The Truth About Amplifier Power Ratings

A

admin

Audioholics Robot
Staff member
Ever wonder why the boom box you bought at Best Buy has a higher power rating than your dedicated two-channel power amplifier? Amplifier power ratings are usually honest in Hi-Fi equipment, but become very silly when it comes to the 'mass market' systems and even some of the latest Class D amplifier offerings. Few amps have a dynamic headroom of better than 1 or 2dB, and the greater the headroom, usually the cheaper the power supply for the rated power. This article explores the history of power ratings for consumer audio and also busts the myth about 'RMS' power.


Discuss "The Truth About Amplifier Power Ratings" here. Read the article.
 
M

markw

Audioholic Overlord
Putting this in one bite sized capsule is long overdue. It should be required reading for anyone in this hobby.
 
Wayde Robson

Wayde Robson

Audioholics Anchorman
Great article, it's very interesting how far they go to pump up those numbers.
 
K

kevon27

Annoying Poster
You need to do an article featuring the other side --- Ultra high End Gear. How much of their claims are true?

This is from Goldmund, describing their Telos 2500 amp.

Telos 2500
MonoPowerAmplifier

A new monster is born. Many of our faithful customers have already successfully adopted the Telos 600, which was received with unanimous acclaim as the best amplifier ever made. However, a few also regretted the extraordinary work of art we made in the Millennium Mechanics and the corresponding authority provided by the ultimate "Mechanical Grounding" construction.

As powerful as it is, the Telos 600 with its incredible dynamics, only tempted users of ultra-low impedance speakers where the current capability of the new amp seemed insufficient.

Now, both have their dreams come true with the new and ultimate Telos 2500 from Goldmund, adding to the incredible accuracy of the Telos 600 much more power on low impedance speakers and the exact mechanical construction of the famous Millennium amplifier. The Telos 2500 is the same size and same finish as the Millenium amplifier, except for the conventional front screen. The Telos 2500 has the final sonic authority given by the most advanced "Mechanical Grounding" construction ever made by Goldmund. And it can play louder.... And better.... A dream come true.
KeyFeatures

* 2500W on 1 Ohms FPP (Fast Pulse Power before clipping) Mono Power Amplifier.
* High-Power newest JOB 5 high-bandwidth circuitry.
* Thermal Grounding full implementation with much increased gold-plated copper buffer plate.
* 12 separate high-power transformers and separate power supply for the safety circuitry.
* Balanced and unbalanced analogue inputs and RCA digital input/output.
* Alize 6 input D/A converter with improved linearity and static protection.
* Full DC, HF and overload speakers and amplifier protection.
* Input attenuation adjustment (-9dB, -6dB, -3dB, 0, +3dB, +6dB, +9dB).
* 3 x Goldmund coaxial speaker cable and 3 x 5-way binding post connectors.
* Millennium amplifier chassis with Goldmund ultimate mechanical grounding.
* Unique circuit time accuracy : better than 100ps.
* Size and weight: 460 W x 515 D x 330 H, 75kg.
 
Z

zapzap

Audiophyte
There are sites on the net where the author(s) seems to think that there is some logic (however perverse) in the PMPO rating.
Well I'll give it a try: "Peak momentary power output" is a perfectly meaningful and useful term.
  • "Peak power" refers to the maximum value of the instantaneous power waveform.
  • "Average power" refers to the average value of the instantaneous power waveform (often incorrectly stated as "RMS power").
"Peak power" is easier to measure accurately and more comparable than average power, since it doesn't require a THD qualifier. The peak of a full-scale square wave and a full-scale sine wave are the same value. Average power ("RMS power") suffers from the flaw of being measured at 0.1% distortion by some people and 10% distortion by others, inflating the values.
  • "Continuous power" refers to the maximum output that can be sustained long-term, limited by the rate at which heat can be removed from the amp or speaker.
  • "Momentary power" refers to the maximum output that can be sustained short-term, in which heat build-up is not a concern.
Since nobody except Merzbow listens to continuous full-power waveforms, "momentary power" is actually the more musically-meaningful measurement. Real music has a crest factor of at least 6 dB, or a peak-to-average power ratio of 4, so "how loud does it get?" is determined by momentary power, not continuous power.

