Audio power amplifier distortion refers to the distortion of the reproduced audio signal waveform, which is usually divided into two categories: electrical distortion and acoustic distortion. Electrical distortion means that the signal current produces distortion during the amplification process, while acoustic distortion means that the signal current passes through the speaker, and the speaker fails to faithfully reproduce the sound. Whether it is electrical distortion or acoustic distortion, according to the nature of the distortion, there are mainly two kinds of frequency distortion and nonlinear distortion. Among them, the relationship between the amplitude and phase of each frequency component of the signal is changed, and only waveform distortion occurs, and no new frequency component is added, which is a linear distortion. Harmonic distortion (THD), intermodulation distortion (IMD), etc. can produce new frequency components, or modulation products of each frequency component. These unwanted products are extremely inharmonious with the original signal, causing sound distortion and harshness. These distortions are Non-linear distortion. Here, we separately discuss harmonic distortion, intermodulation distortion, transient intermodulation distortion (TIM), and AC interface distortion (IHM).
1. Harmonic distortion
Harmonic distortion is a kind of distortion caused by non-linear components in the power amplifier. This kind of distortion causes the audio signal to produce many new harmonic components, which are superimposed on the original signal to form a signal with waveform distortion. Adding up the distortion caused by each harmonic is the total harmonic distortion, and its value is usually expressed as a percentage of the ratio of the root mean square value of all harmonics in the output signal to the effective value of the fundamental voltage. Here, the fundamental signal is the input signal, and all harmonic signals are the harmonic signals of each order introduced by nonlinear distortion. Obviously, the smaller the percentage, the smaller the harmonic distortion and the better the circuit performance. At present, the harmonic distortion of Hi-Fi power amplifiers is generally controlled below 0.05%, and the harmonic distortion of many high-quality power amplifiers is less than 0.01%, and the harmonic distortion of professional audio power amplifiers is generally controlled below 0.03%. In fact, when the total harmonic distortion is less than 0.1%, it is difficult for the human ear to distinguish. It should also be noted that for a designated audio power amplifier, for example, the total harmonic distortion index of an audio power amplifier is expressed as THD<0.009% (1W). At first glance, it seems that the total harmonic distortion is very small, but it is only the total harmonic distortion when the output power is 1W, which is different from the total harmonic distortion value obtained under the measurement conditions required by the relevant standards. Therefore, when marking the total harmonic distortion index of the audio power amplifier, the measurement conditions are generally indicated.
As we all know, the human auditory system is extremely complicated, and sometimes power amplifiers with low harmonic distortion are not as resistant to hearing as those with large harmonic distortion. There are many reasons for this phenomenon. Among them, it is directly related to the different degree of influence of each harmonic component on the sound quality. Although the steady-state test data of the stone machine and the tube amplifier are the same, people always feel that the tube amplifier's bass is mellow and exciting, the midrange is bright and round, and the treble is slender and clear. But the high frequency is hairy, the sound is stiff, and the tone is cold. Through spectrum analysis, it is found that stone machines contain a large number of odd harmonics, which cause a harsh and unpleasant feeling to human ears; tube amplifiers contain abundant even harmonics, and human ears are not sensitive to even harmonics. In addition, the human ear has low resolution for even harmonic distortion, but is very sensitive to higher harmonics, which is also one of the important reasons for the above phenomenon.
The main ways to reduce harmonic distortion are:
1) Apply an appropriate amount of voltage negative feedback or current negative feedback; 2) Use high fT, small NF, and good linearity amplification components; 3) Improve the consistency of the tube in each unit circuit as much as possible; 4) Use class A Amplification method, select excellent circuit program; 5) Improve the power reserve of the power supply and improve the filtering performance of the power supply.
