Fundamental question about power output and speakers

[jude]

Something that has been confusing me since day one of looking at amps is matching up power output with speakers etc. From the 36W forum, I copied this answer which is with regard to a 72W amp:

You will eventually need speakers that can cope with it. Your 8 x EL84 suddenly MUST have a 4x12" of 200-250-300W capability in order for the speakers to survive the square waves.

/Richard

Now, how can someone calculate how much power the speakers have to handle if the amp is supposed to put out a certain number of watts? For a theoretical 72W amp, why would I need 200W+ of capability?

Starting at the beginning, if I were to have a 2xEL84 amp, like ooooh, let's say the 18watt, and it were to put out a theoretical 18watts, what should the speaker rating be to compensate?

Does the amp put out 18watts before clipping? Does it put out more than 18watts after that, or is it all distorted sound at 18watts, square waves and all?

Moving on from that, if I were to have a 2xEL34 Marshall, putting out 50W theoretically, would it cause damage to 2x12 G12M speakers if it were ran at 10 all day? Do you get 50W output at 10? At 7? At 3?


Sorry for such simple questions, but between AX84, ampage and even tone-lizard, I don't see any right way to sort this stuff out. Even Randall Aiken has nothing. Could anyone point me in the direction of a scientific explanation to all this?

Thanks,

Jude

p.s. more annoying question to follow, I'm sure!

[Ballz]

Hmm...yes, it´s a jungle. Let´s see -

Speakers are rated at "nominal" watts, "continous" watts, "sinus" watts , "music" watts/power, "RMS" or "AES" watts, "program" watts and "Peak power" (and a few more "classifications") these days. Most of these ratings are junk, and only there to inflate the numbers to make sellers/manufacturers look better. Ever heard of a 200W computer speaker with a 4" woofer, a plastic enclosure and a 1" tweeter?

Let´s look at a reasonable way to classify what come out of the amp (and what the speaker needs to cope with) - RMS, i e the amount of ENERGY that the amp puts out. Without becoming too much of a math buff, I hereby state that the RMS energy increases a lot as we go from clean (sinus wave) to more or less square wave when running full tilt (distorsion).

In the process (from clean to crunch), the amp can easily go from class A operation via AB, B and even into class C operation for short periods of time, making the 18-watter put out peaks that correspond to maybe 100W (or more) for a split second. This depends on the bias scheme used in the amp and a few other factors - some amps stay where they belong, but at a price.

These peaks are not exactly the wet dreams of your 10W speakers, especially if they have old style paper cones and coils made of very thin copper wire. Add these attack peaks to high average RMS, and run like that for 2 hours or more, then watch paper cones fly and coils burn....

Use this example on a 50W amp. Without doing any math at all. I´d personally go for a safety factor of 4 to 6 if the amp is designed to be a distorsion amp like the TMB 18W. With a clean amp like a Fender Twin, I´d stay on factor 2-4.

Hifi guys recommend a factor 2 or better. Everything below is risking both the amp and the speakers, if you ask me. The only thing that hurts is your pocketbook - upsizing the power handling can be expensive, but buying new amp hardware AND speakers is worse.

I am at work, so have to stop here. Check Google for "speaker power RMS peak amplifier rating" and similar buzzwords, you´ll find lots of stuff. I did, even if it´s mostly Hifi-related.

Hopefully someone else chimes in, so you get some real facts also.

Cheers /Richard

[Wannatone]

Richard,

just my speaker $.02:

A pair of 15 watts rated Blue´s holds up pretty well in an AC30 and there are not too many that blow over the years. That´s how they built amps since the 50´s. A low rated speaker is an essential part of the sound most of us are after, IMHO.

Personally, I don´t like the sound that comes out of a way overrated cab like a JTM45 played through a 300 or 400 watt 4x12. But, I do like the sound of hard driven low rated speakers.

I agree that too much low end response of the amp can be dangerous to light weight speaker cones. But, spikes do generally not hurt voice coils since we are talking heat and heat is a pretty slow process. What kills voice coils is mainly excessive heat over time... we´re talking average power, not peak.

Regards,
Adrian

[Ballz]

Sure a pair of Vox speakers stay alive - I never saw a company in the 60:s that rated their products at true face value. Hammond still underrate their trannies , and I wonder if a Vox speaker wouldn´t stand 30-40-50W without any trouble.

What I am talking about is hours of high RMS/distorsion/squarewave and the combination of mechanical wear and electrical fatigue that evetually makes the speaker POP. SPikes are OK, but not if the speaker cone is on the halfway back into the speaker when the next spike hits..

Guess this can be discussed at length. I have no trouble listening to a speaker cab that is loafing along at safe levels. I also like a pushed speaker, but I always cringe when I hear it.....

Cheers /Richard

[jude]

Thanks for your replies, guys.

First, in response to the AC30 comments: My dad used to have one in his band and they used to blow speakers in them all the time trying to be heard at the back of a full dance hall....and then after lugging around a heavy VOX amp that kept breaking and tearing speakers, they got these great new amps called Peaveys that were a lot lighter and louder....and then he traded the AC30 for a SKIL saw.......... Oh! the horror stories I hear!
Bear in mind, he tells me, that at the time an AC30 was a beginners band's amp, not like a Marshall that the professionals, had, and that getting a SKIL saw and not having to use hand saws anymore for his carpentry work would have been like getting a top-of-the-line DeWalt multi-angle do-everything radial arm saw today..


Anyways, back to speakers and the blowing-up thereof:

If I had an 18Watt and put it through a Celestion G12M (25W) speaker, and ran it at 10 all day and then put a big distortion pedal at the front end, would the speaker be destroyed?

How about if I did the same with a Marshall 2xEL34 (50W?) into two G12M, in parallel?


The basic reason for all this is that I have a cabinet with two Vintage 30 (70W? each) in parallel, which should give me 140W of handling power. I'd like to swap them for the G12M or G12H30 to see what the big deal about these speakers is and would like to know if I could use these type of speakers to give voice to a 2xEL34 (1987) amp.

Any more takers?

Thanks again,

Jude