Macro-Tech 24x6 Reference Manual - page 13
Page 13
Macro-Tech 24x6 & 36x12 Power Amplifiers
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 3.8
shows some capacitor values and how they affect the
frequency response. Use only low-leakage paper,
mylar or tantalum capacitors.
For balanced input wiring use one of the examples in
Figure 3.10. Filters A, B and C correspond to the un-
balanced filters above. Filter D also incorporates the
subsonic filter described in Figure 3.8.
Input Wiring Tips
1. Use only shielded cable. The higher the den-
sity of the shield (the outer conductor), the better
the cable. Spiral wrapped shield is not recom-
mended.
2. When using unbalanced lines, keep the
cables as short as possible. Avoid cable lengths
greater than 10 feet (3 meters).
3. Do not run signal cables together with high-
level wiring such as loudspeaker wires or AC
cords. (This lessens the chance of hum or noise
being induced into the input cables.)
4.
The amplifier should be off for at least 10
seconds
before changing any connections. Re-
member, the amplifier can produce lethal output
energy and can drive loudspeakers to levels
which can cause permanent hearing damage.
Crown is not liable for personal injury or damage
incurred when a transducer or component is over-
driven.
+
–
Balanced In
910
Ω
.003
f
µ
.015
f
µ
.018
f
µ
1.8 mH
2.5 mH
A
C
B
.015
f
µ
1.8 mH
D
Balanced Out
+
–
910
Ω
1.8 mH
2.5 mH
1.8 mH
+
–
Balanced In
Balanced Out
+
–
+
–
Balanced In
Balanced Out
+
–
+
–
Balanced In
Balanced Out
+
–
0.47 Film
0.47 Film
Fig. 3.10 Balanced RFI Filters
Tip:
The P.I.P.-FX that came with your amplifier has
plenty of room on its circuit board for input filters.
A third problem to avoid is
hum
. The two most com-
mon sources of hum in an audio system are
inductive
coupling
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
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
levels can also cause your amplifier to prematurely
activate its protection circuitry, resulting in inefficient
operation. RF can be introduced into the signal by lo-
cal 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 of unbalanced wiring are shown below:
Fig. 3.9 Unbalanced RFI Filters
Fig. 3.8 Subsonic Filter Capacitors
1 Hz
10 Hz
100 Hz
1 kHz
10 kHz
dB
0
–5
–10
–15
1 f
µ
.1 f
µ
.05 f
.01 f
µ
µ
Frequency
4 kHz
10 kHz
40 kHz
100 kHz
Frequency
dB
0
–10
–20
A
B
C
6 dB/octave
12 dB/octave
To
Amp
GND
To
Amp
GND
To
Amp
GND
Source
1.8 K ohm
.003
f
µ
.015
f
µ
.018
f
µ
3.9 mH
5 mH
600 ohm
Source
R
600 ohm
Source
R
A
C
B
Note: A low source impedance (R) can be
increased to 600 ohms with an appropriate resistor.