or 18 dB/octave slopes can be ordered as an option. The 24/octave
slope has equal phase at the crossover frequency region, in
accordance with Linkwitz-Riley. To achieve flatness in the summed
output, the high and lowpass filters meet each other at 6 dB.
Each of the outputs of the ACN 2/3C have been equipped with
shelving equalization, which gives an adjustable bass boost of
upto +12 dB, and a treble boost of upto +12 dB. In the ACN 3C the
mid output has an additional shelving boost, but that works at a
lower corner frequency. With the help of this equalization it is
possible to align the frequency response of the speaker system.
You can, for example, compensate the roll-off in the treble of
so-called "Constant Directivity"-type horns. It also
means you negate the need, for an external (graphic) equalizer, in
several applications.
|