Several commonly used distortions of audio amplifiers
What is distortion? Distortion is the phenomenon that the input signal and the output signal change in amplitude proportional relationship, phase relationship and waveform shape. The distortion of audio power amplifier is divided into two categories: electrical distortion and acoustic distortion. Electrical distortion is caused by the circuit, and acoustic distortion is caused by the speaker of the sound-returning device. The types of electrical distortion are: harmonic distortion, intermodulation distortion, transient distortion. Acoustic distortion is mainly the distortion of the AC interface. According to the nature, there are nonlinear distortion and linear distortion. Linear distortion refers to the change of the amplitude and phase relationship between the frequency components of the signal. Only the amplitude and phase distortion of the waveform appear. The characteristic of this distortion is that no new frequency component is generated. Non-linear distortion refers to the distortion of the signal waveform and the generation of new frequency component distortion. Four main points of audio amplifier distortion: The main points of distortion generated by the audio amplifier are as follows: 1. Harmonic distortion This distortion is caused by nonlinear components in the circuit. After the signal passes through these components, new frequency components (harmonics) are generated. These new frequency components interfere with the original signal. The characteristic of this distortion is the input signal. The waveform of is inconsistent with the waveform shape of the output signal, that is, the waveform is distorted. The main ways to reduce harmonic distortion are as follows: 1. Apply appropriate amount of negative feedback. 2. Amplifier devices with high characteristic frequency, small noise figure and good linearity are selected. 3. Improve the power reserve of the power supply and improve the filtering performance of the power supply. 2. Intermodulation distortion Signals of two or more different frequencies pass through amplifiers or speakers to produce beats and form new frequency components. This distortion is usually generated by active devices (such as transistors and tubes) in the circuit. The magnitude of the distortion is related to the output power. Since these newly generated frequency components have no similarity to the original signal, less intermodulation distortion is also easily noticeable by the human ear. Methods to reduce intermodulation distortion: 1. Use electronic frequency division to limit the working bandwidth of the amplifier circuit or speaker, thereby reducing the occurrence of beats. 2. Choose a tube or circuit structure with good linearity. 3. Transient distortion Transient distortion is an important indicator of modern acoustics, which reflects the ability of the power amplifier circuit to keep track of transient transition signals, so it is also called transient response. This distortion makes the music lack layer or transparency, and there are two forms of expression: A. Transient intermodulation distortion. When a pulsed transient signal is input, due to the capacitance in the circuit, the output terminal cannot immediately get the proper output voltage, and the negative feedback circuit cannot get a timely response. The amplifier is in an open-loop state at this moment, making the output instant Clipping occurs due to overload. This clipping distortion is called transient intermodulation distortion. This distortion is more serious on stone machines. Transient intermodulation distortion is a dynamic indicator of the power amplifier, mainly caused by deep negative feedback inside the power amplifier. It is the culprit that affects the sound quality of stone machines and causes 'transistor sound' and 'metal sound'. The main methods to reduce this distortion are: 1. Choose good devices and adjust the operating point, try to improve the amplifier's open-loop gain and open-loop frequency response. 2. Strengthen the negative feedback of each amplifier stage, and cancel the negative feedback of the large loop. B. Distortion caused by low conversion rate. As mentioned above, high-level input pulses cause the amplifier to clip and cause transient intermodulation distortion. Will low-level input pulses cause distortion? This depends on the response time of the amplifier. Because the response time of the amplifier is too long, the change in the output signal of the amplifier cannot keep up with the rapid change in the input signal, which is called the slew rate distortion. It reflects the speed of the amplifier's response to the signal. This low-distortion amplifier has good resolving power, layering and positioning. Four, AC interface distortion Distortion of the AC interface is caused by the back electromotive force of the speaker (the electric potential generated by cutting the magnetic force lines when the speaker sounds vibrating) to the circuit.