Total harmonic distortion (THD) is an index to characterize the degree of non-linear distortion of ecj1vc1h4r7b amplifier. It is one of the traditional technical indexes of amplifier, and its importance is second only to the frequency response index. When the amplifier amplifies, it will inevitably produce various distortions of the amplified signal. This technical index is used to explain the degree of distortion of the amplifier to the signal. This indicator is a percentage, and the smaller its value, the better.

     (1) Linear distortion. If the frequency in the original signal is lost, this distortion is called linear distortion. It is easy to suppress and has little impact on sound quality, so we generally don‘t care about it.

     (2) Nonlinear distortion. If a new frequency signal is added to the output signal, such distortion is called nonlinear distortion, also known as harmonic distortion. This distortion is difficult to suppress and does great harm to sound quality. Especially, odd harmonics are non musical and do great harm to sound quality. Therefore, the smaller the value of this index is required, the better.

     (3) Various representations of total harmonic distortion. The total harmonic distortion of a machine is expressed as follows: 20Hz ~ 20kHz, 40W / 8q-0.005%, which indicates that the distortion of the amplifier is less than 0.005% when the output power of 8q load is 40W within this frequency range.

     Another expression is "point to area", that is, point frequency (usually at 1kllz), such as 0.01% (1kHz, 100W), which shows the distortion at the frequency of lkl-z when the output power is 100W.

    (4) There is a great relationship between harmonic distortion and signal frequency. In the process of amplifying signals with different frequencies, the harmonic distortion caused by the same amplifier is different. The harmonic distortion of the signal near lkhz is the smallest, and the harmonic distortion of the signal below or above this frequency will increase. Especially for the signal higher than lkhz, the harmonic distortion increases significantly with the increase of frequency, and its distortion can be about 4 times that of lkhz signal.

    It can be seen that among the above two harmonic distortion labeling methods, the first method can more comprehensively characterize the harmonic distortion of the amplifier, and the second method only describes the harmonic distortion of the frequency with the smallest distortion among various frequency signals, and the technical meaning of the index is not comprehensive enough.

    (5) The harmonic distortion is related to the output power of the amplifier. When the output power increases, the harmonic distortion also increases, which is obvious, because when the output power increases, the amplification of the signal increases, and various distortions of the signal also increase. The curve shown in figure 5-63 shows the relationship between them. It can be seen that when the output power is relatively large, the harmonic distortion increases significantly, which is one of the reasons why the amplifier can not operate at full output power in operation.

    (6) The harmonic distortion characteristics of left and right channel amplifiers are different. In the dual channel amplifier, the circuit structure of the left and right channel amplifiers is symmetrical, and their technical performance indexes are required to be consistent. However, the harmonic distortion of the right and left channel amplifiers is still different. As shown in figure 5-64, the harmonic distortion of the left channel is larger than that of the right channel.

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