Audio power amplifier distortion refers to the phenomenon of waveform distortion of replayed audio signal, which is usually divided into electrical distortion and acoustic distortion. Electrical distortion is that the signal current produces distortion in the amplification process, while acoustic distortion is that the signal current passes through the loudspeaker, and the loudspeaker fails to reproduce the sound truthfully.
Whether electrical distortion or acoustic distortion, according to the nature of distortion, there are mainly frequency distortion and nonlinear distortion. Among them, the relationship between amplitude and phase of each frequency component of the signal changes, only waveform distortion occurs, and no new frequency component is added, which belongs to linear distortion. Harmonic distortion (THD) and intermodulation distortion (IMD) can produce new frequency components or modulation products of each frequency component. These redundant products are very disharmonious with the original signal, causing sound distortion, rough and harsh. These distortions belong to nonlinear distortion. Here, we discuss harmonic distortion, intermodulation distortion, transient intermodulation distortion (TIM), AC interface distortion (IHM) and so on.
1. Harmonic distortion
Harmonic distortion is a kind of distortion caused by nonlinear components in power amplifier. This distortion makes the audio signal produce many new harmonic components, which are superimposed on the original signal to form a waveform distorted signal. The distortion caused by each harmonic is the total harmonic distortion. Its value is usually expressed as the 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 harmonic signals 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 amplifier is generally controlled below 0.05%, the harmonic distortion of many high-quality power amplifiers is less than 0.01%, while the harmonic distortion of professional audio power amplifier is generally controlled below 0.03%. In fact, when the total harmonic distortion is less than 0.1%, it is difficult for human ears to distinguish. It should also be noted that for a specified 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 relevant standards. Therefore, when indicating the total harmonic distortion index of audio power amplifier, the measurement conditions will generally be indicated.
As we all know, the human auditory system is extremely complex. Sometimes the power amplifier with small harmonic distortion is not as good as the one with large harmonic distortion. There are many reasons for this phenomenon. Among them, it is directly related to the influence of each harmonic component on sound quality. Although the steady-state test data of stone machine and gall machine are the same, people always think that the bass of gall machine is mellow and stirring, the Alto is bright and mellow, and the treble is thin and clear, which is extremely durable to listen to; The stone machine is powerful in low frequency, transparent and bright in medium and high frequency, but hairy in high frequency, hard in sound and cold in timbre. Through spectrum analysis, it is found that the stone machine contains a large number of odd harmonics, which cause a harsh feeling to human ears; Courage machine is rich in even harmonics, while human ear is not sensitive to dual harmonics. In addition, the human ear has low resolution of dual subharmonic distortion, but is very sensitive to higher harmonics, which is also one of the important reasons for the above phenomenon.
The main methods to reduce harmonic distortion are:
1) Apply an appropriate amount of voltage negative feedback or current negative feedback;
2) Select amplification components with high ft, small NF and good linearity;
3) Improve the consistency of tubes in each unit circuit as much as possible;
4) Class a amplification mode is adopted, and excellent circuit program is selected;
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 with different frequencies modulate each other through the back of the amplifier or when the speaker makes sound, resulting in sum frequency and difference frequency, and the combination of each harmonic generates sum frequency and difference frequency signals.
The nonlinear distortion composed of these newly added frequency components is called intermodulation distortion. Generally, the two high and low frequency signals with a certain proportion of amplitude (4:1 more) are mixed into the circuit. The intermodulation distortion is measured by the percentage of the ratio of the root mean square value of the newly generated nonlinear signal to the amplitude of the original higher frequency signal, that is, the intermodulation distortion can be expressed by the percentage of the intermodulation product level and the rated signal level.
The higher this value, the greater the intermodulation distortion. Obviously, the magnitude of intermodulation distortion is related to the output power.
Because these newly generated frequency components are not similar to the original signal, the small intermodulation distortion is also easy to be detected by the human ear. It sounds sharp and harsh, accompanied by the phenomenon of "acoustic staining". In other words, the influence of intermodulation distortion will make the sound field of the whole playback system lack of hierarchy and reduce the definition.
In hi fi power amplifier, we always hope that the smaller the intermodulation distortion is, the better. It is very difficult to do this. Therefore, the value of high fidelity power amplifier is required to be less than 0.1%. Of course, compared with the courage machine, the intermodulation distortion of the stone machine is larger, which is one reason why the tone of the stone machine is less sweet than the courage machine.
Common methods to reduce intermodulation distortion are:
1) Electronic frequency division is adopted to limit the working bandwidth of amplification circuit or loudspeaker;
2) A high pass filter is added at the input of the audio power amplifier to eliminate the sub-low frequency signal; 3) Select the pipe or circuit structure with good linearity.
3. Transient distortion
Transient distortion is an important index of modern acoustics. It reflects the ability of power amplifier circuit to keep track of transient jump signal, so it is also called transient reflection. In the high fidelity system with transient distortion, the output music signal lacks a sense of hierarchy and transparency. In general, the causes of transient distortion are:
1) The effect of reactance components in the circuit is too large and the frequency range is not wide enough;
2) The action of the loudspeaker vibration system cannot keep up with the change of the transient electrical signal.
There are two main forms of transient distortion, namely transient intermodulation distortion and distortion caused by low conversion rate (SR).
