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16
2.
3.
4.
5.
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
l/4
inch phone jack
and plug when wired for stereo sound.
Connectors which can be plugged into AC power
receptacles should
never
be used.
Connectors having low-current-carrying capacity
are “verboten.”
Connectors having any tendency to short, or having
shorted leads, are unadvisable.
Most commercially-available headphones employ a 3-
circuit
l/4
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.
FIG. 2
-
6
SCHEMATIC OF EARPHONE PAD
2.4.1 MONO OPERATION
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st.r.0
Earphenor
A mono-stereo switch on the rear panel adjacent to the in-
put jacks, allows the DC-300A to be operated normally
(stereo) or in mono, with no internal modification. (See
figure 2-2.) When in the mono position, the input circuitry of
the
DC-3OOA
is changed so that the two amplifiers are
“added” for mono output. (See mono specifications, page
3).
Care must be taken in the external hook-up to assure proper
operation. Proceed as follows:
1
The input line should be plugged into
the
channel 1 input
jack. The level is adjusted with the channel 1 input level
control.
NOTE: The Channel 2 input jack and level control are
not defeated in the Mono mode. However, the Channel 2
input should not be used in this mode. If a Channel 2
input is added to the Channel 1 input, distortion may
result. If Channel 2 input is used alone, very low power
output will result. For best results unplug the input to
Channel 2 when operating Mono.
2.
Connect output lines as per the following drawing, Fig-
ure 2-7. The output from the
DC-3OOA
in Mono is BAL-
ANCED and is isolated from the chassis, and from the
input grounds to the DC-300A.
CAUTION: Be certain that all equipment (meters,
switches, etc.) connected to the Mono output lines is
balanced. Both sides of the line must be totally isolated
from the input grounds, to the DC-300A. If this is not ob-
served, severe oscillation may result.
FIG. 2-7
DC-300A MONO HOOK-UP
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 displacement is
observed, the DC content of the input may be excessive and
require a blocking capacitor. The graph of Fig. 2-8 indicates