Crown DC-300A Instruction Manual page 25

24
. Use the proper line voltage, which is the one for
which the amplifier is connected. The voltage should
be measured throughout the testing with a peak read-
ing meter, and adjusted to the RMS equivalent voltage
(to compensate for line voltage regulation errors
during the course of the measurements). All
measurements should be taken at the power ampli-
fier's plug. When testing for IHF music-power
measurements, the line voltage is to be set at 120V
when the amplifier is connected to 120V, (IHF
standards). If the amp is connected otherwise, the
equivalent test may be given by applying 128 volts
on 128V, 240V on 240V, or 256V on 256V line.
. The load should be resistive, having less than 10%
reactive component at any frequency up to five
times the highest test frequency. The _ resistor
should be capable of continuously dissipating the
full output of the amplifier while maintaining its
resistance within 1% of its rated value. The load
should employ only high-current connectors (if any),
and be connected to the binding-post output ter-
minals. All output measurements should be taken
at the amplifier output terminals, and not anywhere
along the output cable through which the load current
is flowing.
. The input level controls should be set to maximum
for all distortion tests to assure repeatability of
all measurements.
. When measuring hum and noise, all inputs should
be disconnected from the amplifier and the level
controls set to minimum or to maximum, prefer-
ably minimum.
. Whenever possible avoid ground loops in the test
equipment caused by connecting the output ground
to the input ground. Never connect the ground of
the cable going to the load back to the input ground.
Ground loops are especially obnoxious when measur-
ing distortion. An !-M distortion analyzer, for ex-
ample, has its input and output terminals tied to a
common ground. Such a test should use an un-
grounded output return, with the output lead(s)
wrapped around the well-shielded and grounded
input cable.
. Always monitor the test oscillator when measuring
frequency response. Use a wide-band AC voltmeter;
or use the same meter for both input and output
level measurements, if the meter's frequency re-
10.
11.
. Accuracy
sponse is known not to be dependent on attenuator
settings.
in measuring voltages for computing
wattage is critical. For example, a 2% voltage error
together with a 1% resistance error can result in an
error of 10 watts power into 8A.
- Residual distortion and noise levels should be fully
known for all the test equipment in order to ac-
curately evaluate the amplifier.
. Never attempt to measure damping factor by placing
abnormal loads on the output. D-F measurements
taken during clipping, or any other form of overload,
are meaningless. The preferred method is to apply
an externally generated current to the output
terminals and measure the resultant voltage at the
terminals. A convenient current is one ampere — as
the resultant voltage will read directly in ohms for
|Zo|. Damping Factor is defined as Esl. where
12, | is typically 8 ohms. A convenient generator
for the 1A. current is that amplifier channel not
under test. A non-inductive resistance of 8 ohms —
coupled between both channels' output terminals —
will provide 1A. when 8 volts are impressed across
the resistor (by that channel not under test).
Never measure hum and noise when in the presence
of strong magnetic fields. The amplifier should be
at least 4 inches away from any large metallic ob-
jects or shield plates for a reading to be meaningful.
Noise measurements should be taken with a band-
pass filter of 20-20KHz. For audio purposes the
measurement of noise above 20KHz is meaningless.
4.3 SERVICE
Because of the level of circuitry sophistication of the
DC-300A, only the most competent technicians should
be allowed to service it.