Power amplifier usually consists of three parts: preamplifier, driver amplifier and final power amplifier.

1. The preamplifier plays a matching role, its input impedance is high (not less than 10k Ω), it can absorb most of the front signal, the output impedance is low (less than tens of Ω), it can transmit most of the signal. At the same time, it is a kind of current amplifier, which converts the input voltage signal into current signal and amplifies it properly.

2. The driving amplifier acts as a bridge. It further amplifies the current signal sent by the preamplifier and amplifies it into a medium power signal to drive the final power amplifier to work normally. If there is no driving amplifier, it is impossible for the final stage power amplifier to send out high-power sound signal.

3. The last stage power amplifier plays a key role. It will drive the amplifier to send the current signal to form a high-power signal, drive the speaker to sound, its technical indicators determine the technical indicators of the whole power amplifier.

Main technical specifications of power amplifier：

1. Rated power: refers to the continuous sine wave power. Under 1kHz sine wave input and certain load, the output power with harmonic distortion less than 1% is expressed as w / Ch. Generally speaking, the higher the rated power, the higher the cost.

2. Total harmonic distortion (THD): it refers to the percentage of higher harmonics in the fundamental wave. The smaller the thd, the better. At present, the THD of a good power amplifier can reach 0.02%

3. Slow rate: voltage amplitude rising per unit time, unit: V / microsecond. It reflects the ability of power amplifier to track transient sound signal, and is a kind of transient characteristic index.

4. Damping factor: it is defined as the load impedance of the power amplifier (internal resistance of the high-power tube plus the wiring resistance of the speaker), for example, 8 Ω: 0.04 Ω = 200:1. Generally, it is required that the ratio is relatively large, but it should not be too large. If it is too small, it will make the sound turbid, poor sound level and poor sound image distribution.

5. Output impedance (or rated load impedance): it is usually equivalent to 8 Ω, 4 Ω and 2 Ω. The smaller the value is, the stronger the load capacity of the power amplifier is. As far as a single channel is concerned, the power amplifier with rated load of 2 Ω can drive four speakers with impedance of 8 Ω to make sound, and the distortion is very small.