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harmonic distortion in power amplifier


Harmonic distortion: One common type of distortion is harmonic distortion. Harmonics of a signal are signals which are related to the original (or fundamental) by an integer (non decimal) number. A pure tone signal has no harmonics; it consists of only one single frequency. If 100 Hz pure tone signal was applied to the input of an amplifier, we would (upon measurement with special test equipment) find that the output signal of the amplifier was no longer pure. Careful measurements would likely show that several “new” frequencies have appeared. These new frequencies are almost certainly to be integer multiples of the original tone; they are the harmonics of the original signal. In the case of a 100 Hz input tone, we might expect to find tones at 200, 300, 400, 500 (etc.) Hz. We would also probably notice that the odd harmonics are much stronger than the even harmonics (we will not go into the reasons why in this article). In a good amplifier, the harmonics will be much weaker than the original tone. By much weaker, we mean on the order of a thousand times for decent amplifiers.

All amplifiers are generally rated for Total Harmonic Distortion (or THD), usually at full power output, with both channels driven, over a given frequency band (normally 20-20,000 Hz) and with a particular load. Good values are anything less than 0.5 % THD. Some amplifiers have vanishingly low THD ratings, like 0.01%, this is superb but in practice it does not really need to be this low for music reproduction). When an amplifier is measured for THD, a pure tone is applied to the input and the output is measured with special test equipment. The energy of the pure tone is measured, and the energy of the harmonics is measured. Those two values are compared, and a THD rating is calculated. A THD rating of 1% means that the total energy of all the harmonics combined is one one-hundredth of the energy in the fundamental.

Harmonic distortion (although certainly undesirable) is one of the more tolerable types of distortion as long as it is kept reasonably low. Distortion levels of 10% may be very tolerable with music so long as the 10% level is only “occasional” (10% THD on a pure tone can easily be heard by the human ear… but who listens to pure tones?). The reason that a seemingly high value of THD is acceptable for music is partially because many sounds in nature are rich in harmonics. Also, most decent cassette decks (which most people agree sound pretty good) have THD (off the tape that is) of several percent. Worse, even good speakers can have THD up to 10%, especially at low frequencies! All in all, the human ear can tolerate a fair amount of THD before it becomes objectionable. That said, it is preferable to maintain low THD to have the best sound at all times.





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