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?
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?
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
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 said:
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.
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?
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?
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
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
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.
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.
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.
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.
According to my friend who worked at Rockford Acoustics (and is now buying out their equipment from the GR, MI facility), there are set standards when it comes to measuring output power. However, there are no regulations requiring companies to label per specifications. Essentially, an audio company can put whatever number they want on the box, though I'm sure they wouldn't since they'd be sued in a heartbeat for false advertising if anyone with a signal generator and multimeter took a look at it.
Videtich:
Well, it might be possible to sue them if they say it puts out 150 watts RMS and it doesn't come anywhere near that.
If they say an amp that actually is rated at 30 watts RMS puts out 150 watts "music power", "max power" or that oldie but goodie, "PMPO", they probably would not be sued because the amp can put out out that power for a fraction of a second.
Fact is, in the seventies the real cheapie amps with unrecognizable names did list wattages way, way over the RMS rating.
I'm just amazed that decades after the power ratings were standardized in the hifi world, the related PA amp world is still messing around with these silly non RMS ratings.
Well, it might be possible to sue them if they say it puts out 150 watts RMS and it doesn't come anywhere near that.
If they say an amp that actually is rated at 30 watts RMS puts out 150 watts "music power", "max power" or that oldie but goodie, "PMPO", they probably would not be sued because the amp can put out out that power for a fraction of a second.
Fact is, in the seventies the real cheapie amps with unrecognizable names did list wattages way, way over the RMS rating.
I'm just amazed that decades after the power ratings were standardized in the hifi world, the related PA amp world is still messing around with these silly non RMS ratings.
AndrewT said:
Well, I have not designed any large amps like Andrew T, but my understanding is that total wattage is the max available voltage times max available current.
So if the max voltage is eight times the max current, you have an amp that delivers most of it's power into an eight ohm load.
If the max voltage is four times the max current, your amp delivers most of it's power into a four ohm load. So if you hook an 8 ohm speaker up to an amp whose max voltage is four times it's max current, I would imagine you would deliver half the power that you would get if you used a 4 ohm speaker. I think most amps seem to be somewhere in between though, the max voltage is less than eight times but more than four times the max current. So yes, you don't get twice the power if you halve the load.
Incidentally, while we are on the subject, back in the seventies amp manufacturers spoke of "headroom", which meant the amp had extra voltage beyond that required for an 8 ohm load. A friend's big Phase Linear, for example, was really more of a 14 ohm amp than an 8 ohm amp.
I notice that headroom number is not listed these days, so I guess they don't have that extra voltage. Am I correct on this?
actually "dynamic headroom" is a real number expressed in db above continuous rated RMS power, and is measured with a special test signal (which was also standardized by the IHF) where a 1khz signal is applied to the amplifier, sufficient to run the amplifier at 50% power. the input signal is amplitude modulated to produce an input envelope that is constant for 80ms, and increased a variable amount for 20ms. the signal "spikes" are then increased in amplitude until the amplifier clips during the spikes. the RMS power at clipping of this test is compared to the continuous rated power of the amp, and the result is expressed in db.
for instance the dynamic headroom rating of the Apt 1 amp was 3db at 8 ohms and 4 ohms. it was 2db at 2 ohms, and 1db at 16 ohms. dynamic headroom has a lot to do with power supply design, and not so much with the output stage.
the unscupulous take the same test signal, and instead of using the rms value of the clipping point, probably use the peak value (or even worse the peak to peak wattage) and use it as the basis for their "wattage" rating. they may even modify the standard test signal by shortening the spikes to 10 or even 5 ms to get even higher numbers
for instance the dynamic headroom rating of the Apt 1 amp was 3db at 8 ohms and 4 ohms. it was 2db at 2 ohms, and 1db at 16 ohms. dynamic headroom has a lot to do with power supply design, and not so much with the output stage.
the unscupulous take the same test signal, and instead of using the rms value of the clipping point, probably use the peak value (or even worse the peak to peak wattage) and use it as the basis for their "wattage" rating. they may even modify the standard test signal by shortening the spikes to 10 or even 5 ms to get even higher numbers
Attachments
Hi,
I believe that dynamic headroom was invented by designers at the behest of unscrupulous salespersons to make their cheap/underspecified amplifiers seem more powerful.
I further believe dynamic headroom is a measure of the "badness" of an amplifier. i.e. more headroom = more sag in the supply rails when asked to work hard.
It is misused by advertising departments to seem an asset. If you believe in my opinion then low dynamic headroom is the better selection criteria.
Uncle,
I appreciate your contribution on how DH was measured.
Can you reference a way that us DIYers could mimic such a test?
Could a computer generate the envelope for instance?
Is there a software package available as freeware that has such a facility in it?
BTW,
your diagram show a psuedo scope view.
Scaling vertically from that posting, I see a voltage output at the low level of about 25% of maximum, that implies a low power signal of about 10 to 15% of maximum. Similarly the horizontal scale is nowhere near the 20:80 split, it looks more like 6:94. Can you confirm if other ratios are actually specified in the IHF test procedure?
I believe that dynamic headroom was invented by designers at the behest of unscrupulous salespersons to make their cheap/underspecified amplifiers seem more powerful.
I further believe dynamic headroom is a measure of the "badness" of an amplifier. i.e. more headroom = more sag in the supply rails when asked to work hard.
It is misused by advertising departments to seem an asset. If you believe in my opinion then low dynamic headroom is the better selection criteria.
Uncle,
I appreciate your contribution on how DH was measured.
Can you reference a way that us DIYers could mimic such a test?
Could a computer generate the envelope for instance?
Is there a software package available as freeware that has such a facility in it?
BTW,
your diagram show a psuedo scope view.
Scaling vertically from that posting, I see a voltage output at the low level of about 25% of maximum, that implies a low power signal of about 10 to 15% of maximum. Similarly the horizontal scale is nowhere near the 20:80 split, it looks more like 6:94. Can you confirm if other ratios are actually specified in the IHF test procedure?
Hi all
Here's how I think the old 70's amp power ratings went:
unregulated supplies with small caps = higher voltage than sustained on load
say Vcc (quiescent)=20% more than Vcc (average)
music power = clipping level for short burst (peak)
power rating compared with RMS=1.2 *1.2 * 2 = at least 3x higher.
I think sometimes the clipping level (10% distortion) figure for output power was also used as this allowed the peak output to increase even more.
Peak music power out = peak misleading power out.
cheers
John
Here's how I think the old 70's amp power ratings went:
unregulated supplies with small caps = higher voltage than sustained on load
say Vcc (quiescent)=20% more than Vcc (average)
music power = clipping level for short burst (peak)
power rating compared with RMS=1.2 *1.2 * 2 = at least 3x higher.
I think sometimes the clipping level (10% distortion) figure for output power was also used as this allowed the peak output to increase even more.
Peak music power out = peak misleading power out.
cheers
John
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