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Compressor/limiters
A limiter can be used to prevent clipping — to reduce the
level of the signa! so it does not push the amplifier to the
point of clipping. In this way, the limiter protects the speak-
ers from
having to dissipate excessive heat and it subjects
them to less mechanical stress. By avoiding clipping of com-
plex waveforms in the high frequency amp of a biamplified
or triamplified
system, the limiter. can
avoid compression
driver damage from clipping-generated low-frequency energy.
A limiter can also prevent a sudden loud transient, as from a
dropped microphone, from tearing out a woofer suspension.
Thus, when carefully adjusted, a limiter can protect speakers.
Improper adjustment can do speakers more harm than good.
An excessively compressed program, for example, may raise
the average power to the point where thermal! damage occurs.
Remember,
compression
reduces
headroom.
A
250 watt
amplifier operated with 20 dB of headroom feeds an average
of about 2.5 watts to the speakers. Add 17 dB of compres-
sion, and the average power may come up to 125 watts. Be
sure you know what you're doing if you use a compressor/
limiter, and use a security cover where appropriate to keep
unauthorized fingers off the controls.
Equalizers and filters
Equalizers can be used to reduce the amount of "unneces-
sary"
power
dissipation
in the speakers.
If the amplifier
power is "used ир" reproducing unnecessary low frequencies,
then the amplifier wil! reach the clipping point that much
sooner, causing more distortion on loud program peaks. What
do we mean by "unnecessary" frequencies? For instance, by
rolling off the bass response in a microphone that is used for
a flute, the intended signal is not compromised, but leakage
from nearby drums, stage rumble and wind noise are prevent-
ed from
reaching
the speakers. Along the same
lines, it is
advisable to use band-limiting high pass and low pass filters
to block inaudible frequencies from the speakers. Many con-
soles, crossovers and equalizers include high pass filters with
"turnover points" of from 20 Hz to 80 Hz, and these should
be used
(and set at the highest practical
thrnover point)
whenever possible.
Another
important
aspect to using a high pass filter is that
low frequency
driver
(woofer)
suspensions
are
not over-
stressed
by being forced to reproduce
noise and spurious
signals.
Still another
benefit
is that Doppler distortion
is
reduced. This distortion, named for the Doppler effect, is a
form
of intermodulation
induced
when
a speaker cone is
simultaneously reproducing treble and bass notes, Given bass
and treble note of the same volume level, physical laws dic-
tate that the cone
must move
as much
as 4 times further.
(which
requires twice the power)
for every octave lower in
frequency. The travel of the cone as it reproduces the bass
notes causes the treble notes which
"ride on the cone" to
move
up and
down
in pitch,
much
like a train whistle
changes pitch as the train passes by the listener, This Doppler
distortion (intermodulation sidebands) is reduced by block-
ing unnecessary low frequencies at the amplifier input.
High frequency driver protection networks
(filters)
A compression driver or tweeter can be quickly destroyed by
transient
DC surges, DC shifts in amplifier output due to
non-symmetrical
waveforms, or turn-On/turn-Off "thumps"
from system electronics. While the Yamaha
amplifiers pro-
vide DC protection
in the form of an output relay, there is
no protection from somebody mis-wiring the low frequency
amplifier of a multi-amped
system
to the high frequency
driver (or misconnecting the electronic crossover network).
Such protection can only be provided by carefully checking a
system before applying full power, and perhaps by protecting
the drivers themselves.
For this reason, we discuss this alter-
nate method to protect compression drivers.
One
can
insert a capacitor in series with the cable to the
power
amplifier,
and possibly
a resistor shunt
across the
driver, This circuit constitutes a half section filter. Even with
a multi-amplified system, such protection may be necessary
to prevent
low frequency
energy from
damaging
the dia-
phragm
or
suspension
due
to
over-excursion.
The
low
frequency
energy
can
reach
the driver in a multi-amped
system if it is caused by amplifier clipping (see note below).
NOTE
Clipping a single sine wave generates only harmonics that
are higher in frequency than the clipped signal. However,
when
two 5іпе waves are simultaneously clipped, they
not only produce higher frequency harmonics, they also
produce intermodulation products which include lo wer
frequency components. When complex musical signals are
clipped, it is actually possible for full-bandwidth evergy
to be generated. The extent of this effect depends on the
amplifier itself and the degree to which it is overdriven —
and some amplifiers exhibit better behavior than others.
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