October 1960 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Most of us, long before being introduced to the concept of power
in electrical circuits, learn about it in terms of mechanical
power and/or sound power. It takes some doing to abandon the
esoteric nature of power and be trained to grasp the scientific
and mathematical aspects of power in all its forms. When the
driving source is steady state or a pure sinewave, life is relatively
simple, but such is more often than not an exception to the
system being studied. Here is a nice, short treatise on the
concept of sound power that will augment your earlier-learned
knowledge of music power rating.
Music Power Rating - Help or Hindrance?
By Norman H. Crowhurst
This author's provocative view is that this amplifier rating
is a step in the right direction, however it may not tell the
whole story about hi-fi performance.
The question that triggered this article is, "Does the new
Music Power Rating help tell the quality of amplifier ? If so,
how?" The answer to this question will be covered later, but
mainly some concepts, notions, and viewpoints need clarifying
first. There will be some explaining to do - why the particular
Music Power Output definition, as used by the IHFM (Institute
of Hi Fidelity Manufacturers) standard for rating amplifiers,
was chosen, as well as what it can tell us.
For a long while, the accepted methods of rating or specifying
amplifier performance have been recognized as inadequate for
assessing the relative merit of different products. This has
been correctly related to the fact that music, or other program
material, is different in many respects from the pure-tone sine
waves used for testing amplifiers.
These two standards differ in two respects: all test tones,
whether one or two sine waves, or a square wave, are applied
steadily, while the tones in music are constantly changing,
often rapidly. Apart from their transient nature, musical waveforms
are much more complicated than anything used for testing, even
when the tones are steady.
Engineers in the high-fidelity industry have, for some time,
been concerned by this discrepancy. Some have tried to do something
about it and some have even dared to try and make an amplifier
whose first job was to sound good, even if its specifications
didn't read quite as impressively! But these engineers soon
ran into opposition.
Some people either are not able to listen critically or else
don't believe their ears. They want something in print that
they can show around, proving that the product they have is
better. We have no argument with this attitude. The difficulty
is, what figures can we provide that really prove something?
The notion of music power output originated with engineers:
it should be a way of rating amplifiers which will more nearly
evaluate how "loud" they will sound, than does the conventional
power output rating.
One problem is that steady power tests force the amplifier
to deliver its maximum power continuously. Musical program material
calls for maximum power for only short periods of time. Why
not see what an amplifier will produce when the demand is not
If you put just one or two cycles of a frequency through
an amplifier you can see its performance during such a short
burst but you cannot measure it because the meters won't have
time to "get up to" whatever the reading is. To overcome this,
the method specified by the IHFM involves the maintenance (artificially),
from an external source, of the voltages that sag under continuous
power for long enough to get the reading.
This sounds good, when it is explained to you, otherwise
it might appear to be "cheating a bit." Next, you want to know
what the "answers" mean. And that is a good question. Until
most audiophiles and hi-fi equipment makers use the music power
rating, the more familiar continuous output power will also
be given. So your question is: which is better, an amplifier
that has both ratings the same, or nearly the same, or one for
which the music power is appreciably higher than the continuous
Pursuing this question reveals the fact that an amplifier
with good power-supply regulation will have ratings which are
closer together while one whose power supply provides poorer
regulation will show a greater difference between continuous
and music power ratings. Surely the amplifier with the better
regulated power supply is the better amplifier? Put it this
way: do you want quality watts per dollar or are you looking
for the highest power irrespective of cost?
One amplifier may have both ratings at 25 watts. Another
unit, by spending part of what the first spent on the power
supply on other features, may give 25 watts continuous but also
manages 35 watts music power as well. The second amplifier will
undoubtedly provide greater clean music volume - other things
being equal - than the first unit.
But we have no objection to a third amplifier that gives
35 watts measured both ways, if the manufacturer can make it
competitively. It is an even better amplifier than the second
one, but whether it is worth paying the 30% to 40% higher price
is up to the buyer to decide.
This is, roughly, the way the new music power rating works
out. It is a step forward. but some engineers are still no satisfied.
While it does come nearer to some aspects of how an amplifier
performs on music, it still misses some points.
Another Look at Waveforms
What is the true relationship between the power in a musical
program and the maximum power as measured by a single tone?
This is the question the new music power rating does not really
touch because both ratings, by definition, measure average power.
Average music power can have a number of different connotations,
while the average power of a sine-wave output is unambiguous.
It was the transient nature of music power that dictated the
new rating. But even a steady tone, such as that played by a
single instrument, can have quite a different average power
from that of a sine wave, in terms of what an amplifier can
handle. See Fig. 1.
Fig. 1. - Comparison of voltage and power
waveforms for (A, above) simple sine wave and (B, below) musical
tone consisting of a fundamental with considerable third harmonic.
