Page 17
CP660
Six Channel Amplifier
Reference Manual
5 Other Issues
5.1 Solving Input Problems
Sometimes large
infrasonic
(sub-audible)
frequencies
are present in the input signal. These can damage loud-
speakers or step-down transformers by overloading or
overheating them. To attenuate such frequencies, place
a capacitor in series with the input signal line.
Fig. 5.1 Unbalanced RFI Filters
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
Ω
.003
f
µ
.015
f
µ
.018
f
µ
3.9 mH
5 mH
600
Source
Ω
R
600
Source
Ω
R
A
C
B
Note: A low source impedance (R) can be
increased to 600 by an appropriate resistor.
Ω
Another problem to avoid is large levels of
radio fre-
quencies
or RF in the input signal. Although 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 protec-
tion circuitry, resulting in inefficient operation. RF can
be introduced into a 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 input signal, as shown in Fig. 5.1.
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
shielded pair cable is another effective way to reduce
or eliminate hum resulting from inductive coupling.
Ground loops often result when two or more devices
are improperly grounded. This causes undesirable
stray currents that may produce hum in the output. The
best way to avoid ground loops is to ensure
that all system devices are plugged into the same
power strip. In addition, when operating a balanced sig-
nal path, make sure that all cable shields are grounded
at one end only.
Input and output grounds are sometimes tied together
for testing or metering. This can cause
feedback oscil-
lation
from load current in the test loop. In some sys-
tems, even the AC power line may provide this
feedback path. To avoid this problem, use proper
grounding, isolate the inputs and other common AC
devices.