The challenge:
I have to create a protection circuit for an existing installation.
The Setup:
one Professional Power Amplifier with 2x250W output (that is 250W per channel).
per Channel we have 2 4Ohm 40W Ceiling speakers connected in series, so for the Amp it looks like one 80W 8Ohm speaker (am I correct in this?).
Because the users tend to increase the volume beyond the set levels (especially when no one of the managing organisation around), the output of the amplifier gets higher then the 80W's (or 40W?) the speakers can stand and they blow.
These speakers have blown and been replaced now several times. This is not only costly but also a big nuisance.
I have been asked to create a protection circuit such that the speakers wont blow.
I cannot replace the speakers by higher power speakers because I have not been able to find 11 inch ceiling speakers.

Possible Solutions:
1. place a fuse in circuit with each speaker.
The questions is is this effective enough and if so what type of fuse would I need?
2. use 70V or 100V circuitry with transformer
although the speakers can be switched to use this, the AMP does not have any 70V or 100V output.
So not sure if we can use, and how? And is this enough protection?
3. Built a speaker protection circuit
how? any designs? Passive or active protection? where to place, at AMP, or at speaker? I dont have any power point at the speakers so I cannot power any active circuitry.
4. combination of above.
If so how and what is best combination?

Sorry for the long intro but I wanted to set this up correctly such that I can get some help with this.
Hope that there is anybody in this forum who can help me to solve this issue.
Thanks in advance for your input and efforts.

 

Google "Speaker Protection Circuits". Having spent some time in the audio industry I have my doubts about their efficacy. They use fuses and relays and amplitude detectors with low pass filters. Go ahead and do the research and let us know what you find. Most audiophiles I know keep a careful watch on the volume control.

 

A fuse in series with each speaker should provide adequate protection.
Use a regular regular or instrument fuse, although a slo-blo might work better if you are listening to music with a lot of heavy bass.

Below is a fuse selection chart from a Crown amplifier:

 

Google "Speaker Protection Circuits". Having spent some time in the audio industry I have my doubts about their efficacy. They use fuses and relays and amplitude detectors with low pass filters. Go ahead and do the research and let us know what you find. Most audiophiles I know keep a careful watch on the volume control.

Yes have done my research. As this is unknown territory for me I am getting confused. Many claim to be a good or even the best way to to protect the speakers, but somehow none of them have really convinced me, and on top my limitation is that I dont have power where the speakers are fitted, so any protection circuit I will have to design, construct and install near the amplifiers where I have power to power any protection circuit.

I put my trust more in people in this forum who share and support based upon their knowledge and expertise. And after some input I can then hopefully come to some design I feel will work.

 

The real problem with the fuse alone is that you have no effective way of testing it without potentially damaging your expensive equipment. The circuit above for a turn on transient muter is disallowed on this forum because it connects directly to the mains without a transformer. Don't even consider it - it is dangerous and potentially life threatening. Getting in the habit of always turning the volume down before turning the equipment off and on will pay enormous dividends.

 

You can use a PolySwitch resettable thermal fuse.
I have also seen incandescent light bulbs being used to protect loudspeakers.

 

The simplest and cheapest and safest and most reliable answer is to put 8 speakers in the ceiling in series-parallel.

 

The simplest and cheapest and safest and most reliable answer is to put 8 speakers in the ceiling in series-parallel.

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

 

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 ???????????????

 

OR, attenuate the signals into the amplifier so that the outputs are less than 80 W when the knob is turned up to ten.

ak

 

OR, attenuate the signals into the amplifier

How are you going to do that?????????

 

The simplest and cheapest and safest and most reliable answer is to put 8 speakers in the ceiling in series-parallel.

This is for an existing installation.
How can that be the simplest and cheapest?

 

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.

 

A speaker is an inductor.

It's more than an inductor.
The energy converted into sound as well as the speaker winding resistance generates a resistive component to the speaker impedance.
And high peak sounds can physically damage the speaker cone without burning out the speaker.

 

It's more than an inductor.
The energy converted into sound as well as the speaker winding resistance generates a resistive component to the speaker impedance.
And high peak sounds can physically damage the speaker cone without burning out the speaker.

I'm well aware of how a speaker works - The technology isn't new.

I'm still waiting to see more information about the speakers. Are the D/C powered, A/C powered, what voltage, what frequency response are they rated for, part number, stamped value, photograph- anything that would help us determine the limits the component is meant to be operated in.

 

I'm well aware of how a speaker works

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

 

we have 2 4Ohm 40W Ceiling speakers connected in series,

Are the D/C powered,

?

 

I would immediately add 3 ampere non-delay fuses while considering other possibly better options.

3 A into 8 ohms is 72 watts. A 3 A fuse will take some time to blow at 3 A, but be fairly quick at 4 A (128 W). Unless the power applied to a speaker is sufficient to damage the cone or suspension, which is possible, the limitation is in large part temperature rise in the voice coil, which is proportional to the square of the current though it. The fuse responds in the same way. In a situation where someone decides to crank up the volume, the comparatively slow approach to overcurrent has a reasonable chance of blowing the fuse before ripping the speaker to shreds, especially if the amp has a push button volume control that limits the rate of change.

I would be inclined to use two fuses, one accessible if necessary and another, rated for a bit high current, somewhere inaccessible. That way if someone stuffs the accessible fuse holder full of aluminum foil, there is still some measure of protection for the speaker.

A fast "active" circuit to limit voltage or current is probably better than a fuse.

If you just have one set of speakers, going "constant voltage" probably isn't worth the bother. Essentially what you wind up doing is using two transformers to step down the voltage from the amp. You could do the same with a single transformer, but finding a suitable one off the shelf might be impossible.

Resistive attenuator "pads" can be used, but aren't very practical at the power level required.

 

I would go with the lower value fuses rated for about 1/2 the total speaker power or 40W.
It's unlikely that the average music power will exceed that at normal listening levels.
For that a 2A fuse should work.
Only go to a larger fuse if that blows too readily.
Finding out the fuse is too small, is less catastrophic, then finding out it was too large.

 

You can use a PolySwitch resettable thermal fuse.
I have also seen incandescent light bulbs being used to protect loudspeakers.

Problem with them is each time they trip and reset they reset to a different value of resistance

I would go with the lower value fuses rated for about 1/2 the total speaker power or 40W.
It's unlikely that the average music power will exceed that at normal listening levels.
For that a 2A fuse should work.
Only go to a larger fuse if that blows too readily.
Finding out the fuse is too small, is less catastrophic, then finding out it was too large.

What he said -- See post#5