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safety
6
speaker protection
All loudspeakers have electrical, thermal,and physical limits which must be
observed to prevent damage or failure. Cone or compression drivers can
be damaged
(sometimes to the point of failure) from excessive power,low
frequencies applied to high frequency drivers, severely clipped waveforms,
and
DC voltage. All Pro II Series amplifiers automatically protect speakers
from
DC voltages and subsonic signals.
see—TourClass protection
Mid- and high-frequency transducers—compression drivers in particular—
are highly susceptible to damage from overpowering, clipped waveforms,
or frequencies below their rated passband. When using an electronic
crossover, make absolutely certain that the low and mid bands are con-
nected to the correct amplifiers and drivers
—and not accidentally con-
nected to those for a higher or lower frequency band.
The amplifier’s clipping point is its maximum peak output power. At max-
imum peak output power, Crest Audio Pro II Series amplifiers will deliver
more power than many speakers can safely handle. Be sure the peak
power capability of the amplifier is not excessive for your speaker system.
To ensure that the speakers never receive excessive power,and to prevent
amplifier clipping, use a properly adjusted external limiter
(or a compres-
sor with a ratio of 10:1 or higher
) to control power output.Use one com-
pressor/limiter for each frequency band in systems with active electronic
crossovers.
The
ACL clip limiting circuit will automatically limit the duration of
squared-off, continuous waveforms applied to the speakers.The amplifier
will, however, allow normal musical transient bursts to pass. Of course,
when the amplifier does clip, it is operating at its maximum output power.
Note that some speaker systems are packaged with proprietary “proces-
sors” that have power limiting circuits and therefore should not require
additional limiting.Do not drive any low-frequency speaker enclosure with
frequencies lower than its own tuned frequency; the reduced acoustical
damping could cause a ported speaker to “bottom out” even at moderate
power. Consult the speaker system specifications to determine its fre-
quency limits, and employ a roll-off filter if necessary.
recommended speaker cabling
The wire gauge charts will assist you in determining the optimum copper
wire gauge for your speaker cables. Remember that the speaker cable
resistance robs amplifier power in two ways: through power lost directly
to resistance
(often referred to as I
2
R loss), and through increased total
load resistance, which decreases the amount of power available from the
amplifier. Appendix C gives cable length figures in feet/
AWG wire gauges
and in metric values.
see—wire gauge