Power bandwidth

I was reading in The Absolute Sound magazine about power amps and came across this,

Another way that manufacturers exaggerate power ratings is by not specifying the power bandwidth. This term describes the frequency range over which a power amplifier can deliver the power out put specified. A power amplifier delivering 200W only at 1kHz is far less powerful than one that can deliver 200W over the full audio bandwidth of 20Hz-20kHz, the general range of human hearing. You’ll often see mass-market audio/video receivers with power-output ratings specified only at 1kHz, or from 50Hz to 20kHz. Further, stereo power am plifiers can deliver more power when only one channel is driven-look for the words “both chan nels driven.” The maximum power output should also be specified at a certain maximum level of distortion.

After reading this I looked up my M700’s specs and found exactly that. The power rating is at 1 kHz and there is no power bandwidth rating. Does anyone know why? What am I missing. How can I know what power bandwidth my amps are?

Gee …I thought full power bandwidth measurements were taken from
20hz to 20khz both channels driven at the same time…that was the power measurement “gold” standard even as far back as 60’s …

Has this changed?

Stereophile and other reviewers have long measured amplifier power into 8/4/2 ohms at 1kHz.

It is akin to the measurement of a speaker’s sensitivity at 1kHz.

I’ve looked up Adcom, Mark Levinson, Anthem so far and all give output power from 20 to 20k or a power bandwidthin the literature. I’m not sure what the article I mentioned means with the statement “…far less powerful…” but I would like to know and don’t know how to find out.

Pass Labs and Krell do not use the 20 to 20k figure nor the 1k figure. They just give the output power rating with no standards so I’m very confused. Maybe I’m making a mountain out of a mole hill but there should be standards.

I do think you are making mountain/mole hill.
The question you should ask yourself is, how does my system sound? And if there is a flaw, where is the weak link?
And then proceed logically; How big is your room? How efficient are your speakers? How loud do you like to listen? How close do you sit? etc etc etc etc etc…


RonP you bring an excellent point…

However the idea of power measurement made at 1khz only 1 channel driven
falls way short of the standard 20hz all the way up to 20khz and taking it further
again it used to be with a mix of frequencies to boot complicating the load on
the amplifier under test.

Box stores often rate mass market receivers one channel at 1khz…let alone
both or multichannels driven at once…more than likely those receivers
would croak under the sterling test of years past.

I do not think that the major players of highend audio would resort to lower
power measurement standards…

If I am making a mountain/molehill I’m not the only one.

the absolute sound

You can see the potential for misleading claims of
power- amplifier output. At one time, the abuses
were so bad that the Federal Trade Commission
(FTC) stepped in to regulate power claims-the
only example of an audio specification being regu
lated by a governmental body. The FTC mandate
for power ratings requires that the power rated be
continuous (not peak), that the load impedance
and bandwidth be specified, and that the total har
monic distortion (THD) be given at full power and
measured over the audio band. You may see a
power specification that reads “50Wpc continuous
(or RMS) power into 8 ohms, both channels
driven, 20Hz-20kHz, with less than 0.1% THD.” A
power specification including all these conditions is
called an “FTC power rating.” Some manufacturers
no longer adhere to the FTC-mandated power rat
ings, figuring that the issue has blown over and the
regulation is no longer enforced. You see fudged
power ratings for mass-market A/V receivers that
must now power five or seven loudspeakers rather
than two, and in single-ended- triode amplifiers
that can’t meet the FTC’s stringent requirements
for power-output specifications.

1 Like

Understood and agreed. But are you pondering theoretically or practically? Can you hear some weakness in the sound of your system?

1 Like

Hey Ron…

I believe it would be safe to think that the best known and regarded names of high end
amplifier designers and builders do so; the highest and tightest power
output standards…

Great discussion for sure!!!

Best wishes

Most models from any name brand will have ruler flat frequency response over the 20-20k region, and usually wider, so virtually all the rated power is available at those frequencies. In addition, for the sake of the music being played, you do not want to play at full power, or even near it. You want to stay within the linear region of the amplifier, which may be only to first 25-30% of rated power or less, with the remaining power being your headroom. For the vast majority of today’s amps, it’s totally unnecessary, therefore, to rate amps at more that the reference frequency.


This discussion reminds me of the Emotiva BasX A-300 I had for a year, before getting my M700s. Its published specs showed the full 20-20k frequency spectrum for its 8-ohm 150 w/channel rating, but for 4 ohms they only showed 300/channel at 1 kHz. That led me to believe the amp didn’t quite double its output into 4 ohms; when measured like-for-like, it probably produced something less. But that was OK - it still sounded good with my Maggies, and was definitely an improvement over my old ca. 1985 B&K. But not good enough to keep me from trying the M700s and falling in love with them. :grin:

1 Like

Every measurement has limits. Sure, power could be measured with a series of tones from 20Hz to 20 kHz but, in general, hasn’t been measured that way. IMO what matters most is consistency and transparency when comparing power amplifier measurements. JA at Stereophile has been measuring voltage gain and power at 1 kHz for years, he clearly denotes his measurements as being made at 1 kHz and the deep database he has accumulated over the years is a frankly terrific resource for objective apples-to-apples comparison of power amplifiers.

1 Like

I suspect the bass region is what prevented it from being able to double power from 20-20kHz.

I agree. JA’s approach sets the line for the benchmark to be used for all amps. I compare this with BHK Labs measurements of frequency response and distortion at specific impedances, and outputs.

@dconfalone This will give you a good idea of what really matters in the performance of the magnificent M700:
SoundStageNetwork.com | SoundStage.com - BHK Labs Measurements: PS Audio Stellar M700 Mono Amplifiers