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kelticwizard 13th January 2007 02:18 PM

What's With These Inflated Power Ratings For PA Amps?
 
When I first became interested in audio equipment back in the 70's, all the buying guides said that when choosing an amp you should go by the "continuous", or RMS, power rating.

This, we were told, was a fairly new system adopted by reputable audio companies, because before companies were rating amps by "music power", which the amp could only achieve for small fractions of a second. So, I was told, you know you have a quality amp or receiver is the manufacturer gives you the RMS rating instead of the music power rating.

And indeed it seemed to be true, because the only companies which rated their stuff "music power" were cheap units, such as from JC Penny, Yorx, and the like. All the quality stuff used RMS ratings. And when the RMS power and the "music power" were compared, the "music power" was often ten times or so the RMS power.


This system seemed to be true all the way through to today until I entered the world of PA amps. A friend bought a system to do some DJing and the beginner amp that went with it-Gemsound, a real cheapie-rated itself as 1500 watts. That was music power.

I tried to explain to him that was music power, the amp might only be able to deliver that power for a small fraction of a second, but he wasn't listening because his brother down in Florida had a QSC with 3,000 watts and it really filled the room, etc.

Over time, I plan to try to explain things to him so he can grasp this.

Anyway, his crappy Gemsound amp crapped out, and he needed a new one fast. A local store had a Pyle PT-2000, and I figured that since Pyle had a decent rep in the car audio world, their PA amps have to be considered reasonably OK for a guy who does small gigs perhaps once a month or so. Maybe not Crown or QSC, but serviceable until his DJ workload increases.

Looking at the specs of the Pyle, I see the usual RMS specs, but the unit named itself after the "music power" spec it also included. And apparently, DJ's also go by that "music power" spec, not RMS spec.

At least the Pyle's "Music power" specs were only about twice the RMS power, not ten times as the units in the seventies rated it.

All this seems rather odd, as I thought this whole thing was settled back in the seventies.

Am I missing something here? Why would otherwise reputable sound companies start using a long-discredited power rating system for their amps?

GRollins 13th January 2007 02:32 PM

Offhand, I would suggest that it says something about the IQ of the average buyer. I know that's uncharitable, but the fine print, meaning RMS vs. Music Power vs. Average vs. With The Wind At Your Back, is easily ignored in the quest for bigger numbers.
And if you're thinking that I'm short on sleep, hence grumpy, you're right. But I'm right, too...just phrasing it in a more curmudgeonly manner than usual.

Grey

GRollins 13th January 2007 02:36 PM

Oh, and while I'm at it...have you noticed the speaker specs? You'd think that +-3dB would be good enough, but I keep seeing the term "Useable" which means -10dB. So "Useable" to 30Hz, actually translates to something more like -3dB at 80Hz.
Harrumph!

Grey

kelticwizard 13th January 2007 03:15 PM

Quote:

Originally posted by GRollins
Oh, and while I'm at it...have you noticed the speaker specs? You'd think that +-3dB would be good enough, but I keep seeing the term "Useable" which means -10dB. So "Useable" to 30Hz, actually translates to something more like -3dB at 80Hz.
Harrumph!

Grey

Hi Grey, good to see you again. Haven't run into each other much lately :)

Yes, those "frequency range" specs get me too. Back in the late seventies, I was in the town's music store. This store had the best reputation around, and even people from larger cities with music stores in their own town had heard of this place. I was there to get a high efficiency 15 for a hi fi unit I was building.

Anyway, I heard the salesman on the phone to somebody, and the sales man says, "Yeah, they list the frequency range going down to 30 Hz, but that doesn't mean much. The thing could be 20 decibels down at 30 Hz."

Right then, I got a warm feeling that this music store had earned it's reputation.

Cal Weldon 13th January 2007 03:35 PM

I remember once many moons ago, I was in a department store and the customer asked for the wattage rating on some receiver. The salesman looked at the back of the unit and quoted the the VA rating for what the unit draws from the wall.

I have a much different approach. I like to understate the power produced by the amp and the wattage the speakers can handle. Much preferred effect over leaving them wanting. Kinda like Altec used to do with their speakers. Wasn't the model 19 rated at 50 watts?

Zero Cool 13th January 2007 04:06 PM

Pyle is not a name brand in PA amps and frankly they aint what they used to be anyway! they dont have much of a rep in the car audio world anymore either. they have long since changed hands and are now made oversea's someplace. Pyles are Piles now.

For cheap PA amps the haflers seem to be doing ok. Guitar Center has them on sale all the time and they seem to work well enough. I have no idea how well they would stand up to nightly DJ abuse however. For that i say buy a used QSC!

Even the reputable companies play tricks with there numbers. try finding a meaningfull spec for a crown. they rarley use the EIA standard method of rating there amps.

Peavey is the who started all this way back when, they would list there amps with a Voltage rating. 40V rms or they would list there power at 4 ohms. Now most MFG's list there power at 4 ohms instead of the more widely accepted 8.

QSC is about the only PA amp maker turning out a consistant product. But you pay for that quality as well. There specs are real but they are using the peavey trick of rating everything at 4 ohms as well. but there numbers hold up. I can throw a QSC in any job and know that it will still be there in 10 years.


Zc

kelticwizard 15th January 2007 02:16 AM

Thank you all for your responses.

Zero Cool, I appreciate your suggestions. However my friend's first gig with the Pyle amp went flawlessly, and now he's fallen in love with it. I am starting a separate thread in Musical Instruments forum aboutt Pyle PA amps.

About rating the amps at 4 ohms, I can't complain if they are clearly labelled. Not too long ago, audio magazines were listing 4 ohms specs for amps as well. For awhile there, it looked like even home audio was headed 4 ohm.

Anyway, I think a 4 ohm spec is good for PA for another reason. Most PA cabs are 8 ohms. However, if a DJ is playing a larger hall, he will be tempted to bring two sets of speakers, (which give him a 3 dB increase in sensitivity and 6 dB more total output), and when you parallel two sets of 8 ohm PA cabs, you get 4 ohms. So this 4 ohm business makes sense for when you need power the most-when the venue is so big you need two sets of cabs.

GRollins 15th January 2007 02:36 AM

(Assuming the amp can deliver more power into half the load...)

Grey

NVMDSTEvil 15th January 2007 09:05 AM

If they list it as operable at 4 ohms and it isnt, you could always sue them for false advertisement.

At least they dont list "PMPO" specs.... :bigeyes:

AndrewT 15th January 2007 03:12 PM

Hi,
I think Grollins is alluding to the fact that virtually no amplifier can double it's maximum power output into half load impedance.

This applies when both impedances are within specified range.

Some (poor=cheap) amplifiers can just manage the same power into 4r as into 8r, again the amp is safe into 4r.

Once again I refer to an amp I built decades ago. 110W into 8r 200W into 4r but this is exceptional and DIYers continually refer to my strategy as overkill. Few commercial amps can manage better than +2db into half impedance.

That reference to 6db more by doubling up ONLY applies to the lower mid/upper bass frequencies when the distance between drivers fits the model that predicts the efficiency increase. At higher frequencies lobing ruins the even spread of power.


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