Who says the speakers are rated for 80W?
I had a pair of Radio Shack Minimus 7 speakers rated at 40W. I used a 25W per channel amplifier and burned them out. Taking them apart I saw the woofer was stamped 5W Korea.

 

I'm uncertain as what the difficulty is. A speaker is an inductor. A signal is a a waveform of a certain voltage and current level. Low impedance means high-current (which is expected to control the inductor). The speaker has limits it needs to be kept within (aka, the inductor's behavior needs conttrolled to stay within a window)

People need to stop looking at this as a speaker, and thinking of it as an inductor. You control the signal to keep it within parameters when it begins to exceed them (like an opamp providing negative feedback on the signal when it starts moving beyond an acceptable range. You limit the current to the inductor when it begins to exceed a certain range (filters, capacitive & inductive).

What would be useful is any actual information off the speaker itself so it can be identified, and perhaps a datasheet sourced.

See this link for details of the speaker:
https://assets.bose.com/content/dam...ical_data_sheet/tds_fs_ds_40f_loudspeaker.pdf

 

The truthful 40W rating for continuous pink noise (similar to music) is with a 70Hz highpass filter to prevent the speaker from hammering against the magnet since the enclosure is ported.
The speaker system resonates at about 75Hz then two in series will cause the resonance to be uncontrolled and sound boomy, and be even more damaging.

250W into 8 ohms is 44.8V RMS and the rated 40W into 4 ohms is 12.7V RMS. The wrong amplifier for the speaker.

 

According to the linked datasheet, the speakers are 8 ohms each, not 4. For 80 W into 16 ohms the current would be 2.24 A, so if you follow crutschow's advise, a fuse of probably 1.5 A for nominally 36 watts would be a good starting point.

Is there any opportunity to attenuate the input signal to the power amplifier? I've seen some amps where there is an output from the preamp and source selector and an input to the power amp to allow additional processing equipment (e.g. an "equalizer") to be conveniently inserted. If that is the case, then an attenuator and high-pass filter could be added. It isn't foolproof, but might be worth considering.

 

Did you determine for sure if the destroyed speakers were burned up or mechanically damaged from "over-travel?" A fuse can be fair protection against the former, but poor against the latter.

 

Not sure this will work as you may increase the wattage but decrease the Impedance, so your 8Ohm may not work.

What are you talking about ???????????????

The truthful 40W rating for continuous pink noise (similar to music) is with a 70Hz highpass filter to prevent the speaker from hammering against the magnet since the enclosure is ported.
The speaker system resonates at about 75Hz then two in series will cause the resonance to be uncontrolled and sound boomy, and be even more damaging.

250W into 8 ohms is 44.8V RMS and the rated 40W into 4 ohms is 12.7V RMS. The wrong amplifier for the speaker.

I agree the wrong amplifier for these speakers but the customer doesnt want to change the amp. So stuck now and have to deal with the speakers and protect them as much as possible.

 

Did you determine for sure if the destroyed speakers were burned up or mechanically damaged from "over-travel?" A fuse can be fair protection against the former, but poor against the latter.

Yes did several tests, but the simplest of them all is use a multimeter and measured connectivity and resistance. Connectivity is open, and resistance is infinite... so speakers burnt...

 

According to the linked datasheet, the speakers are 8 ohms each, not 4. For 80 W into 16 ohms the current would be 2.24 A, so if you follow crutschow's advise, a fuse of probably 1.5 A for nominally 36 watts would be a good starting point.

Is there any opportunity to attenuate the input signal to the power amplifier? I've seen some amps where there is an output from the preamp and source selector and an input to the power amp to allow additional processing equipment (e.g. an "equalizer") to be conveniently inserted. If that is the case, then an attenuator and high-pass filter could be added. It isn't foolproof, but might be worth considering.

Correct the speakers are 8 ohms and yes if I put to speakers in series that would make them 80W's into 16 ohms? But my replacement speakers are 4ohms and the amp is a 8ohm amp. The customer has the following amps:
1: https://www.bhphotovideo.com/c/product/306256-REG/Tapco_JUICE_J_800_J_800_Two_Channel.html
and 2: https://www.bhphotovideo.com/c/product/306258-REG/Tapco_JUICE_J_1400_J_1400_Two_Channel.html
Amp 1 is used to power the Ceiling speakers (2 speakers in series per channel).

Unfortunately cannot attenuate the input singal. There is a mixer before the amp, but people keep changing settings so we need a kind of "people safe" solution, something they cannot alter easily or cannot get to (in the ceiling for instance).

 

Hello,

Unfortunately cannot attenuate the input singal.