So, yes, if measured honestly, PMPO would be a better measurement than "continuous average sine wave power". That the value is usually pulled out of thin air does not invalidate the concept.
 
slipperybidness

slipperybidness

Audioholic Warlord
Well I'll give it a try: "Peak momentary power output" is a perfectly meaningful and useful term.
  • "Peak power" refers to the maximum value of the instantaneous power waveform.
  • "Average power" refers to the average value of the instantaneous power waveform (often incorrectly stated as "RMS power").
"Peak power" is easier to measure accurately and more comparable than average power, since it doesn't require a THD qualifier. The peak of a full-scale square wave and a full-scale sine wave are the same value. Average power ("RMS power") suffers from the flaw of being measured at 0.1% distortion by some people and 10% distortion by others, inflating the values.
  • "Continuous power" refers to the maximum output that can be sustained long-term, limited by the rate at which heat can be removed from the amp or speaker.
  • "Momentary power" refers to the maximum output that can be sustained short-term, in which heat build-up is not a concern.
Since nobody except Merzbow listens to continuous full-power waveforms, "momentary power" is actually the more musically-meaningful measurement. Real music has a crest factor of at least 6 dB, or a peak-to-average power ratio of 4, so "how loud does it get?" is determined by momentary power, not continuous power.

So, yes, if measured honestly, PMPO would be a better measurement than "continuous average sine wave power". That the value is usually pulled out of thin air does not invalidate the concept.
I agree with most of what you say here, and you obviously understand much of the technical aspects of electronics in general and measuring output in particular.

In my opinion, and the opinion of many authors of books on building and designing audio power amplifiers (Sloan, Cordell), the CONTINUOUS POWER is the design parameter that will give you the best representation of amplifier performance.

When I design an amp power supply, I design it to be robust to be able to supply continuous power.

Edit--Good luck getting an honest or any continuous power spec from a vendor! You will almost certainly have to measure this yourself!
 
P

PENG

Audioholic Slumlord
Edit--Good luck getting an honest or any continuous power spec from a vendor! You will almost certainly have to measure this yourself!
You got that right, most lab measurements found online referred to continuous power as sustained power for a short duration, 1 minute to a few minutes, may be ten? I am not sure but not truly continuous for sure. I suppose there are a few manufacturers that probably do rated their amp correctly, or almost correctly as "continuous", Krell is likely one of those. Emotiva, Parasound, NAD and Anthem's, definitely not. In real world, even continuous for a couple of minutes is good enough to pass the point of diminishing return for normal hifi music enjoyment.
 
slipperybidness

slipperybidness

Audioholic Warlord
You got that right, most lab measurements found online referred to continuous power as sustained power for a short duration, 1 minute to a few minutes, may be ten? I am not sure but not truly continuous for sure. I suppose there are a few manufacturers that probably do rated their amp correctly, or almost correctly as "continuous", Krell is likely one of those. Emotiva, Parasound, NAD and Anthem's, definitely not. In real world, even continuous for a couple of minutes is good enough to pass the point of diminishing return for normal hifi music enjoyment.
Yeah, to be clear, driving an amp with a sine wave is going to be way more strain on an amp than any real world program material.

And, when you are doing that, you are almost always driving the amp right up to the point of clipping, that's kind of the point of that test measurement.
 
M

Mr Moscode

Audiophyte
I sold stereos at a Lafayette store in Ithaca, NY the summer of 1970. At that time there were FTC rules that prohibited lying about power ratings and only RMS values were permitted.

However the forces of commerce found their way into undoing truth in advertising, as they have found their way into undoing truth itself. Now you can say anything about anything. I say this as I listen to my 5kw cell phone. Which is better than a 4kw cell phone.

