The function of the audio power amplifier is to amplify the signal input from the sound source and then output the drive speaker. There are many types of sound sources, such as microphones, record players, recorders (playing heads), CD players, and line transmission. The voltages of the output signals of these sound sources vary greatly, from a few millivolts to several hundred millivolts. Generally, the input sensitivity of the power amplifier is certain. If these different sound source signals are directly input into the power amplifier, the output power of the power amplifier is insufficient for the input signal that is too low, and the function of the power amplifier cannot be fully exerted; if the amplitude of the input signal If the value is too large, the output signal of the power amplifier will be severely overloaded, which will lose the meaning of audio amplification.
Therefore, a practical audio power amplification system must be equipped with a preamplifier to adapt the amplifier to different input signals, or to amplify, or attenuate, or to perform an impedance transformation to match the input sensitivity of the power amplifier. In addition, in various sound sources, in addition to the difference in amplitude of the signals, their frequency characteristics are also different. For example, the output signal of the record player and the output signal of the tape playback are up-warped, that is, the bass is attenuated, and the treble is Upgrade. For such an input signal, before the power amplifier is performed, frequency compensation is required to restore the frequency characteristic curve to a nearly flat state, that is, to add a frequency equalization network amplifier.
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