A. Transient intermodulation distortion
When inputting impulse transient signal, due to the existence of capacitance in the circuit (such as lag compensation capacitance, tube inter pole capacitance, etc.), the output terminal can not immediately obtain the due output voltage (i.e. phase lag), so that the input stage can not obtain the due negative feedback in time. At this moment, the amplifier is in the open-loop state, causing the input stage to overload instantaneously. At this time, the input voltage is dozens of times higher than that under normal conditions, This leads to severe clipping at the input stage, which is called transient intermodulation distortion. It is essentially a transient overload phenomenon.
Due to its strong anti overload ability, low magnification, no deep inter stage negative feedback and only some local negative feedback, it is not easy to produce transient intermodulation distortion. The general stone machine adopts the large loop deep negative feedback network to meet the requirements of low distortion and broadband. It can be seen that the transient intermodulation distortion mainly occurs in the stone machine. In addition, the program source with large volume, high frequency and large dynamic range is most likely to produce transient intermodulation distortion. The reason is that the time change rate of music near the zero signal level is the largest, which will make the sound not completely clear.
Especially the medium and low-grade stone machine often appears in the high-frequency part, producing a sharp, hard and harsh feeling, that is, the so-called "transistor sound" and "metal sound".
1) The open-loop gain and negative feedback of the amplifier are controlled at about 50dB and 20dB respectively;
2) Select tubes with high ft, tubes with FT greater than 100MHz shall be used in the front stage, and the FT of the last stage power tube shall be greater than 20MHz. Try to broaden the open-loop frequency response of the circuit, increase the current negative feedback of each level, and cancel the large loop negative feedback. At present, the last stage current spreading circuit of some power amplifiers (such as bell ja-100) does not intervene in loop negative feedback, which is one of its purposes;
3) A fully complementary symmetrical circuit is adopted to improve the working current of the power output stage, and a buffer amplifier stage is added in front of the output stage to improve the transient response of the circuit;
4) Cancel the phase lag capacitor and change the lag compensation to lead compensation, that is, a small capacitor with appropriate capacity is connected in parallel on the feedback resistance of the large loop without the lag compensation capacitor;
5) Appropriately increase the static current of the input stage, increase its dynamic range, and set a low-pass filter in its input circuit to eliminate the high-frequency clutter signal above 80kHz and prevent the instantaneous overload of the input stage caused by the high-frequency interference signal.
B. Distortion caused by low conversion rate
Conversion rate refers to the ability of audio equipment to track or respond to sudden sound signal or pulse signal. It is an important parameter to reflect the transient strain ability of power amplifier circuit. The transient distortion caused by low conversion rate is caused by the change of amplifier output signal can not keep up with the rapid change of input signal. If a large enough pulse signal is input to the amplifier, the maximum change rate of its voltage shall be the ratio of the rise in voltage to the required time, and the unit is how many volts rise per second. The digital expression is sR = v/ μ ｓ。 SR for high fidelity power amplifier, it directly affects the transient response and reaction speed of the amplifier. Power amplifier with high SR value has good analytical power, sense of hierarchy and positioning, and good listening feeling, especially when replaying pop music. The value of SR is related to the output voltage and high-frequency cut-off frequency of the power amplifier. If the output power is large, the SR value is large; The SR value of high-frequency cutoff frequency is also large, and the SR value of high-quality power amplifier can reach 100V/ μ ｓ。 In order to improve the SR value of the power amplifier, the tube with ultra-high speed and low noise is usually used, but the SR value is too high, which is easy to cause the circuit self excitation and poor stability. In addition, the SR value of the front stage circuit shall not be higher than that of the rear stage circuit, otherwise it is easy to cause transient intermodulation distortion. Incidentally, the SR of the power amplifier can be estimated by oscilloscope. The method is to first feed a square wave signal to the audio power amplifier as the input signal. The time required for the front edge of the output signal waveform to rise to the rated value is v/ μ S is the conversion rate. Obviously, if the audio power amplifier can process the square wave signal well, it shows that it has good conversion rate and wide frequency characteristics.
4. AC interface distortion
The distortion of AC interface is caused by the back EMF of loudspeaker fed back to the circuit through the line. The methods to improve this distortion are as follows: 1) reduce the number of circuit stages and appropriately increase the static working current of the circuit; 2) Select suitable speakers to make the damping coefficient more reasonable; 3) Large capacity and high-quality power transformer shall be adopted, and the capacity of filter capacitor shall be appropriately increased. Small capacity CBB capacitor shall be connected in parallel on the filter capacitor.
In addition, due to the improper selection of DC working point of the circuit or the low quality of components, there will be other nonlinear distortions, such as cross distortion and clipping distortion, which can cause harmonic distortion and intermodulation distortion. Cross distortion is also called cross distortion. For push-pull power amplifier, it is mainly caused by the initial conduction nonlinearity of power tube in class B push-pull power amplifier. Especially in the case of small current, its output current produces nonlinear distortion at the junction, and the smaller the signal amplitude is, the more serious the distortion is. Clipping distortion is caused by the insufficient dynamic range of the power amplifier tube and the large signal is clipped by limiting amplitude due to saturation conduction. Clipping distortion produces a large number of ultrasonic waves, which makes the sound blurred and jittery. Listening for a long time makes people headache. The common method to reduce cross distortion is to appropriately increase the DC working point of push-pull output tube; The measure to improve clipping distortion is generally to appropriately increase the linear working range of the circuit.
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