Input Connection; Input Wiring; Subsonic Filter Capacitors; Unbalanced Rfi Filters - Crown DC-300A II Reference Manual

DC-300A II Power Amplifier
outputs will receive the channel 1 input. The channel 2
output is inverted so it can be bridged with channel 1.
There are two different ways to connect Bridge-Mono
wiring. The most common method is to connect the
positive (+) output of channel 1 to the positive (+) loud-
speaker lead, and the positive (+) output of channel 2
to the negative (–) loudspeaker lead (see Figure 3.2).
The negative amplifier outputs are not used.
In Bridge-Mono mode, it is also possible to connect a
loudspeaker to each output channel, however, the out-
put of channel 2 is inverted. To compensate for this,
you can invert the polarity of the channel 2 output wir-
ing. First, connect a loudspeaker to channel 1 as you
would normally. Then, connect a loudspeaker to chan-
nel 2 so its positive (+) output goes to the negative (–)
loudspeaker terminal, and negative (–) output goes to
the positive (+) loudspeaker terminal.
CAUTION: Only connect balanced loads to a bridge-
mono output. Output lines must be isolated from
ground or severe oscillations may occur.

3.3.2 Input Connection

The unbalanced ¼ inch phone inputs have a typical
impedance of 25 K ohms. They accept the line level
output from most devices. Figure 3.3 shows how to
properly wire both balanced and unbalanced lines.
+
SHIELD
FROM
UNBALANCED
SOURCE
–
+
FROM
DROP
SHIELD
BALANCED
SOURCE
Fig. 3.3 Input Wiring
SOLVING INPUT PROBLEMS
Sometimes large subsonic (subaudible) frequencies
are present in the input signal. These can damage
loudspeakers by overloading or overheating them. To
attenuate such frequencies, place a capacitor in se-
ries with the input signal line. The graph in Figure 3.4
shows some possible capacitor values and how they
affect frequency response. Use only a low-leakage pa-
per, mylar or tantalum capacitor.
Another problem to avoid is the presence of large lev-
els of radio frequencies or RF in the input signal. Al-
though high RF levels may not pose a threat to the
amplifier, they can burn out tweeters or other loads that
dB
0
–5
–10
–15
0.1 Hz
are sensitive to high frequencies. Extremely high RF
levels can also cause your amplifier to prematurely ac-
tivate its protection circuitry, resulting in inefficient op-
eration. RF can be introduced into a signal by local
radio stations and from the bias signal of many tape
recorders. To prevent this from happening, place an
appropriate low-pass filter on the input(s). Some ex-
amples are shown below for unbalanced wiring.
1.8 K ohm
Source
A
3.9 mH
600 ohm
R
Source
B
5 mH
600 ohm
R
Source
C
Note: A low source impedance (R) can be
increased to 600 ohms with an appropriate resistor.
+
For balanced input wiring, use one of the examples in
UNBALANCED
INPUT
Figure 3.6. Filters A, B and C correspond to the unbal-
anced filters above. Filter D also incorporates the sub-
+
sonic filter described previously.
UNBALANCED
INPUT
1 Hz 10 Hz
Frequency
Fig. 3.4 Subsonic Filter Capacitors
To
Amp
.003
f
GND
To
Amp
.015
f
GND
To
Amp
.018
f
GND
4 kHz 10 kHz
Fig. 3.5 Unbalanced RFI Filters
910
+
.003
A
Balanced In
f
–
910
1.8 mH
+
.015
B
Balanced In
f
–
1.8 mH
2.5 mH
+
.018
C
Balanced In
f
–
2.5 mH
0.47 Film 1.8 mH
+
.015
D
Balanced In
f
–
0.47 Film 1.8 mH
Fig. 3.6 Balanced RFI Filters
100 Hz 1 kHz
dB
0
6 dB/octave
A
12 dB/octave
–10
B
C
–20
40 kHz 100 kHz
Frequency
+
Balanced Out
–
+
Balanced Out
–
+
Balanced Out
–
+
Balanced Out
–
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