2. Intermodulation distortion
Two or more signals of different frequencies are modulated by each other after the amplifier or the loudspeaker is sounding to produce the sum frequency and difference frequency, and the combination of each harmonic produces the sum frequency and difference frequency signal. These newly added frequency components constitute a non- Linear distortion is called intermodulation distortion. Usually, two high and low frequency signals with a certain ratio of amplitude (4:1 more) are mixed into the circuit, and the ratio of the root mean square value of the newly generated nonlinear signal to the amplitude of the original higher frequency signal is measured as a percentage Intermodulation distortion, that is, the magnitude of intermodulation distortion, can be expressed as a percentage of the intermodulation product level to the rated signal level. The greater the value, the greater the intermodulation distortion. Obviously, the degree of intermodulation distortion is related to the output power. Since these newly generated frequency components have no similarity with the original signal, the smaller intermodulation distortion is easily noticed by the human ear, and it sounds sharp and harsh, and is accompanied by a phenomenon of "sound coloring". In other words, the effects of intermodulation distortion will make the sound field of the entire playback system lack a sense of hierarchy and degrade clarity. In Hi-Fi power amplifiers, it is always hoped that the degree of intermodulation distortion is as small as possible. It is very difficult to achieve this. Therefore, high-fidelity power amplifiers require this value to be less than 0.1%. Of course, the intermodulation distortion of the stone machine is larger than that of the tube amplifier, which is one reason why the sound of the stone machine is not as sweet as the tube amplifier.
The common methods to reduce intermodulation distortion are:
1) The electronic frequency division method is used to limit the working bandwidth of the amplifier circuit or speaker; 2) A high-pass filter is added to the input of the audio power amplifier to eliminate the sub-low frequency signal; 3) The tube or circuit structure with good linearity is selected.
3. Transient distortion
Transient distortion is an important indicator of modern acoustics. It reflects the ability of the power amplifier circuit to keep track of transient signals, so it is also called transient response. A high-fidelity system with transient distortion, the output music signal lacks layering and transparency. Generally, the causes of transient distortion are:
1) The effect of the reactance components in the circuit is too large, and the frequency range is not wide enough; 2) The action of the speaker vibration system cannot keep up with the change of the transient electrical signal. There are two main manifestations of transient distortion, namely, transient intermodulation distortion and distortion caused by too low slew rate (SR).
A. Transient intermodulation distortion
When inputting a pulsed transient signal, due to the existence of capacitors in the circuit (such as lagging compensation capacitors, inter-electrode capacitors, etc.), the output terminal cannot immediately obtain the due output voltage (that is, the phase lag) and the input stage cannot be obtained in time Due to the negative feedback, the amplifier is in an open loop state at this moment, causing the input stage to instantaneously overload. At this time, the input voltage is several tens of times higher than normal, resulting in severe instantaneous clipping of the input stage. Wave distortion is called transient intermodulation distortion. It is essentially a transient overload phenomenon.
Because the amplifier has strong anti-overload ability, low magnification, no deep inter-stage negative feedback, and only some partial negative feedback, it is not easy to produce transient intermodulation distortion. Generally, stone machines use a large loop deep negative feedback network to meet the requirements of low distortion and broadband. It can be seen that transient intermodulation distortion mainly occurs in stone machines. In addition, program sources with high volume, high frequency, and large dynamic range are most likely to produce transient intermodulation distortion. The reason is: the time change rate of music is the largest near the zero signal level, which will make the sound incompletely clear, especially in the low-end stone machines, which often appear in the high-frequency part, resulting in a sharp, harsh feeling, the so-called "Transistor sound" and "metal sound".
