Infrasonic Filter Capacitors; Unbalanced Rfi Filters - Crown Macro-Tech MA-1200 Reference Manual

Macro-Tech 600/1200/2400 Power Amplifiers
SOLVING INPUT PROBLEMS
Sometimes large infrasonic (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 3.10
shows some capacitor values and how they affect the
frequency response. Use only low-leakage paper,
mylar or tantalum capacitors.
dB
0
–5
–10
–15
1 Hz 10 Hz
Fig. 3.10 Infrasonic Filter Capacitors
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
are sensitive to high frequencies. Extremely high RF lev-
els can also cause your amplifier to prematurely acti-
vate its protection circuitry, resulting in inefficient
operation. RF can be introduced into the signal by local
radio stations and from the bias signal of many tape
recorders. To prevent high levels of input RF, install an
appropriate low-pass filter in series with the the input
signal. Some examples of unbalanced wiring for low-
pass filters are shown in Figure 3.11.
1.8 K ohm
To
Amp
.003
Source
µ
F
GND
A
3.9 mH
To
Amp
.015
600 ohm
R
µ
F
Source
GND
B
5 mH
To
Amp
.018
600 ohm
R µ
F
Source
GND
C
4 kHz
Note: A low source impedance (R) can be
increased to 600 ohms with an appropriate resistor.
Fig. 3.11 Unbalanced RFI Filters
For balanced input wiring use one of the examples in
Figure 3.12. Filters A, B and C correspond to the unbal-
anced filters above. Filter D also incorporates the infra-
sonic filter described previously.
Reference Manual
100 Hz 1 kHz
Frequency
6 dB/octave
A
12 dB/octave
B
C
10 kHz 40 kHz
Frequency
+
A Balanced In
+
B
Balanced In
+
C
Balanced In
+
D
Balanced In
10 kHz
Fig. 3.12 Balanced RFI Filters
Tip: The P.I.P.-FX included with your amplifier has plenty
of room on its circuit board for input filters.
A third problem to avoid is hum. The two most common
sources of hum in an audio system are inductive cou-
pling and ground loops.
Inductive coupling can occur when input cables are
subjected to a magnetic field from a power cord or
power transformer. One way to prevent inductive cou-
pling is to lace the input cables together along their
length and route them as far away as possible from
power transformers and power cords. The use of
Input Wiring Tips
1. Use only shielded cable. Cables with
dB
higher density shields are better. Spiral
0
wrapped shield is not recommended.
2. When using unbalanced lines, keep the
cables as short as possible. Avoid cable
–10
lengths greater than 10 feet (3 meters).
3. Do not run signal cables together with
–20
high-level wiring such as loudspeaker wires or
AC cords. This greatly lessens the chance of
hum or noise being induced into the input
100 kHz
cables.
4. Turn the entire system off before changing
connections. Turn level controls down
completely before powering the system back
up. Crown is not liable for damage incurred
when any transducer or component is
overdriven.
910 Ω
+
.003
Balanced Out
µ
F
– –
910 Ω
1.8 mH
+
.015
Balanced Out
µ
F
– –
1.8 mH
2.5 mH
+
.018
µ Balanced Out
F
–
–
2.5 mH
0.47 Film 1.8 mH
+
.015
Balanced Out
µ
F
– –
0.47 Film 1.8 mH
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