Page 20
Com-Tech Power Amplifiers
Reference Manual
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
Fig. 3.11 Unbalanced RFI Filters
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. Some examples of
unbalanced wiring for low-pass filters are shown in Fig-
ure 3.11.
+
–
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. 3.12 Balanced RFI Filters
For balanced input wiring, use an example from Figure
3.12. Filters A, B, and C correspond to the unbalanced
filters shown in Figure 3.11. Filter D also incorporates
the infrasonic filter in Figure 3.10.
Tip:
The standard PIP2-BB has space on its circuit
board for the addition of input filter circuitry.
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 sys-
tem devices are plugged into the same power strip. In
addition, make sure that all cable shields are grounded
at one end only.
Input Wiring Tips
1. Use only shielded cable. Cables with
higher density shields are better. Spiral
wrapped shield is not recommended.
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 reduces the chance of hum
or noise being induced into the input cables.
4. Turn the entire system off before changing
connections. Turn level controls down before
powering the system back up. Crown is not
liable for damage incurred when any trans-
ducer or component is overdriven.
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
Fig. 3.10 Infrasonic Filter Capacitor Values
