Subsonic Filter Capacitors; Unbalanced Rfi Filters - Macro-Tech 24x6 Reference Manual

Macro-Tech ® 24x6 & 36x12 Power Amplifiers
SOLVING INPUT PROBLEMS
Sometimes large subsonic (subaudible) frequencies
are present in the input signal. These can damage loud-
speakers by overloading or overheating them. To at-
tenuate such frequencies, place a capacitor in series
with the input signal line. The graph in Figure 3.8 shows
some capacitor values and how they affect the fre-
quency response. Use only low-leakage paper, mylar
or tantalum capacitors.
Fig. 3.8 Subsonic 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 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
For balanced input wiring use one of the examples in
Figure 3.10. Filters A, B and C correspond to the unbal-
anced filters above. Filter D also incorporates the sub-
sonic filter described in Figure 3.8.
Reference Manual
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 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. 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.
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