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[Johannes Halvorsen]

Now, this is not realy pro sound by a long shot, but anyway:

I recently bought a couple of ye big ol' Cerwin Vega home speakers for a song and change. The venerable pair of VS-150s is now pulling back yard duty providing summer entertainment for me, 3 neighbours and the occasional deer or boar skulking around the area.

The speakers is powered by a dirt cheap "professional" amplifier (a König Amp 20000), and today I got a little carried away and ran the amp into audible clipping for a few minutes. The CVs are equiped with a fuse, and this eventualy blew on one of the speakers.

I picked out the blown glass tube fuse, and here comes the question:

The fuse is marked with "32V 3A". The speakers have a stated power handling of 400W. I haven't done much calculation of power/current/resistance since I was in uni, and I DO know that speakers are not linear ohmic resistors, but shouldn't 3 amps at 4 ohms be closer to 3A^2 x 4ohms = 36W...?

If someone knowledgeable would educate me, I'd be very happy!

[John Roberts {JR}]

And that is why the fuse blew... The important spec for a fuse is current not voltage, when working the voltage across the fuse is almost nothing.

Music is not steady state and constant but more dynamic,, loud then soft again. The fuse and the loudspeaker voice coils only care about the average power, while the amp must be capable of peak power. The fuse will open for more than 3A continuous, so will pass much higher short term peaks, until the average is enough to open the fuse.

JR

[Johannes Halvorsen]

Yes, this I under stand. The fuse is a slow blow type, so higher peak powers shouldn't be a problem. It blew because the amp was pushing to much nasty square wave signal down it's throat, thus delivering much more energy per time than the 200W or so RMS it's designed to deliver.

What I DON'T understand is the diffetence between the speakers 400W continous rating (√(400W/4ohms)=10A) and the fuse 3A rating.

Cerwin Vega, at least in the 90s, was hardly known for owerspecing power handling, where they?

[Ivan Beaver]

Yes, this I under stand. The fuse is a slow blow type, so higher peak powers shouldn't be a problem. It blew because the amp was pushing to much nasty square wave signal down it's throat, thus delivering much more energy per time than the 200W or so RMS it's designed to deliver.

What I DON'T understand is the diffetence between the speakers 400W continous rating (√(400W/4ohms)=10A) and the fuse 3A rating.

Cerwin Vega, at least in the 90s, was hardly known for owerspecing power handling, where they?

You also have to realize that the impedance rating of the speaker is only a "nominal" rating.  Often over much of the freq range the actual impedance is higher than the rating.  So less current is flowing.

There is no "simple answer".  The actual current is much less than the "wattage rating" would dictate.

As John said- music is very dynamic and not constant-so the "simple formulas don't work well.

THat is what makes it so hard to set proper limiters, figure out amp size etc.  MANY variables.

[Bob Leonard]

The fuse is not protecting the entire cabinet. The fuse is protecting the compression driver or tweeter after the crossover. The fuse rating you state, and without doing any math, should protect a 10 or 15 watt compression driver just fine, maybe even a 20 watt driver, but nothing more.. Once you clipped the amp you were asking the cabinet to withstand at least twice the amps power rating and the results can be predicted.

[Ivan Beaver]

The fuse is not protecting the entire cabinet. The fuse is protecting the compression driver or tweeter after the crossover. The fuse rating you state, and without doing any math, should protect a 10 or 15 watt compression driver just fine, maybe even a 20 watt driver, but nothing more.. Once you clipped the amp you were asking the cabinet to withstand at least twice the amps power rating and the results can be predicted.

I would argue about that.

Yes the "math" says it is not so-but due to the dynamic effect of music, and the heating effect on the fuse of the dynamic signal, a MUCH SMALLER fuse would be required than the "math" would say.

Do the  test on a sub that has no tweeter and the results will be the same.

My experience has been that a 400 watt cabinet would "draw" power about equal to a 3 or 4 amp rating on a fuse/PTC/circuit breaker type device.

This is with both noise and music.

This is done by simply inserting a "fuse" and cranking up the levels to an average of 400 watts (as read on an analog volt meter) and seeing what values it takes to blow it at some specific time period.

Yes it will vary depending on the type of fuse (slow or fast blow) etc, but the 3-4 amp range would be correct.

Put a sine wave in (at the point of course that the impedance is actually the same as the rating) and it is a different story.

[David Sturzenbecher]

I would argue about that.

Yes the "math" says it is not so-but due to the dynamic effect of music, and the heating effect on the fuse of the dynamic signal, a MUCH SMALLER fuse would be required than the "math" would say.

Do the  test on a sub that has no tweeter and the results will be the same.

My experience has been that a 400 watt cabinet would "draw" power about equal to a 3 or 4 amp rating on a fuse/PTC/circuit breaker type device.

This is with both noise and music.

This is done by simply inserting a "fuse" and cranking up the levels to an average of 400 watts (as read on an analog volt meter) and seeing what values it takes to blow it at some specific time period.

Yes it will vary depending on the type of fuse (slow or fast blow) etc, but the 3-4 amp range would be correct.

Put a sine wave in (at the point of course that the impedance is actually the same as the rating) and it is a different story.

Is this whole idea still in practice?   I can't say I have ever seen any speaker with a fuse in line with the drivers. Light bulbs yes, fuses no.

[Ivan Beaver]

Is this whole idea still in practice?   I can't say I have ever seen any speaker with a fuse in line with the drivers. Light bulbs yes, fuses no.

I haven't seen a fuse in years, and then mostly on consumer or "prosumer" products.

I used fuses for a short period of time-a couple of decades ago.

They got quite "irritating" very quickly.  Either by the fuse holder failing-or the fuses blowing and stopping the sound.

[John Roberts {JR}]

I haven't seen a fuse in years, and then mostly on consumer or "prosumer" products.

I used fuses for a short period of time-a couple of decades ago.

They got quite "irritating" very quickly.  Either by the fuse holder failing-or the fuses blowing and stopping the sound.

While this is TMI for the OP, fuses in series are generally not considered high fidelity. As the fuse heats up in use it's resistance changes, this can actually cause measurable distortion at LF... Perhaps not the top concern for applications that use fuses for protection.

Lamps change resistance even more that fuses but are generally used in HF paths where the time constants are shorter.

Peavey made a decent sounding HF speaker protection circuit where the lamp was shunted by a PTC fuse (that looks like a short circuit when cold). So the protection lamp was out of the audio path, until it was actually protecting, and at that point any distortion from the lamp resistance was a lesser evil than the driver releasing smoke.

JR   

[Ivan Beaver]

While this is TMI for the OP, fuses in series are generally not considered high fidelity. As the fuse heats up in use it's resistance changes, this can actually cause measurable distortion at LF... Perhaps not the top concern for applications that use fuses for protection.



 

Maybe you just need a "better sounding" fuse?

http://samtechonline.com/futhiendpefu.html?utm_source=googlepepla&utm_medium=adwords&id=18283950120

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