Crown dc-300a ii 参考指南

  Power Amplifier

Page 12

A third problem to avoid is 

. The two most com-

mon sources of hum in an audio system are 

 and 

.

Inductive coupling can occur when input cables are
subjected to a magnetic field from a power cord or
power transformer. One way to prevent inductive cou-
pling is to lace the input cables together along their
length and route them as far away as possible from
power transformers and power cords. The use of
shielded pair cable is another effective way to reduce
or eliminate hum resulting from inductive coupling.

Ground loops often result when two or more devices
are improperly grounded. This causes undesirable
stray currents that may produce hum in the output. The
best way to avoid ground loops is to ensure that all
system devices are plugged into the same power strip.
In addition, make sure that all cable shields are
grounded at one end only.

Input and output grounds are sometimes tied together
for testing or metering. This can cause 

 from load current in the test loop. In some

systems, even the AC power line may provide this feed-
back path. Proper grounding, input isolation and isola-
tion of common AC devices in the system is good
practice.

3.3.3 Output Connection
Consider the power-handling capacity of your load be-
fore connecting it. Crown is not liable for damage in-
curred at any time due to a load being overpowered.
The use of loudspeaker protection fuses is highly rec-
ommended (see Section 3.3.4). Also, please pay close
attention to the precautions in Section 4.1.

1. Use only shielded cable. Cables with

higher density shields are better. Spiral
wrapped shield is not recommended.

2. Avoid unbalanced lines longer than 10 feet

(3 meters).

3. Keep signal and high level wiring apart.

4. Turn off the unit before making connec-

tions. Crown is not liable for injury or dam-
age due to overdriven components.

1. Note the load resistance of the loudspeakers connected to each

channel of the amplifier. Mark this value on the “Load Resistance”
line of the nomograph.

2. Choose an acceptable system damping factor and mark it on the

“Damping Factor” line. Your amplifier can provide an excellent
damping factor of 750 or more from DC to 400 Hz in Dual mode
with an 8 ohm load. In contrast, typical damping factors are 50 or
lower. Higher damping factors yield lower distortion and greater
motion control over the loudspeakers. A common damping factor
for commercial applications is between 50 and 100. Higher
damping factors may be desirable for live sound, but long cable
lengths often limit the highest damping factor that can be
achieved practically. (Under these circumstances, Crown’s

IQ System

®

 is often used with Crown’s premium 

Macro-Tech

,

®

Com-Tech

®

 and 

Reference

 series amplifiers for easy monitoring

and control of units located very near the loudspeakers.) In
recording studios and home hi-fi, a damping factor of 500 or more
is desirable.

Fig. 3.7  Wire Size Nomograph

HOW TO  DETERMINE

APPROPRIATE WIRE GAUGE

It is important to use loudspeaker cables of sufficient
gauge (thickness) for the length used. The resistance
introduced by inadequate loudspeaker cables will re-
duce both the output power and the motion control of
the loudspeakers. The latter problem occurs because
the damping factor decreases as loudspeaker cable
resistance increases. This is very important because
the amplifier’s excellent damping factor can easily be
negated by using insufficient loudspeaker cables.

Use the nomograph and the procedure that follow to
find the recommended wire gauge (AWG or American
Wire Gauge) for your system.