Crown DC-300A, Instruction Manual

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the
effect of the size of the blocking capacitor on the
frequency response. Only a low-leakage type paper, mylar,
or tantalum capacitor should be used for this purpose.
TABLE FOR SELECTION OF
INPUT DC BLOCKING CAPACITOR
If large amounts of ultrasonic or RF frequencies are found
on the input, such as bias from tape recorders, etc., a
pass filter should be placed on the input. While
obtainable RF input levels will not damage the amplifier,
they may cause burn-out of tweeters or other sensitive
loads, activate the amplifier’s protective systems, or cause
general overload in the controlled-slewing-rate stage of the
amp (which is employed to provide RF overload protection).
The following filters are recommended for such applica-
tions.
FIG. 2-9
LOW-PASS FILTER FOR SEVERE RF AT INPUTS
A
second problem area is “ground loops” electronic
jargon for undesirable circulating currents flowing in a
grounding system. A common form of loop (possibly result-
ing in hum in theoutput)isapairof inputcableswhosearea
is subjected to a magnetic hum field. In practice, both
cables should lie
along
their length, and
away
from
the power transformer. Tying the input and output grounds
together may also form a ground loop.
A third problem (with input and output grounds together, as
in testing or metering) is feedback oscillation, from load
current flowing in the loop. In industrial use, even the AC
power line may provide this feedback path. Proper
grounding, and isolation of inputs of common AC line
devices is good practice. Refer to Section 4.2, par. 5
for
testing precautions.
An extremely common form of this problem is encountered
when using electrostatic loudspeakers or any other kind of
load that is joined to the AC power mains. Capacitive
coupling through the load’s supplies may allow the
amplifier’s output to be fed through the AC mains and into
the grounds of input equipment resulting in a system
oscillation.
To combat the problem, first try reversing the speaker leads
on all channels if possible. If this does not solve the
problem, try grounding the power amplifier to the AC
ground with its 3 wire plug. (A ground loop may result
through an
FM tuner with an earth-grounded antenna
system when deploying the 3 wire plug. The use of ,005
disc capacitors in series with the FM antenna leads will
eliminate
should of course be as short as reasonably possible, and a
turntable baseplate ground should always be broughtto the
phono preamp with a separate ground wire, never via the
input cable ground.
If the oscillation still persists, start
removing input devices, working towards the amplifier until
the oscillation disappears. This will identify the point of
feedback. If an offending piece of equipment is found it
should be inspected for unnecessary AC line-to-ground
capacitances such as
line filters,
etc. If possible such should
be removed. Such devices should never be found in the
load, although it is true that some RF loudspeakers have
used such filters to reduce
2 . 6 C O N N E C T I N G P O W E R
The amplifier is furnished with a three-wire AC plug as
standard equipment. Adapters are readily available com-
mercially for adapting this to a two-wire system if neces-
sary.
The amplifier offers five standard line-voltage connec-
tions: 100, 120, 200, 220, and 240VAC. The tag attached to
the line cord indicates for which voltage the amplifier is
connected. Most units are connected for
When
changing the voltage to 200V or above, the external fuse,
should be changed from
1
OA to 5A. Relabel the line tag
to indicate the proper voltage.
When testing the amplifier, the line voltage must be the
peak equivalent to a sinusoid of the indicated line voltage
when at full load. Line regulation problems can introduce
serious errors in the measurements on an amplifier of this
size.
Only a competent technician should attempt alteration of
the line voltage connections.