Yamaha P2075 Operating Manual page 29

The meaning of various amplifier power
rating methods
There are a number of different ratings or specifications that
describe amplifier power. A given amplifier's specifications
can vary by a factor of 2, 3 or 4 depending on the method
used. It is important to understand just what you are reading
when you see a power rating. A number of years ago, the
United States Federal Trade Commission (FTC) established
standard methods of rating amplifiers sold in the consumer
(Hi-Fi) market to avoid the abuse of non-technical consumers
by amplifier manufacturers who cited misleading or meaning-
less tests to persuade consumers that their amplifiers were
more powerful than others. However, the FTC ratings them-
selves are simplistic and do not necessarily describe the "real
world" performance of an amplifier. Since professional
sound system designers can understand and make better use
of several types of power ratings, these other ratings continue
to be used.
Amplifiers intended for professional use are not required to
carry FTC ratings (though some do, anyway, and these amps
are measured after meeting the FTC pre-conditioning criteria,
which tends to cause significantly more heating than full
power operation.)
PEAK POWER
This is a measurement of the maximum undistorted power an
amplifier can deliver to its load. Usually the load specified in
such measurements is a non-reactive, laboratory load resistor
(an ideal load). The test is usually done with one channel
operating if the amplifier is a 2-channel model, and with a
1000 Hz sine wave input signal (i.e., a pure tone in the
middle of the audio frequency spectrum). Peak Power is use-
ful to determine the ability of the amplifier to reproduce
peaks and transient signals that occur in musical program
material, Since Peak Power is calculated using the peak-to-
peak (or zero-to-peak) output voltage instead of the RMS
output voltage of the amplifier, the peak rating can be several
times higher than the Continuous Average (RMS) power even
if there is no difference between the maximum and continu-
ous output power levels.
CONTINUOUS AVERAGE SINE WAVE POWER
(RMS POWER)
"Continuous Average Power" used to be known as "RMS
Power." "RMS" is an abbreviation for "root mean square," а
mathematical term. The RMS value of a sine wave signal is
the peak value multiplied by half the square root of two; the
RMS value cannot be so easily computed for complex wave-
forms. RMS ratings are convenient because the RMS voltage
or current value of an AC or audio signal, when applied to a
resistor, will accomplish the same heating as an equivalent
DC voltage or current, so one can measure RMS power by
measuring the heating of a resistor (though this is seldom
done in practice).
Continuous Average Power ratings are used in amplifier test-
ing as a. convenience; test equipment measures the RMS
values of AC signals. Mathematically, there is no such thing
as "RMS Power." If RMS current is multiplied by RMS vol-
tage, the result is NOT RMS power. Instead, power is general-
ly calculated squaring the RMS voltage and dividing the
result by a given load impedance; such calculations yield
"Average power," not "RMS power." "Average sine wave
power" is the proper way to express what many people think
of as "RMS power."
IHF POWER & DYNAMIC HEADROOM
"(MUSIC POWER)
The IHF (Institute of High Fidelity), a trade organization,
defined a power rating method that is essentially the same as
"Continuous Average Power" or "RMS Power." They also
defined a secondary rating known as "Dynamic Headroom."
The Dynamic Headroom of an amplifier is based on a 20
millisecond, 1 kHz burst of higher power within a half
second after operating for 480 milliseconds at the nominal
level (approximately the same as the old "music power"
method). Instead of specifying a value in watts, however, the
IHF calculates the ratio of that midband peak burst power
output to the continuous average power output and states
the result in dB of Dynamic Headroom.
Given two amplifiers with equal continuous average power
ratings, the amplifier with a larger dynamic headroom specifi-
cation may sound louder — if the difference is sufficient.
About 3 dB of difference is needed in order to hear a clearly
audible advantage. For example, an amplifier rated at 100
watts average power, and having 3 dB of dynamic headroom,
would probaly have carried an old-style music power rating
of 200 watts.
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