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Topic No. 6

Loudspeaker Protection
by
John L. Murphy
Physicist/Audio Engineer

 

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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