Audio power amplifier distortion and common improvement method

Author:　News Source:　Click：　Update Time：2007-09-07

Audio Amplifier Distortion refers to replay the audio signal waveform distortion phenomenon, usually divided into two major categories of electric distortion, harmonic distortion. Distortion is the electric current in the amplification signals generated in the process of distortion, while the sound of distortion is the signal current through the speakers, speakers can not faithfully reproduce the sound.

Whether electric or acoustic distortion, distortion, according to the nature of distortion points, mainly two kinds of frequency distortion and nonlinear distortion. Among them, the frequency component of the signal amplitude and phase relationship between the change appears only waveform distortion, no new frequency components belonging to the linear distortion. The harmonic distortion (THD), intermodulation distortion (IMD) such as generate new frequency components, or the frequency components of the modulation product of these extra products very discordant with the original signal, causing the sound distortion, rough harsh, these distortion non-linear distortion. Here, the respective pairs of harmonic distortion, intermodulation distortion, transient intermodulation distortion (TIM), the exchange of interface distortion (IHM), etc. were discussed.

1. THD

Harmonic distortion is the non-linear components from the power amplifier caused by a distortion. This distortion audio signal generator to make a lot of new harmonic components, superimposed on the original signal, forming a waveform distortion of the signal. The harmonic distortion caused by the superposition of them, that is, total harmonic distortion, the value of all common output signal with the fundamental harmonic rms value of the ratio of rms voltage expressed as a percentage. Here, the fundamental signal is the input signal, all the harmonic signal nonlinear distortion introduced in the grounds of various harmonic signal. Clearly, the percentage is smaller, harmonic distortion, the smaller the better circuit performance. Currently, Hi-Fi power amplifier harmonic distortion of 0.05% or less general control, and many high-quality power amplifier harmonic distortion has been less than 0.01%, while the professional-grade audio power amplifier harmonic distortion 0.03% or less general control. In fact, when the total harmonic distortion less than 0.1%, the human ear is difficult to distinguish the. And it takes another explanation is that, for a specified audio power amplifier in terms of, for example, an audio amplifier's total harmonic distortion index is expressed as THD <0.009% (1W). At first glance, it seems that the total harmonic distortion is small, but it is only when the output power 1W total harmonic distortion, which is the relevant measurement standards obtained under the condition of total harmonic distortion values are different. Therefore, indicating the total harmonic distortion audio power amplifier targets, usually indicate measurement conditions.

As we all know, the human auditory system is extremely complex, and sometimes a small power amplifier harmonic distortion harmonic distortion than large Nai Ting, this phenomenon causes are manifold. Among them, with the sub-harmonic components of the sound effects are directly related to varying degrees. Although Stone Machine and Tube Amplifier Steady-state test data the same, but people always feel full-bodied bass Tube Amplifier agitation, bright and mellow tenor, treble delicate clear, very Naiting; Stone plane was a strong low-frequency strong, transparent high-frequency pass bright, but the high-frequency scared, sound blunt, sound Pianleng. The spectrum analysis showed that stone machine contains a lot of odd harmonics, odd harmonics gives the ear caused by harsh nasty feeling; Tube Amplifier is rich in even harmonics, while the human ear is not sensitive to the even harmonics. In addition, the human ear to the resolution of even relatively low harmonic distortion for high-order harmonics are very sensitive, this is a major reason for the above-mentioned phenomena.

Ways to reduce the harmonic distortion are:

1) the imposition of an appropriate amount of voltage or current negative feedback negative feedback; 2) to adopt an fT high, NF is small, a good linear amplification devices; 3) as much as possible to enhance the cell circuit in the right tube consistency; 4) using a Class-way, use good circuit program; 5) increase power supply power reserve to improve the power supply filtering performance.

