About--Basic knowledge of power amplifiers

                        About--Basic knowledge of power amplifiers

The working principle of the power amplifier is actually very simple. Intuitively, it amplifies various sound signals played by the audio source to push the speaker to produce sound. From a technical point of view, a power amplifier is like a current modulator. It converts alternating current into direct current, and then is controlled by the sound signal played by the audio source. It transmits currents of different sizes to the speakers at different frequencies, so that the speakers emit corresponding amounts of electricity. , the sound of the corresponding frequency.

When the volume is low, the sound is weak, thin, unable to come out dynamically, dull, with a significant lack of low frequency and poor fullness, and the sound seems to be shrunk inside and unable to come out. When the volume is appropriate, the sound will be natural, clear, mellow, soft, full, powerful, and dynamic. But when the volume is too high, the sound is stiff, not soft, rough, and feels piercing the roots of the ears.

Fixed resistance amplifier:

It is required that the rated output impedance of the power amplifier should be consistent with the rated impedance of the speaker. At this time, the power amplifier is in a better designed load line state, so it can provide greater undistorted power. If the rated impedance of the speaker is greater than the rated output impedance of the power amplifier, the actual output power of the power amplifier will be less than the rated output power. If the rated impedance of the speaker is less than the rated output impedance of the power amplifier, the sound system can work, but the power amplifier is in danger of overloading, which requires the power amplifier to have complete overcurrent protection measures to solve the problem. For tube power amplifiers, the impedance matching requirements are more stringent.

Constant voltage power amplifier:

The most important feature of the broadcast power amplifier is that it has 70V and 100V constant voltage output terminals. This is because broadcast lines are usually quite long and require high-voltage transmission to reduce line losses. The most important indicator of a broadcast amplifier is its rated output power. How much rated output power should be selected depends on the total power of the broadcast loudspeaker. For broadcast systems, as long as the total power of the broadcast speakers is less than or equal to the rated power of the power amplifier and the voltage parameters are the same, they can be connected at will. However, considering factors such as line loss and aging, an appropriate power margin should be left. If it is a background music system, the rated output power of the broadcast amplifier should be about 1.3 times the total power of the broadcast speaker.

In the above system, all connected are constant-voltage power amplifiers. Due to the need for remote transmission, in order to avoid line losses during the transmission process, after the audio signal is amplified by the power amplifier tube, it must first be boosted by a transformer to make it High-voltage and low-current signals are transmitted in the line. When the signal reaches the terminal, it is first step-down and impedance matched by the audio transformer, and then the signal is fed to the speaker to drive it. Therefore, the constant-voltage power amplifier is suitable for long-distance transmission. At the same time, the speakers can be connected arbitrarily through the audio transformer. Unlike the fixed-impedance power amplifier, which requires strict impedance matching calculations to be connected to the speakers, otherwise the power amplifier or speaker will be damaged. In the constant-voltage system, only the speaker is required. The total power does not exceed 90% of the rated power of the amplifier

      The bias of the Class-B amplifier causes the push-pull transistor (or tube) to be in a low current state when there is no drive signal. When the drive signal is applied, the current of one of the tubes in the pair will increase within half a cycle. rises, while the other tube tends to cut off, and in the other half cycle, the situation is opposite. Since the two tubes work in turns, a push-pull circuit must be used to maximize the complete signal waveform. The advantage of Class B amplifiers is higher efficiency, which can theoretically reach 78%, but the disadvantage is greater distortion.

When the Class A and B (Cass-AB) amplifier is driven at a low level, the amplifier operates as Class A, and when the drive level is increased, it switches to Class B operation. The advantage of Class A and B amplifiers is that it improves the efficiency of small signal input compared to Class A. As the output power increases, the efficiency increases. Although the distortion is larger than Class A, it is still the most widely used transistor power amplifier program. The trend is to increasingly use high bias current Class A and B to reduce the distortion of low-level signals.