Amp head and speaker mach

[J M Fahey]

Thanks again for your answers.
But, to be honest, I'm starting to getting more confused..
The article that jazbo8 posted says that the speakers must have more Watts than the amp.
This guy here says the opposite
Pro Audio 101: Matching Amps To Speakers And How Not To Blow Them Up. - YouTube
Any opinions?

Jazbo8 is right

Rod Elliott is right and explains it in far more detail, showing pictures of damaged voice coils and why it happened.

The guy in Pro Audio 101 is wrong.

So is JBL in *some* brochures stating the same (and where the Pro Audio guy probably picked the idea) .

That wrong idea applies to a problem which may happen in Home Hi Fi where a way too small amp may clip all the time (which by itself shows listener is tone deaf, continuously clipped "clean" music is horrible) and **tweeters** get burnt.
Not your case.

It also assumes that you will listen to music 10dB (1/10th the power) of what your amp is capable of.

THIS DOES NOT HAPPEN IN MUSICAL INSTRUMENT AMPLIFIERS, PA OR DJ DUTY where the NORM is to clip them often or all the time.
Which IS your case.

So get speakers which can stand at least 30% over what your amp puts out.

And if you want louder bass, leave that 8 ohms cabinet as is, get/make another cabinet similar size and with a similar speaker and plug it into the extra output in the back of your amp.

Now you will have roughly 50% more electrical power (which as noted above is not *that* much), plus DOUBLE the cone surface and DOUBLE the cabinet volume, which pushed by that 50% extra means you will move 2*1.5=TRIPLE as much air as before.

THAT is a noticeable increase, your pants will flap a lot more, you´ll drown the drummer, etc.

[Mick Bailey]

The guy in Pro Audio 101 is wrong.

Agreed. It's a risky business to rate the speakers exactly to the nominal output of the amp. Particularly a bass amp.

I can see the context where his assertion kind of holds up, but it relates to perception and misuse more than anything technical; This year I had a guy with a 1200w power amp (for PA use) and he'd blown a speaker. He said he was usually careful but on this occasion had ran the amp a little too hard because it was a much bigger venue. The music shop advised him to get higher-rated speakers to cope with the power. He did this and proceeded to run the amp flat-out for every show, with the limiter switched off. Now he had another burnt out speaker and a blown channel on the amp.

So the higher rated speaker gave him the false assurance that he could run the amp hard. Probably no where near enough thermal capacity to cope with the continually clipping signal. I wonder what that sounded like.

[jaysgr]

Thanks again everyone for your time.
I think I have got the idea.
Very helpful answers!
Thanks!

[vintagekiki]

I'm "old school electronics" (telecomunication engineer, 66 old) and for some years of experience, in terms matching speakers to the power amplifiers can be concluded

Speaker Power RMS = 1.5 x RMS power for clean rhythm or sound amps
Speaker Power RMS = 2 x RMS power for bass and keyboard amp
Speaker Power RMS = 2.8 - 3 x RMS power for MV or distortion sound amps

If you use 3 way crossover, distribution of the sound spectrum by the power would be approximately

100% for the woofer
40% for midrange
20% for tweeter

If you use two or more speakers it is desirable to be approximately the same efficiency (dB)

The apparent greater loudness of sound can be accomplished by using of two or more speakers the same efficiency.

my knowledge of the language https://translate.google.com/

sorry

[okcrum]

vintagekiki's numbers match up pretty well with the real upper limit on drivers. That is when they start to break up, because they've reached their mechanical limits. For bass/keys that's around 1/2 the RMS (thermal) power rating. If you stay below this you can use any amp with any cab. If you can't hear the speakers break up, you'll blow a lot of drivers no mater what rig you use.

[vertamps]

*piggy back thread question and thoughts

I have a guy with an Ampeg SVT 350H that suggest a minimum 4ohm load on the output.
Recently i updated the fan circuit on his amp (per newer schematics) for more reliable and faster fan operation.
He has an 8 x 6's cab that blew the 4 bottom speakers. I originally wired the remaining top 4 speakers to 8ohms which he then ran this in parallel with an 8ohm 15" speaker equipped cab to bring it to 4ohms load. He wanted the cab with the 6's modified so we agreed on putting a 15" Peavey Scorpion on the bottom (A jig saw would have been my best friend). The Scorpion is 8ohm so in parallel with the 6's his modified cab is now back to its original 4ohm impedance. We test ran his 8ohm extention cab with his 4ohm cab and he loves the sound. So at 2.7 ohms impedance i know his amp is working harder, i just told him to test it and listen for the fan to kick in high gear and let me know if it stays that way/overheating. Anyone have experience with with people pushing an Ampeg 350H beyond its suggested limits? The specs of the output mosfets are pretty darn good.

[J M Fahey]

Personally I use 4 x IRFP250 or 240 in my 300W Bass amps, with +/-65V rails and 4 ohm loads, without any problem at all, so these 6 "should" be able to handle 50% higher load (the added extension speaker) , as far as transistor specs go.

BUT there´s other factors: heatsink dissipation, you increased fan speed but max capacity is still the same.
Also you are pulling 50% more current from the same old transformer.

So it might hold, why not? , but it´s not guaranteed.

I´m a big fan of *powered* extension cabinets, the proper way to add *real* power, beyond the smaller increase gotten by overloading the original amp, so maybe you could consider getting some power amp kit, or repurpose any old head you have around or get for pennies at Craigslist or, say, an old Peavey CS800 or similar, which "nobody wants" and use it to drive the new speaker(s) .

I bet your friend will love that

[vertamps]

If he is willing to carry around another cab getting him to use an extra power amp for his extra 15" is a good option.
I can probably find a 4ohm 15" to replace his new 8ohm and do the wiring to make it work for an 8ohm total cab but then im stuck with the extra 15" to resell (or use).
I will be on the lookout.
Everything does go through the PA for shows and i believe his amp is only really' cranked for band practice. On stage his cabs act more as sweet monitors i believe.
But as you say, if holds is holds!