How does the power amplifier amplify the audio signal?

　　The "source" of the audio system, whether in analog or digital form, is a block that receives or plays music content for the audio system that cannot produce music by itself. Only with the source playing or receiving music signals can we listen to the audio system. to music. However, the source circuit or equipment cannot provide enough power to drive speakers or even headphones, so the audio system also needs a "power amplifier" to drive the speaker device to produce sound.

　　As the name implies, "power amplifier" is to amplify the signal of the source to drive the speaker device, but in fact, the work performed by the power amplifier is not to amplify the signal. To put it more clearly, "the signal itself cannot be amplified." We can "loss" the strength of the original signal through passive attenuation components such as resistors, etc., but in fact, there is no component or even circuit that can "amplify" the signal; the speaker manufacturer must have friends who are suspicious, otherwise how does the power amplifier amplify? What about the signal?

　　In other words, the real working method of the expansion circuit is not to directly amplify the signal, but to “copy” the signal with higher voltage or higher current according to the original signal, and the output of the signal depends on the setting of the circuit. Generally speaking, in order to drive the speaker load, the voltage and current of the source signal must be increased. The circuit design of the power amplifier will increase the voltage and current when the signal is copied, which is called "gain" in electronics. .

　　However, in order to avoid making circuits and components work under too harsh conditions for better performance, such as low distortion, low noise, etc., the voltage gain and current gain circuits are usually fabricated separately. In addition, the common operation functions of power amplifiers, such as source selection, volume control, and high-bass sound quality control, are planned in the block of the voltage gain circuit; because the music signal is processed in this low-current stage, it will be better than in the case of high current. The next step is to get better results and lower costs.

　　Therefore, Guangzhou manufacturers will plan the voltage gain and control circuit in the front section of the circuit structure, which used to be called the "pre-stage" circuit. The most important thing in the pre-stage circuit is responsible for the volume control of the audio system, and the volume control depends on this. The simple thing is actually the most learned part of the audio circuit, and we will discuss it in another article in the future. Although the front-end circuit can copy the signal of the source into a signal with sufficient voltage, the weak current is still not enough to drive the speaker with low impedance and heavy load, so "current gain" is also required.

　　The circuit responsible for the current gain is also to copy the signal output by the front stage, but it is copied into a "high current" signal, which is generally called a power amplifier or a "power amplifier"; in most cases, the power amplification circuit A small voltage gain is also provided to improve dynamic performance. Generally speaking, the higher the output current of the power amplifier circuit, the better, and the more it can drive the loudspeaker with heavy load (the lower the impedance, the heavier the load), especially the impedance of the loudspeaker will change with frequency.

　　Therefore, the power amplification circuit needs a power supply with sufficient voltage and current. Therefore, a large transformer and a large number of filter capacitors are very important, but it will also increase the production cost. That is one of the reasons why the performance of the power amplifier is generally measured by weight. ; Power amplifiers with huge output power are usually relatively huge in size and emit a certain amount of heat when working. However, the technology of switching power supply and class D power amplifier is becoming more and more mature, and some high-power power amplifiers can already be light and cool.

　　The other is the advanced power amplifier style, which is a product that separates the pre-stage circuit and the power expansion circuit into a single piece of pre-stage power amplifier and post-stage / power amplifier, so that audio players can match each other more freely to obtain the required functions, Sound performance and adaptability of speaker collocation. However, due to changes in consumer trends, there are also advanced integrated power amplifiers with performance that is no less than the separation of the front and rear stages.

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