When static, Class A and Class B amplifiers are connected to purely resistive loads, the test may have similar indicators, and even thermal noise class A is larger. But in actual application, it is connected to a real load (dynamic load)-the speaker, and the impedance of the speaker is different at different frequencies. The comprehensive electroacoustic index at this time will be inferior to that of a purely resistive load, resulting in transient distortion. Since the presence of negative feedback will feed back to the front stage, the key to this transient distortion is that the quality-off design of the speaker system is caused by effective and uninterrupted damping (control), and the higher the voltage rise rate of the signal, this The more severe the distortion. For high fidelity, it is important that the mass inertia of the speaker system can be effectively damped (controlled) by the amplifier.
The damping of the Class B amplifier cannot effectively control the speaker. Only one arm output is working, or pushing or pulling for any half cycle, but it cannot work at the same time, so its damping is unidirectional, that is, whether it is positive or negative, he only has To generate the power to drive the speaker to work, but not to produce the pulling force that is controlled back, to all-round damping, the drive current must be commutated in time, the problem is here. Taking the input square wave as an example, the input signal may be more complicated than the square wave during operation. When the signal rises, the speaker can work according to the signal waveform, but when the signal suddenly stops, the speaker will not stop immediately due to the inertial effect of mass. At this time, the back electromotive force of its voice coil causes the arm A output tube that is conducting to be reversed and cut off, while the arm B that was originally cut off is turned on. At the same time, this back electromotive force is sent back to the front stage by negative feedback. Therefore, the output arm of the B arm is excited to accelerate conduction, and the special damping of the Class B power amplifier is completed together, because this process has to cross the zero point, and the damping free oscillation is lost for a moment. After this process is completed, the arm B conduction is turned off, the arm A output tube that was originally turned on and reversed is restored to conduction, and the damping is restored at the moment when the zero crossing point loses the damping. Therefore, the damping of the Class B amplifier is unidirectional at any instant, and the damping of the speaker is achieved by repeated zero-crossing commutation, and distortion occurs almost at all times.
Both the positive and negative arms of the Class A amplifier are conducting, and the damping coefficient is bidirectional. When the sudden high voltage rises, the voice coil operates according to the waveform. When the signal stops, the back EMF completes the path through the conducting B arm. Inertia Being damped, it cannot produce free oscillation, and the back EMF cannot be established. This kind of omnidirectional damping of the Class A amplifier forces the vibration of the Yang Zhichao replacement to always vibrate according to the waveform of the signal. This is like a motorcycle that is preparing for momentum.
From the above analysis, you can understand the reason why the sound quality of the Class A amplifier is impressive.
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