In high-fidelity audio equipment, the power amplifier is used to process and amplify the audio signals output by various audio sources without distortion, so that it can reach a certain power to promote the sound of the speaker. Among them, how to amplify the power of the audio signal to achieve high power, high efficiency and low distortion is the most important problem to be solved by the power amplifier.

Basic Requirements for Power Amplifiers

The output power should be large. In order to get enough output power, the working voltage and current of the power amplifier tube are close to the limit parameters. The maximum allowable power dissipation of the collector of the power amplifier tube is related to the heat dissipation condition of the power amplifier tube. Improving the heat dissipation condition of the power amplifier tube can increase its maximum allowable power dissipation. In actual use, the power amplifier tube must be installed with heat sinks as required.

Efficiency is higher. The ratio of the power drawn by the speaker to the power provided by the power supply is called the efficiency of the power amplifier. The output power of the power amplifier is provided by a DC power supply. Since the power amplifier tube has a certain internal resistance, it will have a certain power loss. The higher the efficiency of the power amplifier, the better.

Non-linear distortion is small. Due to the large dynamic range of the signal in the power amplifier, the power amplifier tube works close to the cut-off and saturation state, which exceeds the linear range of the characteristic curve, and must try to reduce the nonlinear distortion.

Basic Components of a Power Amplifier

In high-fidelity audio circuits, power amplifier circuits are usually composed of two or more audio channels. Each channel is divided into two main sections, the preamp and the power amp. The two parts of the circuit can be divided into two chassis, can also be assembled in the same chassis, the latter is called a comprehensive amplifier.

Since the left and right channels are exactly the same, only one of them will be introduced in the two-channel circuit. The circuit composition block diagram is as shown in the figure below. The left side of the picture is the preamplifier, and the right side is the power amplifier.

The composition of the preamplifier. It has dual functions: select the desired audio source signal and amplify it to the rated level; perform various sound quality controls to beautify the sound. These functions are completed by circuits such as equalization amplification, audio source selection, input amplification and sound quality control.

(1) Audio source selection. The function of the audio source selection circuit is to select the desired audio source signal and send it to the rear stage, while closing other audio source channels. The output of various audio sources is different, usually divided into two categories: high level and low level. The output signal level of tuners, recorders, CD players, VCD/DVD players and other audio sources reaches 50~500mV, which is called a high-level audio source and can be directly sent to the audio source selection circuit; The output level is only 0.5~5mV, which is called a low-level audio source, which must be balanced and amplified before it can be sent to the audio source selection circuit. Line inputs, also known as auxiliary inputs, increase the use and flexibility of preamplifiers for connecting TV signals and other high-level audio sources.

(2) Input amplification. The function of the input amplifier is to amplify the audio signal to the rated level, usually around 1V. The input amplifier can be designed as an independent amplifier, and the required amplification can also be completed in the sound quality control circuit.

(3) Sound quality control. The purpose of sound quality control is to make the frequency characteristics of the audio system controllable to achieve high-fidelity sound quality; to modify and beautify the sound according to the listener’s preferences. A separate equalizer can sometimes be inserted to further enhance the sound. Sound quality control includes volume control, loudness control, tone control, left and right channel balance control, low frequency noise and high frequency noise suppression, etc.

2. The composition of the power amplifier. Although there are many circuit types of power amplifiers, they are basically composed of excitation stage, output stage and protection circuit.

(1) Incentive level. The excitation stage can be divided into input excitation stage and push excitation stage. The former mainly provides sufficient voltage gain, while the latter also needs to provide enough power gain to excite the power amplifier output stage.

(2) Output stage. The role of the output stage is to generate sufficient undistorted output power. In order to obtain satisfactory performance indicators such as frequency characteristics, harmonic distortion and signal-to-noise ratio, negative feedback can be introduced between the output stage and the excitation stage.

(3) Protection circuit. The protection circuit is used to protect the output stage power tube and speaker from overload damage.

In addition, a complete high-fidelity power amplifier must also set the level display circuit of the DC stabilized power supply.

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