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5. Amplification and Power Selection (Low Impedance Mode)
As with all professional loudspeaker systems, the power handling is a function of voice coil
thermal capacity. Care should be taken to avoid running the amplifier into clip (clipping is the
end result of overdriving any amplifier). Damage to the loudspeaker will be sustained if the
amplifier is driven into clip for any extended period of time. Headroom of at least 3 dB should
be allowed. When evaluating an amplifier, it is important to take into account its behaviour
under low impedance load conditions. A loudspeaker system is highly reactive and with
transient signals it can require more current than the nominal impedance would indicate.
Generally higher power amplifier running free of distortion will do less damage to the
loudspeaker than a lower power amplifier continually clipping. It is also worth remembering
that a high powered amplifier running at less than 90% of output power generally sounds a lot
better than a lower power amplifier running at 100%. An amplifier with insufficient drive
capability will not allow the full performance of the loudspeaker to be realised.
It is important when using different manufacturers amplifiers in a single installation that they
have very closely matched gains. The variation should be less than +/-0.5dB. This precaution
is important to the overall system balance when only a single compressor/limiter or active
crossover is being used with multiple cabinets. It is therefore recommended that the same
amplifiers be used throughout.
6. Voltage and Power Selection (Distributed Line Operation)
When using 70.7V or 100V distributed-line systems, the transformer can be tapped at 30W,
15W and 7.5W, with an extra 3.75W tapping for 70.7V line systems. The taps are selected via
a rotary switch, as shown in Figure 6a, and is located on the front of the unit, behind the grille,
Figure 6b.
When the relevant voltage taps have been selected, add the individual power in Watts at
all speakers and select and amplifier with a rating equal to or exceeding the total power in
Watts. All of the transformer primaries should be connected in parallel to the output of this
amplifier. If for example, you select the 7.5-watt transformer tap, it means that at full rated
amplifier output the speaker will receive the full 7.5 watts. If the amplifier gain is reduced each
speaker will receive a proportional amount of power, maintaining the overall system balance.
It is recommended that a generous power safety margin (3dB of headroom) be left so that the
system does not have to operate continuously at its full rated output power.
Figure 6a
In order to comply with relevant fire safety regulations (i.e. BS 5839:1998), it is required
that in the event of fire, that failure of the circuit to which the loudspeaker is connected
does not occur before evacuation of the building is complete. Suitable measures include:
a)
use of terminal blocks (for connection to primary) with a melting point of not less
than 650°C, for example constructed from ceramic materials;
c)
use of terminal blocks of a lower melting point but protected with thermal insulation;
d)
use of terminal blocks such that, on melting, an open-circuit or a short-circuit does
not occur.
Figure 6b
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