The distortion is the phenomenon that the input signal and the output signal are in the amplitude proportional relation, the phase relation and the waveform shape change. The distortion of audio power amplifier is divided into two categories: electrical distortion and harmonic distortion. Electrical distortion is caused by the circuit, the sound distortion is caused by the sound device speaker. The types of electrical distortion are: harmonic distortion, intermodulation distortion, transient distortion. Acoustic distortion is mainly the distortion of AC interface. By nature, there are non-linear distortion and linear distortion.
Linear distortion is the variation of amplitude and phase relationship between signal frequency components, and only the amplitude and phase distortion of waveform, which is characterized by not producing new frequency components. The nonlinear distortion is the distortion of the signal waveform, and the distortion of the new frequency component is generated. The main points of distortion generated by audio amplifiers are as follows:
This distortion is caused by nonlinear elements in the circuit, after the signal is passed through these components, a new frequency component (harmonic) is generated, and these new frequency components interfere with the original signal, which is characterized in that the waveform of the input signal is inconsistent with the waveform of the output signal, which is distorted by the waveform.
The main methods to reduce harmonic distortion are:
1. apply the appropriate amount of negative feedback.
2. choose the characteristic frequency high, the noise coefficient small and the linear good amplification device.
3. improve power supply reserve, improve the filter performance of the power supply.
Second, intermodulation distortion
Two or more different frequencies of the signal through the amplifier or the loudspeaker to produce the difference between the new frequency components, the distortion is usually the circuit of the active devices (such as transistors, tubes) produced. The size of the distortion is related to the output power, because the new frequency components are not similar to the original signal, so less intermodulation distortion is easily detected by human ears.
Ways to reduce intermodulation distortion:
1. the use of electronic frequency divider, to limit the magnification of the circuit or the work of the speaker bandwidth, so as to reduce the production of poor.
2. the use of linear good pipe or circuit structure.
Third, transient distortion
Transient distortion is an important index of modern acoustics, which reflects the ability of power amplifier circuit to keep track of transient transition signal, so it is also called transient response. This distortion makes music lack of hierarchy or transparency, and there are two forms of manifestation:
A. transient intermodulation distortion
In the input pulse transient signal, because the capacitance in the circuit so that the output end can not immediately get the due output voltage, and the negative feedback circuit can not get timely response, the amplifier in this moment in the open loop state, so that the output instantaneous overload and produce a cutting wave, this distortion is called transient intermodulation distortion, this distortion in the stone machine performance is more serious.
Transient intermodulation loss is a dynamic index of power amplifier, which is mainly caused by the deep negative feedback inside the amplifier. is to affect the quality of stone machine, leading to the "transistor sound" and "Metal sound" of the culprit. The main ways to reduce this distortion are:
1. select good devices and adjust the working point, as far as possible to improve the amplifier's open-loop gain and open-loop frequency response.
2. Strengthen the negative feedback of each amplification level and cancel the negative feedback of the big loop.
b. distortion caused by low conversion rate
As mentioned above, the high level input pulse causes the amplifier to produce a cutting wave and a transient intermodulation distortion. Does the low level input pulse cause distortion? This depends on the response time of the amplifier, because the response time of the amplifier is too long to change the output signal of the amplifier with the rapid change of the input signal caused by the transient distortion, called conversion rate too low distortion. It reflects the response speed of the amplifier to the signal, the small distortion of the amplifier, the playback of the quality of the analytical power, layering and positioning sense are good.
IV. Distortion of AC interface
The AC interface distortion is caused by the feedback of the speaker's Counter electromotive force (the electric potential generated by the cutting of the magnetic field lines when the speaker sounds vibrating).
The improvement methods are:
1. Reduce the output impedance of the circuit.
2. select the appropriate speakers, so that the damping coefficient more reasonable.
3. Reduce the internal resistance.