That said the most brutal, power wise, test of an analogue amp is 1/3 power as it creates the most waste heat in the output stage. My Moscode amps could always do any output all day long with normal cooling. I never liked to over rate the amplifiers. The Moscode 300 actually does 165 wpc into 8 or 330 both channels driven. RMS, of course.

I see plenty of cheap car amps that claim to put out more power than the fuse lets in. A shame, really.

Class D - Digital amps - that's a whole other world of misinformation. If you really want to know what is what you have to read the paper that comes with the modules that the companies use. One of your other articles mentions IcePower and points to that.
 
M Code

M Code

Audioholic General
I sold stereos at a Lafayette store in Ithaca, NY the summer of 1970. At that time there were FTC rules that prohibited lying about power ratings and only RMS values were permitted.

However the forces of commerce found their way into undoing truth in advertising, as they have found their way into undoing truth itself. Now you can say anything about anything. I say this as I listen to my 5kw cell phone. Which is better than a 4kw cell phone.

That said the most brutal, power wise, test of an analogue amp is 1/3 power as it creates the most waste heat in the output stage. My Moscode amps could always do any output all day long with normal cooling. I never liked to over rate the amplifiers. The Moscode 300 actually does 165 wpc into 8 or 330 both channels driven. RMS, of course.

I see plenty of cheap car amps that claim to put out more power than the fuse lets in. A shame, really.

Class D - Digital amps - that's a whole other world of misinformation. If you really want to know what is what you have to read the paper that comes with the modules that the companies use. One of your other articles mentions IcePower and points to that.
What is pertinent for an audio amplifier be it analog or class D..
Is "What are its output capabilities as the load impedence decreases..."
Note that loudspeakers are not like the load resistors (only resistance) used in a lab test, as the impedence decreases as the amplifier is drawing more current.. So the power supply and its capabilitites must be capable to deliver enough voltage/current to support teh amplifier stage or else the resulting power output drops big time. The other part of the equation is @ what frequency is the amplifier being asked to deliver and for how long.. The lower the frequency and the longer the time is demanded of the output amplifier/power supply the more demanding the requirements..

Just my $0.02... ;)
 
M

Mr Moscode

Audiophyte
In general, you are right.

There's more to it than that. Electrostatic's impedance drops the most at high frequencies but there usually isn't that much power demand in the upper 2 octaves. Efficiencies of speakers in general are not measured broadband so that info is a missing variable.

Some speakers like Magnepans are essentially resistive loads.

Usually the impedance goes up as you approach the resonant F of the speaker.

There are so many things against music being rendered realistically it's amazing it works at all.
 
Z

zapzap

Audiophyte
Edit--Good luck getting an honest or any continuous power spec from a vendor! You will almost certainly have to measure this yourself!
Yeah, with most manufacturers it doesn't matter whether they spec PMPO or average power; they're both lies. :D
 
I

IrwinElectronics

Audiophyte
Well of course the output is more than the fuse lets through or the limits of the power supply. It's an Amplifier. It's job is to convert a low power signal into something much higher!
Hey, it follows the same logic as the mathematical equations, does it not?
 
slipperybidness

slipperybidness

Audioholic Warlord
Well of course the output is more than the fuse lets through or the limits of the power supply. It's an Amplifier. It's job is to convert a low power signal into something much higher!
Hey, it follows the same logic as the mathematical equations, does it not?
Output power is ALWAYS limited by the power supply..........output power must ALWAYS be less than input power, the loss of power for biasing the semiconductor devices manifests as heat.
 
highfigh

highfigh

Seriously, I have no life.
Output power is ALWAYS limited by the power supply..........output power must ALWAYS be less than input power, the loss of power for biasing the semiconductor devices manifests as heat.
If they're making power, I think the Department of Energy will want to have a chat and the Physics community will disagree with them.
 
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