Audio power amplifier distortion and common improvement methods

Regardless of electrical distortion or acoustic distortion, according to the nature of 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, only the waveform distortion occurs, and the new frequency component is not added, which is linear distortion. Harmonic distortion (THD), intermodulation distortion (IMD), etc. can generate new frequency components, or modulation products of various frequency components. These superfluous products are extremely inconsistent with the original signal, causing sound distortion and rough and harsh. These distortions belong to Nonlinear distortion. Here, we discuss harmonic distortion, intermodulation distortion, transient intermodulation distortion (TIM), and AC interface distortion (IHM). 1. Harmonic distortion

Harmonic distortion is a distortion caused by nonlinear components in a power amplifier. This distortion causes the audio signal to generate many new harmonic components that are superimposed on the original signal, creating a waveform-distorted signal. The distortion caused by each harmonic is superimposed, which is the total harmonic distortion, and its value is expressed as a percentage of the ratio of the rms 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 introduced by the 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%, and the harmonic distortion of professional-grade audio power amplifiers is generally controlled below 0.03%. In fact, when the total harmonic distortion is less than 0.1%, the human ear is difficult to distinguish. It should also be noted that for a given audio power amplifier, for example, the total harmonic distortion index of an audio power amplifier is expressed as THD < 0.009% (1 W). At first glance, it seems that the total harmonic distortion is small, but it is only the total harmonic distortion at an output power of 1 W, which is different from the total harmonic distortion value obtained under the measurement conditions required by the relevant standards. Therefore, when the total harmonic distortion index of the audio power amplifier is indicated, the measurement conditions are generally indicated.

As we all know, the human auditory system is extremely complicated. Sometimes the power amplifier with small harmonic distortion is not as good as the harmonic distortion. The reason for this phenomenon is many. Among them, there is a direct relationship between the degree of influence of each harmonic component on the sound quality. Although the steady-state test data of the stone machine and the amplifier are the same, people always feel that the bass of the amplifier is mellow, the midrange is bright and round, the treble is fine and clear, and it is extremely resistant; the stone machine is powerful and low-frequency, and the medium-high frequency is transparent and bright. But the high frequency hair, the sound is hard, the sound is cold. According to the spectrum analysis, the stone machine contains a large number of odd harmonics, and the odd harmonics cause a harsh feeling to the human ear; the amplifier is rich in even harmonics, and the human ear is not sensitive to even harmonics. In addition, the human ear has a lower resolution of even harmonic distortion and is very sensitive to higher harmonics, which is one of the important reasons for the above phenomenon.

The main ways to reduce harmonic distortion are:

1) Apply appropriate voltage negative feedback or current negative feedback; 2) Select amplifying components with high fT, small NF, and good linearity; 3) Improve the consistency of the tubes in each unit circuit as much as possible; 4) Use Class A The amplification method selects 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 different frequency signals are modulated by the amplifier or the sound of the speaker to produce a sum and difference frequency and each harmonic combination produces a 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 (take 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. Intermodulation distortion, the amount of intermodulation distortion, can be expressed as a percentage of the intermodulation product level and the nominal signal level. The larger the value, the greater the intermodulation distortion. Obviously, the magnitude of the intermodulation distortion is related to the output power. Since the newly generated frequency components have no similarity with the original signal, the small intermodulation distortion is also easily detected by the human ear, which sounds sharp and harsh, and is accompanied by "acoustic staining". That is to say, the influence of intermodulation distortion will result in a lack of layering in the sound field of the entire playback system and a decrease in definition. In Hi-Fi amplifiers, it is always desirable to make the intermodulation distortion as small as possible. It is very difficult to do this, so high-fidelity power amplifiers require that the value be less than 0.1%. Of course, compared with the amplifier, the former has a larger intermodulation distortion, which is one reason why the sound of the stone machine is not as sweet as the amplifier.

The methods to reduce intermodulation distortion are:

1) Adopt electronic frequency division method to limit the working bandwidth of the amplifier circuit or speaker; 2) Add a high-pass filter at the input end of the audio power amplifier to eliminate the sub-low frequency signal; 3) Select a linear tube or circuit structure.

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 transition signals, so it is also called transient reflection. A high-fidelity system with transient distortion, the output music signal lacks layering and transparency. In general, the causes of transient distortion are:

1) The function of the reactive 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 a pulsed transient signal is input, due to the presence of capacitance in the circuit (such as hysteresis compensation capacitor, tube inter-electrode capacitance, etc.), the output terminal cannot immediately obtain the desired output voltage (ie, phase lag), so that 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 be instantaneously overloaded. At this time, the input voltage is several times higher than normal, resulting in severe clipping of the input stage instantaneously. Wave distortion is called transient intermodulation distortion. It is essentially a transient overload phenomenon.

