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PROTECTION FOR SPEAKERS
Theoretically,
a power
amplifier
is capable
of damaging
speakers in many ways. Better amplifiers, such as this Yamaha
amp, are designed to eliminate some of these damage modes.
Output relay
A turn-on transient (a low frequency "thump'') may occur as
the power
supplies in signal processing equipment
activate
internal circuits, but before the circuits reach a stable operat-
ing status, These "thumps" are then amplified by the power
amp. The safest approach here is to disconnect the speakers
from the amplifier until all equipment has stabilized. In this
amplifier,
a relay keeps the amp output disconnected until
after power
has been on a few seconds and everything is
stable. This protection only works, however, if you turn on
the amplifier
LAST
— after first turning on the console,
electronic crossover, and any signal processing equipment,
If DC were to appear at an amplifier's output, it would cause
speaker voice coils to move to one position and stay there,
which can quickly lead to voice coil overheating and prema-
ture failure of the speaker. Even relatively tow levels of DC
can cause this problem. DC can appear at a direct coupled
amplifier's output when the input signal has just a few milli-
volts
of DC
offset;
in AC
coupled
amplifiers,
like this
Yamaha amp, it is conceivable that DC would appear with
assymmetrical,
very tow frequency
input signals. High DC
levels, as might be caused
by a short between the power
supply rails and the output, can instantly destroy a speaker
suspension from over-excursion, followed by voice coil burn-
: out, For this reason, circuitry
in the amplifier senses the
29
presence
of any significant
DC component
at the output
(more than 2 volts), and opens the speaker relay immediately,
When
this happens,
the front
panel PROTECTION
LED
turns on. Given that the average 8-ohm speaker has a DC
resistance of about 5 ohms, this means that the relay would
shut down the output before so much as 1 watt of DC power
reached the speaker load.
Current limiting
If a short circuit in the speaker or cable or a very low impe-
dance speaker load overloads the amplifier, current limiting
circuitry in the amplifier prevents excessive power from be-
ing delivered to the load.
HINT
USE
THE RIGHT
TYPE AND
NUMBER
OF SPEAK-
ERS.
The best speaker protection is obtained using am-
plifiers of sufficient power
to avoid clipping, speaker
components designed to handle large amounts of power,
and using these speakers
within their rated frequency
band. In other words, don't plan to push any compo-
nents to their absolute limits if you expect reliability.
Speaker fuses
Fuses offer protection against known currents that flow for a
certain amount of time — two criteria that cannot be estab-
lished for most musical signats. Even if every note were at the
exact same volume (it's possible with a lot of compression),
the changing frequencies themselves will cause changes in the
current flowing through the speaker voice coil. To carefully
calculate
the power
level which
should blow a protection
fuse, you have to use a given impedance, say 8 ohms. How-
ever, speaker loads are frequency dependent, and the fuse
may regularty blow. when a note is played that corresponds
to the minimum
impedance
of the speaker (say 5 ohms in-
stead of the assumed 8-ohm rating).
It may seem that the correct fuse has been selected because
no program frequency causes it to blow ... until a long-time
surge of higher-than-average
power
is sent to the speaker.
Such surges can occur if a performer holds the head of a mic
and
induces feedback,
or if a keyboard
player leans on à
particular synthesizer note for "effect."
Incidentally, while
slo-blo fuses offer a little bit better protection from such
surges, they do so at the cost of poor voice coil protection;
they are too slow to protect compression drivers and tweeters
against high power transients.
Fast blowing fuses can be used to prevent amplifier DC faults
from destroying speakers in the event an amplifier "goes DC"
(which is not likely due to protection circuits in the ampli-
fier). If you do wish to. use fast blowing DC fuses, select a
current rating that will pass the speaker's rated power handl-
ing capacity.
№ the driver
is rated
at 100 watts
and 4
ohms, that corresponds to 5 amps (current = square root of
power divided by impedance). If the speaker is а multi-way
system containing a high level passive crossover, then system
power capacity for the entire system should be used in the
calculation,
Remember
that while the fuse adds a certain
amount
of protection for the woofer; it offers little protec-
tion for a compression driver or tweeter in the same system
because those component
are generally capable of handling
less power than the woofer, and the amount of padding for
the high frequency drivers provided by the crossover may or
may not balance the decreased
power handling capacity of
those drivers.
In general, we don't recommend fuses for speaker protection.
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