Why can't you attenuate the input signal?
Is there no line between the mixer and the amplifier?
If there is such a line, you can attenuate the mixer signal going to the amplifier.

Bertus

 

Could you add a trimpot to the input of the amp?

 

Could you add a dummy load in series with the speakers? 8 ohms would need to be a 60W rating with about 60W going to speakers. I realize the damping factor will be horrible but it already is bad with the speakers in series. Now the amp will be cranked up past the clipping level and the speakers destroyed with the square wave (clipping) output.

 

Guess what? Each Bose speaker has a 70V/100V transformer like on store ceiling speakers.
If the speaker is set for 40W/70V then the impedance is 122.5 ohms. The 250W into 8 ohms amplifier has an output of 44.7V and will power the transformer'd speaker with 16.3W which is only a little less loudness than 40W.

 

The Bose speaker already has a PTC resistor on it for overload protection.
Is it a cheap Chinese knock-off missing the PTC resistor and missing the transformer and not a real Bose speaker?

 

The Bose speaker already has a PTC resistor on it for overload protection.
Is it a cheap Chinese knock-off missing the PTC resistor and missing the transformer and not a real Bose speaker?

Nope this is a real Bose, I double checked. But the PTC didnt do its job.

 

I really don't think anyone knows what they are doing or what they are talking about.
The capability of the speakers MUST match the output of the amplifier.
To say you cannot put speakers in parallel / series is a typical reply from a HIGHLY QUALIFIED TECHNICIAN.
If you say a channel is capable of delivering 250 watts, the only thing you have to know is the impedance required. The amplifier has AN OUTPUT IMPEDANCE.
You must keep to this impedance. Then you must know what each speaker is capable of DISSIPATING.
It is then a simple matter to match these parameters. This can be done with parallel/series connection.
Only an idiot would even mention the use of a fuse.
The only sensible thing one person said was: a speaker can be damaged by over-travel as well as overheating of the voice coil. The only way to guarantee that nothing fails is to match the output of the amplifier with the wattage of the speakers.

 

"A speaker is an inductor."

Let me inform you. A speaker is not an INDUCTOR. It has inductance but it is actually a MOTOR. A LINEAR MOTOR and that's why the voice coils don't burn out. Because 90% of the energy into the speaker gets converted to movement of the cone. This only applies when the cone is moving and if it exceeds its excursion distance, the coil can heat up or be jarred against the end-stop. That's why the coil separates from the cone. And it can separate instantly on a strong excursion. That's the problem with a high power amplifier.

 

Well Colin55, you're only showing us that you too don't know what you're talking about. It's clear you've never operated or installed such a system, and know nothing about what experienced professional sound system operators or installers do.

Back to the OPs questions:

Fuses aren't used to control gain or SPL, they're used to limit current, primarily for safety, and are very effective and efficient devices for such.

It should be noted that any such system installed in a building in any place covered by building codes and other safety codes, MUST include current limiting devices such as fuses or other current limiting or automatic circuit opening devices, simply meet minimum legal requirements.

It appears to me, and apparently other respondents, that both fuses and signal attenuation devices should be used. If unauthorized, or unqualified operators have access to the system, you probably should install a dummy load wired in series/parallel with the other installed speakers, and let the chips fall. Other options have been presented, but the only really good one is to install the proper equipment, and restrict it's use.

 

The speaker is rated for 40W of continuous pink noise. It is supposed to be connected to its built-in transformer that has a max setting of 40W when the amplifier output is 70V or 100V RMS. But the 250W amplifier has a max undistorted output of only 44.7V RMS then the speaker gets only 16.3W from its transformer.
BUT maybe the transformer in the speaker is disconnected or maybe the users are having acoustical feedback howling that delivers much more power than music or pink noise. Maybe the recommended lowpass filter is not used and the bass is boosted instead?

Maybe using the speakers in series causes each one to resonate badly and smash against the magnet structure, breaking its voice coil. An audio amplifier has an extremely low output impedance that damps speaker resonance when there is nothing in series with it.

The transformer on the speaker has a few settings to reduce its power, why weren't they used??

What does Bose say about their speakers being damaged?

 

Fuses aren't used to control gain or SPL, they're used to limit current, primarily for safety, and are very effective and efficient devices for such.
A totally irrelevant answer.

 

Great.
But that wasn't totally apparent when you described them as an inductor.

Well, that may be because you're thinking of a different speaker than I. Any speaker with a coil attached to a paper cone, where the magnetic core is inserted into the coil (paper-sleeve bearing may exist from the cone), is by definition an inductor.

This is why I want more information from the OP, we don't know what kind of speaker he has.