2. IMD

Of two or more different frequency signals through the amplifier or loudspeaker after the sound that arises when the mutual modulation and frequency and difference frequency as well as the combination of various harmonics generated and the frequency and difference frequency signal, the increased frequency of these new compositions, nonlinear distortion is called intermodulation distortion. Typically, the two amplitudes according to a certain percentage (to take on more 4:1) high-frequency signals, mixed into the circuit, the new nonlinear signal generated by the rms value of the original higher frequency signal amplitude ratio of the percentage of to measure the intermodulation distortion, intermodulation distortion that is the size of the intermodulation products can be used with the nominal signal level expressed as a percentage level. This value is larger, the greater the intermodulation distortion. Obviously, the size of intermodulation distortion and output power of the. As the new frequency components generated by the similarity with the original signal is not, so smaller intermodulation distortion can easily be aware of the human ear, sounds are also sharp, but also ear-piercing, and accompanied by "coloration" phenomenon. In other words, the impact of intermodulation distortion will make the entire playback system, the lack of layering of sound field, clarity declined. In Hi-Fi amplifier, the Director hoped that the smaller the better intermodulation distortion, to do this is very difficult, and therefore requested that the value of high-fidelity power amplifier can be less than 0.1%. Of course, stone machine, compared with the Tube Amplifier, the former intermodulation distortion to be larger, which is why the stone machine Tube Amplifier sweet sound less than one reason.

Method of reducing intermodulation distortion, common are:

1) Use of electronic sub-band mode, limiting amplifier or speakers operating bandwidth; 2) in the audio amplifier input additional high-pass filter to eliminate second low-frequency signal; 3) selection of a linear structure of a good pipe or circuit.

3. Transient Distortion

Transient distortion is an important indicator of Modern Acoustics, which reflects the transient jump amplifier circuit to maintain signal tracking capabilities, it is also known as transient reflection. The occurrence of transient distortion of high-fidelity system, the output of the music signal a lack of layering and transparency. In general, the occurrence of transient distortion of reasons:

1) The circuit reactance of the role of components is too large, the frequency range is not wide enough; 2) the action speaker vibration system can not keep up transient signal changes.

Transient distortion, there are two main forms, namely, transient intermodulation distortion and slew rate (SR) caused by low distortion.

Transient intermodulation distortion

Transient nature of the input pulse signal, due to the circuit capacitance (such as lag compensation capacitor, the tube very inter-electrode capacitance, etc.) the presence of the output can not be due immediately the output voltage (ie, phase lag) Ershi input stage can not be timely deserves the negative feedback amplifier, in this moment is in the open-loop state, so that instantaneous overload the input stage, this time the input voltage is higher than normal several times, resulting in a serious moment input stage clipping, this cut wave distortion, known as transient intermodulation distortion. It is essentially a transient overload.

Tube Amplifier overload due to strong resistance, low magnification, there is no depth of inter-stage negative feedback, only some local negative feedback, so difficult to produce transient intermodulation distortion. General stone machines used a large depth of the negative feedback loop network to meet the low distortion, wide bandwidth requirements. Can be seen, transient intermodulation distortion mainly occurred in the stone machine. In addition, the volume of large, high frequency, high dynamic range of the program source of the most prone to transient intermodulation distortion. The reason is: Music in the zero signal level near the time of the greatest rate of change will make the sound becomes not entirely clear, especially in middle and low stone machine, often appear in the high-frequency, resulting in sharp hard, harsh feeling, and that the so-called "transistor sound" and "metal sound."

