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The
type of speaker protection a designer chooses will depend on
the application of the speaker. A fuse may be fine for home
hi-fi applications but most performers would rather have a
speaker die a slow tortured death than to ever have a blown
fuse shut down their performance before a live audience. In
general, the problem of how to protect speakers is a
difficult one with no universal solution. A well designed
crossover provides the upper frequency drivers the protection
they require from full range signals but cannot protect the
drivers from excessive input power in their frequency range.
I’ll identify some of the protection methods in use and
comment on each.
Fuses:
In line speaker fuses can be useful in non performance
applications but are rarely used in live sound because they
protect the speaker at the expense of the performance. In
selecting a fuse value remember that a nominal 1 Amp fuse
doesn’t "blow" at 1 Amp but, rather, is
guaranteed to hold at 1 Amp and blow at some higher current
level.
The speaker power rating that corresponds
to the nominal fuse rating can be calculated as follows:
Power = current X current X resistance
( P = I^2 * R , for math weenies )
Where: current = the fuse rating, in amps
resistance = the speaker’s impedance (
4, 8, 16 Ohms etc.)
Examples:
For a 1 Amp fuse with an 8 Ohm speaker
P = 1 x 1 x 8 = 8 Watts
For a 2 Amp fuse with an 8 Ohm speaker
P = 2 x 2 x 8 = 32 Watts
For a 3 Amp fuse with an 8 Ohm speaker
P = 3 x 3 x 8 = 72 Watts
Remember, the fuse will HOLD up to these
levels and blow at somewhat higher power levels. Also, the
powers calculated here are continuous (or "rms" if
you must) powers NOT peak power. Slow blow type fuses are a
pretty good idea for speaker protection.
One practical way to implement fuse
protection is to start with a very low value fuse and
progress to higher values until the fuse holds for your
application. Fuses are cheap, speakers are usually not. Fuses
have no (or minimal) impact on perceived audio quality.
Light bulbs:
Over the years various manufacturers have employed various
types of "light bulbs" wired in series with the
speaker as a means of speaker protection. As the load current
increases the bulbs filament heats up and the resistance of
the light bulb increases thereby attenuating the power
delivered to the speaker. The bulb actually acts like a
"compressor" by turning down the peaks. Selecting
the correct bulb is tricky and best left to professional
designers who can conduct careful lab tests to select the
right bulb. Not that you shouldn’t try it if you want.
Just don’t expect some nice rule for making it work!
Bulbs can be successfully used for live sound because they
will not interrupt a performance. Hi-fi purists would likely
be offended by the potential effects of bulbs on audio
quality.
PTC Devices:
Positive Temperature Coefficient (PTC) devices or thermistors
can be used to protect speakers. They behave somewhat like a
cross between fuses and bulbs. The company Raychem makes a
line of PTC devices named "PolySwitch" that are
specifically intended for speaker protection. I have used
these devices and can recommend them for live sound
applications. Contact Raychem at 1-800-227-4866 and ask them
to send you their application notes on their PolySwitch
devices. They also make a line of light bulb type protection
devices and provide selection charts for specifying the
protection power level.
Relays:
Relays can be employed in conjunction with some signal
rectifier circuitry to form a protection system. They have
the advantage on being able to momentarily disconnect (or, if
you wish, attenuate) the drive to the speaker and then
restoring the signal immediately when the over power
condition is corrected. This makes them useful for live sound
applications. Rather than shut down the show they just
momentarily interrupt it!
Subsonic Filters:
If your speakers are experiencing mechanical failure then a
possible cause of the failure is excessive low bass energy
being delivered to the system. In this case a good subsonic
filter can help quite a bit by trimming out the most damaging
frequencies. The filter should have at least 18dB per octave
cutoff slopes. Locate the cutoff frequency somewhere above
about 30Hz but below 70Hz. The higher the cutoff frequency
the more protection the filter provides.
In Summary
The best method of protecting your speakers is to know their
limits and stay within them. Blown speakers tend to be more
of a live sound problem than a hi-fi problem. In live
performance situations speakers are routinely operated at the
threshold of death for extended periods of time. If the
overall sound system is correctly designed and operated
within it’s limits then speaker protection will not
usually be an issue. If your system is plagued with speaker
component problems then it may be time for an overall system
review with a professional audio engineer.
jlm
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