The limitation to the handling capacity of an amplifier is
the instantaneous peak power it can deliver. The peak power
of a 10-watt sine wave is 20 watts. But a musical tone with
20 watts peak may very well have an average power of less than
5 watts - even as a single continuous tone. That is really the
first basis for differences.
It affects the rating question in two ways. First, the lower
"waveform factor" means the regulation will not hurt even a
sustained tone's power as much as with the test sine waves (unless
the sustained tone is sinusoidal, approaches a square shape,
or has a "waveform factor" such that its average value is higher
than a sine wave - Editor.) Second, it alters the picture as
to what "average power" itself really means.
Next point: music consists of many tones played at any single
instant - most of the time. Each tone has a different frequency.
At a frequency corresponding to this difference, the peaks of
two tones will coincide. At each such coincidence, it is the
voltage, not the power, that adds to determine the total peak.
If peak voltage doubles, peak power is quadrupled, as shown
in Fig. 2.
Fig. 2. - The effect of combining two tones
of different frequency on the relationship between the peak
and average power.
Suppose each of three tones has an average power of .5 watt
and a peak power of 2 watts. If the impedance is 8 ohms, each
peak will be 4 volts. Three of them will reach occasional "spikes"
of 12 volts (Fig. 3). This is a power of 18 watts, although
the average is only 1.5 watts. If a pedal tone is also present,
it may need a further 5-watts average with perhaps IS-watts
peak, also 12 volts. Now we need 24 volts, which is 72-watts
peak, for just a pedal tone and trichord, totaling an average
power of 6.5 watts. See Fig. 4.
Fig. 3. - Voltage and power relations that
exist in an audio amplifier when three different tones are combined.
Fig. 4. Voltage and power levels that exist
in musical phrase in which there are 1 bass and 3 mid-range
The more complex the music, the greater the basic factor
between average and peak power. You may have noticed that orchestral
music at a certain nominal output power does not seem as loud
as, say, a jazz combo.
This because the ear recognizes the sum of all the average
powers. To make the reckoning easy, let's assume each
instrument contributes 0.5 watt average power and needs 2 watts
or 4 volts peak.
A four-piece combo will give 2 watts average, but needs 16
volts or 32 watts peak power. A 40-piece orchestra will give
20 watts average, but needs 160 volts or 3200 watts peak power!
Nobody has that kind of power, so assume we play with the same
margin of safety against possible distortion: the four-piece
combo can be played at a realistic level of 2 watts average
from a 16-watt amplifier (32 watts peak) ; but the orchestra
will have to be turned down to only one-fifth of a watt total
average, to stay within 32 watts peak.
This explains why a much bigger ampliifier is needed to handle
good orchestra music, even though the sound doesn't seem any
louder - if as loud. It also explains why no single music power
rating, based on an average, can ever be completely practical.
With so many variables, the only point of common reference
between amplifiers and the music they handle is peak power.
This is what determines when the amplifier starts to distort
the music. The average power that corresponds to this peak power
depends entirely on the music, not the amplifier. The "waveform
factor" may react on the power-supply regulation to modify the
peak capability according to what the average power is. But
even the worst power-supply regulation usually makes little
difference on this, with most kinds of music.
There was a move, some years ago, to rate amplifiers by peak
power. Proposed with honest intentions, it was based on similar
reasoning but the simple two-to-one relationship for test sine
waves led to its serious misuse.
Give a Dog a Bad Name!
The intention was not to just double the number already on
the amplifier nameplate, but this is what many companies did.
The peak power intended was what the amplifier could handle
on peaks, representative of the relative duration encountered
in music. But insufficient explanation, plus the urge by some
just to use bigger numbers, led to confusion .
Some firms measured peak power the way the IHFM now defines
music power. It was really a short-term average power. Others
double the number they already had, based on the mathematical
sine-wave relationship. Yet others doubled the short-term average
power, giving the instantaneous peak value of such a wave -
and the highest number of all.
This last figure was really the most logical and, if everyone
used it, would be the most informative regarding amplifier performance
because it does relate directly to the musical program the amplifier
But the fact that so many methods of rating were used, led
many to believe that the "good old standard" watts were "honest
watts", with the obvious implication that the others were cheating
- "just doubling the numbers to make the amplifier look bigger!"
This was understandable. If the new number really told something
extra about the amplifier's performance, it was useful. But
most often it was obtained by the arithmetical operation of
multiplying by two. This had no value, except to further confuse
the already confused consumer.
Experiences like this are difficult to live down. On the
committee that discussed the new IHFM music power rating, several
engineers favored the use of a peak music power rating. This
would double the figure as presently defined. But they remembered
the hangover of adverse criticism from the last attempt and
compromised by using the present definition.
