Crown DC-300A - 5.4.3 REPLACEMENT OF COMPONENTS ON DCA MAIN PC BOARD ASSEMBLY; 5.4.4 REPLACEMENT OF LEVEL POTENTIOMETER (R100, R200); 5.4.5 REPLACEMENT OF POWER SWITCH, SW-1; 5.4.6 REPLACEMENT OF THERMAL SWITCH (SW-2, SW-3)

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2. Connectors which might accidentally cause the two
channels to be tied. together during making and
breaking of connection should not be used. A common
example is the standard 3-circuit %4 inch phone jack
and plug when wired for stereo sound.
3. Connectors which can be plugged into AC power
receptacles should never be used.
4. Connectors having low-current-carrying capacity
are “‘verboten.”
5. Connectors having any tendency to short, or having
shorted leads, are unadvisable.
Most commercially-available headphones employ a 3-
circuit % inch phone plug which violates condition number
2. This is no handicap if a pad is inserted between the
amp and jack, which is the only sensible thing to do,
when such a large amplifier is coupled to such a small
transducer. If this precaution is ignored, not only may
the transducer be burned out but permanent hearing loss
could result. The recommended pad is shown in Fig. 2-6.
from 75a, 20W 100,1W
Amp To Stereo
Outputs Earphones
100, 1W
750, 20W
FIG. 2-6
SCHEMATIC OF EARPHONE PAD
2.5 CONNECTING INPUT LINES
Connecting the inputs will require avoiding three basic
dangers: Undesirable signals to the inputs, ‘‘ground
loops,’” and feedback from output(s) to input(s).
For loudspeaker-driving applications, the input should
be free of any DC, as this could cause overheating of the
loudspeaker voice coil. A simple visual test for DC on
the inputs (providing the woofer is visible) is to slowly
turn up the input level control with the amp on and watch
for any displacement of the cone. If very much displace-
ment is observed, the DC content of the input may be
excessive and require a blocking capacitor. The graph
of Fig. 2-7 indicates the effect of the size of the block-
ing capacitor on the frequency response. Only a low-
leakage type paper, mylar, or tantalum capacitor should
be used for this purpose.
NOTE: Graph
denotes 10Kn
| input Z (Amp
LEVEL controls
| full clockwise)
t
| mati L
ARTs WH 1OHz
FREQUENCY
TOO Hz VKH
FIG. 2-7
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 low-pass filter should be placed on the input.
While practically-obtainable RF input levels will not
damage the amplifier, they may cause burn-out of
tweeters or other sensitive loads, activate the ampli-
fier’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 follow-
ing filters are recommended for such applications.
1.8 Ka To
Amp
1OKH:
FREQUENCY
NOTE: A lo-Z R&R, can be a0KMs WOKM:
Increased te 6001 by an
epprepriete resistor,
FIG. 2-8
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 re-
sulting in hum in the output) is a pair of input cables
whose area is subjected to a magnetic hum field. In prac-
tice, both cables should lie together 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