Transient intermodulation distortion is 70 years in the 20th century, put forward a dynamic target, mainly by the depth of the internal audio amplifier caused by negative feedback. Been recognized as affecting the quality stone machine, leading to "transistor sound" and "metallic sound," the main culprit, it attached great importance. To improve the formation mechanism of TIM by its start, and often used methods are:

1) The amplifier open-loop gain and the amount of negative feedback control of 50dB and 20dB respectively, around; 2) to adopt a high fT of the tube, the former class is greater than 100MHz with fT tube, the end-stage power tube fT should be greater than 20MHz, as far as possible broaden the circuit open-loop frequency response, and increase the levels of their current negative feedback, the scrapping of the negative feedback loop. There are currently part of the power amplifier (such as the bell JA-100) of the final stage of expansion flow loop negative feedback circuit does not get involved, its one of the aims will be in this; 3) using full-complementary symmetry circuit improve the power output stage of the work of current, and the output level before the addition of a buffer amplifier stage to improve the circuit's transient response; 4) the removal of the phase lag capacitors, changing the compensation lag lead compensator, that is, do not lag compensation capacitor, while in a large loop feedback resistor in parallel with an appropriate capacity of the small capacitance; 5) appropriate to increase the input stage of the quiescent current, increasing the dynamic range and input circuit is set in its low-pass filter to eliminate high-frequency clutter signals above 80kHz to prevent the high-frequency interference signals lead to transient overload the input stage.

Conversion rate is too low distortion caused by

Conversion rate refers to the audio equipment to burst acoustic signal or pulse signal tracking or response capacity, to reflect the power amplifier circuit transient response capabilities important parameter. Conversion rate is too low due to transient distortion is due to changes in amplifier output signal not keep up with rapid changes in the input signal caused. If you give a sufficiently large amplifier input pulse signal, the voltage should be the biggest change in rate of voltage rise and the ratio of the time required, the unit increase in the number of volts per second and write the number of expressions for the SR = V / μs . SR for high-fidelity power amplifier, it's a direct impact on the amplifier transient response and reaction speed, SR value is a high power amplifier, resolving power, a sense of layering and positioning is good, listen to a sense of good, especially in pop music playback . SR values of size and power amplifier output voltage and output high-frequency cut-off frequency of the output power of a big, SR value is large; high-frequency cut-off frequency of high, SR value is also large, high-quality power amplifier SR values of up to 100V / μs. In order to improve power amplifier of the SR value, usually ultra-high speed, low noise tube, but the SR value is too high, easy to make the circuit self-oscillation, stability deteriorated. In addition, the front-end circuit of the SR should not exceed the value of the circuit after the class, otherwise easily cause transient intermodulation distortion. Incidentally, to say a few words, power amplifier oscilloscope can be used to estimate the SR, is initially for audio amplifier feeds one wave signal, as input signal, the output signal wave front up to the time required rating, the results with V / μs slew rate that is the size. Obviously, if the audio amplifier square wave signal can be a good deal, it would show that it has a good conversion rate and wide frequency characteristics.

4. Communication interface distortion

Communication interface distortion is the anti-EMF from the loudspeaker back to the circuit through the lines caused. To improve the distortion methods are: 1) reducing the circuit series, appropriate to increase the circuit's quiescent operating current; 2) select the appropriate speaker, so that more reasonable damping; 3) using large-capacity high-quality power transformer, and the appropriate filter capacitor to improve the capacity of the filter capacitor in parallel with small capacity CBB capacitors.

In addition, the circuit DC operating point, or inappropriate choice of component quality is not high, there will be some non-linear distortion, such as crossover distortion and clipping distortion, they all can cause harmonic distortion and intermodulation distortion. Crossover distortion, also known as crossover distortion, it is a push-pull power amplifier is concerned, mainly from the B push-pull tube power amplifier in the initial turn-on caused by non-linear, especially in the small current circumstances, its output current is at its junction with the nonlinear distortion, and the signal amplitude smaller, more serious distortion. Clipping distortion is not enough dynamic range power amplifier tube, caused by the saturated conduction is limiting the large signal caused by clipping, clipping distortion produced a large number of ultrasound, so that blur the jitter of the voice and listen for a long time people headaches. Reduce the crossover distortion common method, it is appropriate to enhance the push-pull output transistor's DC operating point; while improving the clipping distortion measures, the general is appropriate to increase the linear range of the circuit