Due to the strong anti-overload capability of the amplifier, the magnification is low, there is no negative feedback between the depths, and there is only some local negative feedback, so it is not easy to generate transient intermodulation distortion. The general stone machine adopts a large loop deep negative feedback network to meet the requirements of low distortion and wide frequency band. It can be seen that the transient intermodulation distortion mainly occurs in the stone machine. In addition, program sources with large volume, high frequency, and large dynamic range are most prone to transient intermodulation distortion. The reason is that the music has the largest time change rate near the zero signal level, which makes the sound become completely unclear. Especially the middle and low grade stone machines often appear in the high frequency part, which produces a sharp and harsh feeling, 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 amplifier. It is widely recognized as the chief culprit influencing the sound quality of stone machines and leading to "transistor sound" and "metal sound". People attach great importance to this. Improving TIM can start with its formation mechanism. The commonly used methods are:

1) Control the open-loop gain and negative feedback of the amplifier to 50dB and 20dB respectively; 2) Select the tube with high fT, the front stage adopts the tube with fT greater than 100MHz, and the fT of the final power tube should be greater than 20MHz, try to widen the circuit. The open loop frequency response, and increase the current negative feedback of each level, cancel the large loop negative feedback. At present, some of the power amplifiers (such as the bell-sound JA-100) do not intervene in the loop negative feedback, which is one of the purposes; 3) using a fully complementary symmetrical circuit to improve the operating 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 proper capacity in parallel with the large loop feedback resistor. Small capacitor; 5) Properly 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, preventing high-frequency interference signals from causing input stages Instant overload.

B. Distortion caused by too low conversion rate

The conversion rate refers to the ability of the audio device to track or respond to the squeaking or pulse signal, and is an important parameter reflecting the transient strain capability of the power amplifier circuit. The transient distortion caused by the low conversion rate is caused by the change in the output signal of the amplifier that 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 change of its voltage should be the ratio of the voltage rise value to the required time, in units of how many volts per second, written as a digital expression of SR = V / μs. For high-fidelity power amplifiers, SR directly affects the transient response and response speed of the amplifier. The power amplifier with high SR value has good resolution, layering and positioning. It has good listening and replay of pop music. The magnitude of the SR value is related to the output voltage of the power amplifier and the output high-frequency cutoff frequency. If the output power is large, the SR value is large; if the high-frequency cutoff frequency is high, the SR value is also large, and the SR value of the high-quality power amplifier can reach 100V/μs. . In order to increase the SR value of the power amplifier, an ultra-high speed, low noise tube is usually used, but the SR value is too high, which makes the circuit self-excited and the stability deteriorates. In addition, the SR value of the pre-stage circuit should not be higher than that of the latter stage circuit, otherwise it will cause transient intermodulation distortion. By the way, the SR of the amplifier can be estimated by the oscilloscope by first feeding the audio amplifier with a square wave signal as the input signal, and the output signal waveform front edge rises to the rated value, and the result is V/μs. The representation is the size of the conversion rate. Obviously, if the audio amplifier can handle the square wave signal well, it shows that it has a good conversion rate and a wide frequency characteristic.

4. AC interface distortion

The AC interface distortion is caused by the back electromotive force of the speaker being fed back to the circuit through the line. The methods to improve this distortion are as follows: 1) reduce the number of circuit stages, appropriately increase the static working current of the circuit; 2) select a suitable speaker to make the damping coefficient more reasonable; 3) use a large-capacity high-quality power transformer, and appropriately improve For the capacity of the filter capacitor, a small-capacity CBB capacitor is connected in parallel with the filter capacitor.

In addition, due to improper selection of DC operating points or low component quality, other nonlinear distortions, such as crossover distortion and clipping distortion, can cause harmonic distortion and intermodulation distortion. Cross-distortion, also known as crossover distortion, is a push-pull amplifier that is mainly caused by the non-linearity of the power transistor in the B-type push-pull amplifier, especially in the case of small currents. The current produces nonlinear distortion at the junction, and the smaller the signal amplitude, the more severe the distortion. The clipping distortion is caused by the insufficient dynamic range of the power amplifier tube, and the large signal is clipped by the saturation conduction. The clipping distortion generates a large number of ultrasonic waves, which makes the sound blur and shake, and it is a headache for a long time. A common method for reducing cross-distortion is to appropriately increase the DC operating point of the push-pull output tube; and measures to improve clipping distortion generally increase the linear operating range of the circuit.

Technology sharing: audio power amplifier distortion and common improvement methods

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