Yamaha P-2200 - Manual (Page 14)

POWER OUTPUT
Types of Power Ratings
Peak power refers to the maximum undistorted power
output of an amplifier. Most amplifiers cannot sustain
their peak power ratings for long periods of time without
external cooling fans. Because there are many different
methods of rating an amplifier's peak power, it is hard to
objectively compare the peak power ratings of two
amplifiers. The peak power rating is primarily useful
for determining an amplifier's ability to reproduce the
peaks and transients in a musical program, peaks which
may be 20dB or more above the average power level.
The ability to accurately reproduce these high power
peaks in a musical program is one of the most important
advantages of the P-2200 as compared to a smaller
power amplifier.
"RMS"power is actually a misnomer for average
power. Average power is usually measured with a sine
wave input signal, and is equal to the amplifier's RMS
output voltage squared and then divided by the load
impedance (see Appendix). Because RMS voltage is used
in the formula, the resulting power rating is commonly
called "RMS power." While it means the same as "RMS
power," to be more accurate, the P-2200 is rated in watts
of "continuous average sine wave power."
Since the P-2200 is a professional power amplifier,
not sold for home hi-fi use, it is not required to meet the
power rating standard set by the FTC (Federal Trade
Commission), a standard meant for consumer power
amplifiers. However, the P-2200 is measured under
severe conditions which simulate the most demanding
professional usage. Thus, the P-2200 would easily meet
the FTC ratings for consumer amplifiers. In addition,
the P-2200 user has the benefits of professional features
and reliability.
Reasons for a High Power Amplifier
An interesting characteristic of the human ear is
described by the "Weber-Fechner" law. In its general
form, the law applies to all our senses:
The amount of additional stimulus needed to
produce a perceptible change is dependent on the
amount of stimulus already present.
In mathematical terms, the Weber-Fechner law
suggests that the human ear responds to changes in
sound level in a logarithmic manner. More simply this
means that for a sound to seem twice as loud, it requires
approximately ten times as much acoustic power (and
therefore ten times as much amplifier power). Thus, the
P-2200's high power output capabilities are extremely
valuable.
One of the other benefits of high power output is the
ability of the amplifier to easily reproduce high peak
power transients (which may be 100 times the average
program power, or even more). This subject is discussed
further on Pages FIVE 2 and FIVE 4.
Power Output versus Load
Within its maximum limits, the P-2200 acts like a
perfect voltage source (see Appendix), that is, its power
output rises with decreasing load impedance. When the
load impedance drops below 2.5 ohms, the P-2200's
protection circuits begin to limit the power, resulting
in the curve shown in Figure 4 (normal operation) and
Figure 1 5 (mono operation).
DISTORTION (Refer to Figures 6A-B, 7, 17, 18)
The P-2200 is designed to have the lowest possible
distortion. There are many different forms of distortion,
however, and comprehensive distortion ratings offer a
means to compare the performance of different
amplifiers.
Harmonic Distortion, is characterized by the appear-
ance at the amplifier output of harmonics of the input
waveform which were not present in the original input
waveform. Total Harmonic Distortion, or T.H.D. is the
sum total of all of these unwanted harmonics expressed
as a percentage of the total signal.
Harmonic distortion, in an amplifier, can be created
in any of several ways. The T.H.D. rating of a power
amplifier refers to creation of unwanted harmonics by
the amplifier during "linear" operation (normal input
and output levels, impedances, etc.). Harmonic distortion
is also created by "clipping," a form of "non-linear"
operation, which occurs when the signal level at an
amplifier's input is high enough to drive the amplifier
beyond its rated maximum output. The amplifier, in
attempting to reproduce this signal, reaches its maximum
output voltage swing before it reproduces the top of the
signal waveforms. Since the output voltage cannot rise
any farther, the tops of the waveform are "squared off,"
or clipped, as that shown in Figure 65. Clipping dis-
tortion adds odd upper harmonics (3rd harmonic,
5th, etc.) to the original signal. (Input clipping would
be similar, where the input stage of the amplifier is
overdriven by a high level input signal.) The P-2200 has
wide input headroom and extremely high peak power
output capabilities (headroom) to help avoid the pro-
blems of clipping distortion.
Another form of harmonic distortion that occurs in
some power amplifiers is called crossover distortion. *
Crossover distortion can be caused by improper bias in the
output transistors of an amplifier. The amount of cross-
over distortion stays the same whether the signal is large
or small, so the percentage of distortion goes down as
the signal level goes up. Thus, an amplifier with crossover
distortion may sound relatively distortion free at high
output levels, yet sound "fuzzy" at low levels. Some
amplifiers have internal adjustments which enable a
service technician to control the amount of output
transistor bias, and therefore control the distortion. The
P-2200 has automatic biasing circuitry which needs no
adjustment and avoids crossover distortion under all
operating conditions.
Fig. 28A - Large Amplitude Sine Wave with Crossover
(notch) Distortion.
Fig. 28B - Smaller Amplitude Sine Wave with same amount
(higher %) of Crossover (notch) Distortion.
"Crossover," in this case. refers to the transition between the
positive half and the negative half of the output voltage wave-
form in a "push-pull" class B or AB power amplifier: it has
nothing to do with the crossover used to divide frequencies in
a speaker system. See Figure 28.