Sound System Design 1279
f is a frequency chosen as follows:
- If the system is two way (or three way, and so on),
choose f equal to one-half the system crossover
frequency. If the capacitor is to be used as a high-pass
protection device in a voice-only system, choose f
equal to the desired high-pass frequency, remem-
bering that a single series capacitor provides about a
6 dB/octave slope rate.
- Choosing a low-frequency value for f in the equation
results in a very large capacitor. Thus, a capacitor is
usually an impractical method of protecting a
low-frequency loudspeaker. A high-pass filter prior
to the power amplifier is better for a low-frequency
loudspeaker.
- For a high-frequency driver, using the driver’s rated
impedance for Z may result in errors since the actual
impedance can be much higher at frequencies below
crossover. Thus, it may be a good idea to actually
measure the driver’s impedance at the chosen
frequency f and use this in the equation.
- Choose a capacitor with a voltage rating at least equal
to the maximum peak-to-peak voltage output of the
power amplifier. This will be the sum of the absolute
values of the positive and negative power supply
voltages for a direct-coupled amplifier and can be
approximated from the following equation for either
direct-coupled or transformer-coupled amplifiers:
(34-31)
where,
VP–P is the peak-to-peak voltage output of the power
amplifier,
P is the rated output power of the amplifier,
Z is the rated load impedance of the amplifier,
the value of 2.828 is twice the square root of two.
The value of V resulting from this equation may be
conservative, since most amplifiers can produce power
output in excess of their rated value for short periods.
Thus, the actual voltage rating for the capacitor should
probably be somewhat higher.
The capacitor must be nonpolarized. Motor run types
are considered good choices for sound system applica-
tions. Motor start capacitors may be used. Standard
electrolytic capacitors, if nonpolarized, can be used, but
these capacitors normally have a very poor tolerance in
actual capacitance value and, thus, may not provide the
expected protection.
34.3.5.3.3 Limiters
A limiter is not normally considered a loudspeaker pro-
tection device, but it may be one of the best and most
practical. The limiter, Fig. 34-35, can be adjusted to pre-
vent the system power amplifier from exceeding its
power output capabilities and can help prevent
high-power peaks from reaching the loudspeakers. In
systems where sound quality is a primary consideration,
adjust the limiter so that its threshold is high and its
compression ratio is high. This way, the limiter will not
be in operation until a potentially dangerous peak is
detected. Then, the high compression ratio of the limiter
will clamp the peak and help prevent loudspeaker
damage.
34.3.5.3.4 Other Protection Devices
Transformers can help protect loudspeakers because
they cannot pass dc. In the past, some transformers
included series capacitors to limit the low-frequency
energy to a high-frequency driver. Autotransformers, on
the other hand, may pass dc to a loudspeaker since they
have only a single winding.
Passive crossover networks, because they include
one or more series capacitors, provide good protection
for high-frequency drivers and some protection for the
low-frequency loudspeaker (against excessive
high-frequency power levels). The passive crossover
networks used in some packaged loudspeaker systems
include sophisticated protection circuitry. The manufac-
turer will normally specify this in its sales literature and
intruction manuals.
High-pass and low-pass filters, similar to those often
found in a DSP or on a graphic equalizer, are valuable
in any system. A high-pass filter helps keep out
unwanted low frequencies that could cause overexcur-
sion. A general rule is that, except for subwoofer
systems, a 40–160 Hz high-pass filter should be used in
all systems. Even for subwoofers, a 10–20 Hz (or
higher) high-pass filter can help prevent dangerous
overexcursion. High-pass filters are often available on
mixer input channels. Using them here can help reduce
damage from dropped microphones or other problems.
Low-pass filters help prevent heat-producing
radio-frequency energy (picked up from outside sources
or from faulty system electronics) from reaching the
loudspeakers. Low-pass filters also keep out audio
frequencies above the loudspeaker’s range (which
would also cause unwanted heating).
VPP– =2.828 PZ
