24
4.2 TEST PROCEDURES
The sole function of this section is to list precautions
essential to obtaining accurate measurements when deal-
ing with high-power, high-purityamplifierssuch
astheDC-
300A.
1.
2.
3.
4.
5.
6.
7.
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 reading
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 amplifier’s plug. When
testing for IHF music-power measurements, the line
voltage is to be set at 12OV when the amplifier is con-
nected to 12OV, (IHF standards). If the amp is con-
nected otherwise, the equivalent test may be given by
applying the appropriate voltage.
The load should be resistive, having less than 10% re-
active 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 terminals. All output measure-
ments should be taken at the amplifier output ter-
minals, 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 meas-
urements.
When measuring hum and noise, all inputs should be
disconnected from the amplifier and the level controls
set to minimum or to maximum, preferably 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 I-M distortion analyzer, for example,
has its input and output terminals tied to a common
ground. Such a test should use an ungrounded output
return, with the output lead(s) wrapped around the
well-shielded and grounded input cable.
Always monitor the test oscillator when measuring fre-
quency response. Use a wide-band AC voltmeter; or
use the same meter for both input and output level
measurements, if the meter’s frequency response is
known not to be dependent on attenuator settings.
Accuracy 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
lOwatts
power into 8 ohms.
8.
9.
10.
11
Residual distortion and noise levels should be fully
known for all the test equipment in order to accurately
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
direc;ly
in ohms for
[Zd.
Damping
Factor is defined as
w
,
where
IZLI
is typically 8
ohms. A convenient
gengrator
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 noisewhen in the presence of
strong magnetic fields. The amplifier should be at least
4 inches away from any large metallic objects or shield
plates for a reading to be meaningful.
Noise measurements should be taken with a bandpass
filter of
20.20KHz.
For audio purposes the measure-
ment of noise above
20KHz
is meaningless.
4.3 SERVICE
Should service other than routine fuse replacementever be
required, it is recommended that the unit be returned to the
factory in the original packing (or replacement, if damaged).
For warranty service the machine must be returned to the
factory or warranty service station. The CROWN warranty is
detailed on page 12.
Because of the level of circuitry sophistication of the DC-
300A,
only the most competent technicians should be
allowed to service it.
Many of the parts are standard items stocked by most
supply houses. However, there are several which appearto
be standard parts but are actually different. Although
standard parts may be used in an emergency, best results
will be with factory parts. A number of the parts are
available only from CROWN.
When ordering parts, be sure to give the amplifier serial
number as well as the part number and description. Rated
firms will be billed, otherwise shipments will be C.O.D.
Before returning an amplifier to the factory for service,
authorization should first be obtained from the service
manager. Shipments may be sent UPS or truck freight,
prepaid and insured at total value. The factory will return
your serviced unit by UPS or truck freight, collect. and will
add C.O.D. charges in the event that the cost is not covered
by registered warranty.