Transient intermodulation distortion is a dynamic indicator proposed in the 1970s, mainly caused by deep negative feedback inside the audio power amplifier. It is recognized as the culprit that affects the sound quality of stone machines and causes "transistor sound" and "metal sound", and people attach great importance to this. Improving TIM can start from its formation mechanism, and the commonly used methods are:
1) Control the open-loop gain and negative feedback of the amplifier at about 50dB and 20dB respectively;
2) Choose high-fT tubes, and use tubes with fT greater than 100MHz in the front stage. The fT of the final power tube should be greater than 20MHz. Broaden the open-loop frequency response of the circuit as much as possible, and increase the current negative feedback of each stage to cancel the large loop. Road negative feedback. At present, there are some power amplifiers (such as the bell JA-100) of the final stage expansion circuit that does not involve loop negative feedback, one of its purposes is this;
3) Adopt a fully complementary symmetrical circuit to increase the working current of the power output stage, and add a buffer amplifier stage before the output stage to improve the transient response of the circuit;
4) Cancel the phase lag capacitor, change the lag compensation to lead compensation, that is, do not use the lag compensation capacitor, and connect a small capacitor of appropriate capacity in parallel with the large loop feedback resistor;
5) Appropriately increase the quiescent current of the input stage, increase its dynamic range, and set a low-pass filter in its input circuit to eliminate high-frequency clutter signals above 80kHz and prevent high-frequency interference signals from causing instantaneous overload of the input stage.
B. Distortion caused by too low conversion rate
The conversion rate refers to the ability of audio equipment to track or respond to burst sound signals or pulse signals, and is an important parameter that reflects the transient capacity of the power amplifier circuit. Transient distortion caused by too low slew rate is caused by the fact that the change of the output signal of the amplifier cannot keep up with the rapid change of the input signal. If a large enough pulse signal is input to the amplifier, the maximum rate of voltage change should be the ratio of the voltage rise value to the required time. The unit is how many volts rise per second, written as a digital expression as SR=V/μs. For high-fidelity power amplifiers, SR directly affects the transient response and response speed of the amplifier. A power amplifier with a high SR value has good resolution, sense of layering and positioning, and a good sense of hearing, especially when replaying popular music. The value of SR is related to the output voltage of the power amplifier and the output high-frequency cut-off frequency, etc. The output power is large, the SR value is large; the high-frequency cut-off frequency is high, the SR value is also large, and the SR value of a high-quality power amplifier can reach 100V/μs . In order to increase the SR value of the power amplifier, ultra-high-speed, low-noise tubes are usually used. However, if the SR value is too high, the circuit is easy to self-excite and the stability becomes poor. In addition, the SR value of the pre-stage circuit should not be higher than that of the post-stage circuit, otherwise it is easy to cause transient intermodulation distortion. By the way, the SR of the power amplifier can be estimated with an oscilloscope. The method is to first feed a square wave signal to the audio power amplifier as an input signal. The time required for the leading edge of the output signal waveform to rise to the rated value, the result obtained is V/μs Represents the size of the conversion rate. Obviously, if the audio power amplifier can handle square wave signals well, it shows that it has a good conversion rate and wide frequency characteristics.
4. The AC interface is distorted
The distortion of the AC interface is caused by the back-EMF of the speaker being fed back to the circuit through the line. The methods to improve this distortion include: 1) reduce the number of circuit stages and appropriately increase the quiescent operating current of the circuit; 2) select suitable speakers to make the damping coefficient more reasonable; 3) use large-capacity high-quality power transformers and increase them appropriately For the capacity of the filter capacitor, connect a small-capacity CBB capacitor in parallel with the filter capacitor.
In addition, due to the improper selection of the DC operating point of the circuit or the low quality of the components, other nonlinear distortions, such as crossover distortion and clipping distortion, will appear, which can cause harmonic distortion and intermodulation distortion. Crossover distortion is also called crossover distortion. It is for push-pull power amplifiers. It is mainly caused by the non-linearity of the initial conduction of the power tube in the class B push-pull power amplifier, especially in the case of low current. The current produces nonlinear distortion at the junction, and the smaller the signal amplitude, the more serious the distortion. Clipping distortion is caused by insufficient dynamic range of the power amplifier tube, and the large signal is clipped by the saturated conduction. The clipping distortion generates a lot of ultrasonic waves, which makes the sound fuzzy and jittery. It is a headache for people to listen to it for a long time. The commonly used method to reduce crossover distortion is to appropriately increase the DC operating point of the push-pull output tube; and the measure to improve clipping distortion is generally to appropriately increase the linear operating range of the circuit.
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