Yamaha P-2200 Owner's Manual - Page 14
POWER OUTPUT, Types of Power Ratings, Reasons for a High Power Amplifier, DISTORTION, Harmonic - for home use
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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 appearance 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 distortion 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 problems 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 crossover 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.