Pots and Kettles
This compromise is not unreasonable when you consider some
other factors. Most important among these: what does the power
rating of a loudspeaker mean? It is intended to be the electrical
input power the speaker can handle, as delivered by the amplifier.
A 10-watt speaker with 10% efficiency (and that's high) should
be able to take in 10 electrical watts and deliver 1 acoustic
watt - maximum. But it is called a 10-watt speaker.
That's not all. Few loudspeakers, rated at 10 watts, will
handle this much input power at a single sinusoidal frequency,
through the range for which they are designed - which most good
amplifiers do with ease. But the same loudspeakers are quite
happy handling the maximum musical power a 10-watt amplifier
can deliver, which is a mixture of many frequencies, with a
low waveform factor.
Taken in conjunction with this fact, and other inconsistencies
we haven't space to explain here, it is not so illogical to
rate amplifiers according to the new IHFM music power definition.
So let's take another look at that question, "Does the new Music
Power Rating help tell the quality of an amplifier?"
What Good is MPR?
If you want to use all the figures you can get hold of about
every available amplifier, you will probably try to make some
deductions for which the published information was never intended.
In introducing music power rating, the intention is to bring
into use a figure for comparison that is more realistic than
the one now used.
During a transitional period, progressive manufacturers will
have to use both ratings, not to provide more information about
their own product, but so that comparison can still be made
with other products that do not yet use the new rating. It is
hoped that all firms will ultimately use music power rating
- at least as the main figure.
For simplicity, most people do not want to digest a whole
catalogue of specifications about each product. From this standpoint,
music power rating is a more informative single figure than
continuous power rating. If you are sufficiently interested,
continuous power rating, as a second figure, will convey an
additional measure of merit.
We should not compare amplifiers with the same continuous
power ratings and different music power ratings. When both figures
are given, we would compare amplifiers first by music power
rating; if two amplifiers have the same music power, we may
then compare continuous power ratings.
This is important. Viewing music power rating as the secondary
figure, we are tempted to conclude that a bigger difference
between the two numbers represents a better amplifier. Our intuition
favors the amplifier with better power-supply regulation, so
we suspect the whole notion of music power rating.
But starting on the other foot, with music power rating as
the primary figure and continuous power rating secondary, our
intuition supports the rating inference. It is legitimate, to
get reasonable quality at low cost, to put music power ahead
of continuous power, by using an inexpensive power supply. But
using a better power supply results in an amplifier with "more
We would not say music power rating completely solves the
problem of providing a relative evaluation of an amplifier,
though. Properly used, it is a good step forward but there are
still many things the present method of specifying performance
doesn't tell. Let's put it this way:
Provided the amplifier does not handle musical transients
in some peculiar fashion the specification does not show, and
provided you always work it so musical peaks never exceed twice
the rated music power output, this rating does give a relative
indication of how much power amplifiers will deliver.
We can still have amplifiers that do strange things on certain
kinds of musical sound. None of the figures currently published
shows what is likely to happen if a momentary peak overshoots
the peak music power (twice the music power rating). One amplifier
may handle these things without "batting an eyelid," while another
has electronic convulsions. That the specs do not tell us even
now. This difference can often account for one amplifier seeming
to give a lot more undistorted power than another of the same
or similar rating, even when both the ratings are fully supported
We still have the effect that loudspeaker loads can have
on an amplifier, as compared with the "dummy" resistance load,
used for testing. The low distortion figures are always obtained
with the dummy load. Nobody publishes, even if he measures,
distortion with a loudspeaker load, because this depends, in
an amplifier, on how much reactance the loudspeaker has - and
no two loudspeakers are alike.
A good amplifier may give up to twice the test distortion
when a loudspeaker is connected. A poor one may give many times
as much distortion as soon as a little reactance enters the
It should be possible, for the benefit of those who really
care, to find a standard means of evaluating these various differences.
It would be good to see the manufacturers make a move, possibly
through the IHFM, in this direction. Meanwhile, it remains true
that the specifications are a good starting point in judging
an amplifier: but the real test is how it performs in your system.
1. Crowhurst, Norman H.: "Why Do Amplifiers Sound Different?",
Radio & Television News, March, 1957.
2. Crowhurst, Norman H.: "Some Defects in Amplifier Performance
Not Covered by Standard Specifications," Journal of the Audio
Engineering Society, October, 1957.
3. Van Recklinghausen, D. R.: "Mismatch Between Power Amplifiers
and Loudspeaker Loads," Journal of the Audio Engineering Society,
4. Crowhurst, Norman H.: "The Amplifier Distortion. Story,"
Audio, April and May 1959.
Posted March 7, 2014