【 Weak current College 】 audio amplifier distortion and improve method

Audio power amplifier distortion refers to replay audio signal waveform distortion of phenomena, usually divided into electric distortion and acoustic distortion. Electric distortion is the signal current amplification process produces a distorted sound distortion, which is the signal current through the speakers, speaker fails to faithfully reproduce the sound.
Whether electric distortion or sound distortion, according to the nature of the distortion of the main frequency distortion and two non-linear distortion. Among them, causing signal between the frequency component of amplitude and phase relationship changes only occur waveform distortion, not adding new frequency components that belong to the linear distortion. And harmonic distortion (THD), intermodulation distortion (IMD), which may give rise to a new frequency components, or the frequency components of a product, these extra products and original signal very harmonious, causing the sound distortion, rough harsh, these distortions are nonlinear distortion. Here we are on the harmonic distortion, intermodulation distortion, transient intermodulation distortion (TIM), communication interface distortion (IHM), etc.
1. harmonic distortion
Harmonic distortion by amplifier nonlinear components in an induced a distortion. The distortion makes the audio signals produced many new harmonic constituents, superimposed on the original signal, a signal waveform distortion. The harmonic distortion caused by together, that is, total harmonic distortion, its value is common output signal on all harmonic RMS value and fundamental voltage RMS ratio expressed as percentages. Here, the radical wave signal is input signal, all harmonic signals nonlinear distortion introduced by the harmonic signals. Obviously, the smaller the percentage, harmonic distortion, circuit performance better. Currently, Hi-Fi amplifier of harmonic distortion General 0.05% following, many high-quality amplifier of harmonic distortion is less than 0.01%, while the professional-grade audio power amplifier of harmonic distortion General 0.03%. In fact, when the total harmonic distortion is less than 0.1%, the human ear is very difficult to distinguish. Take note that for a specified audio amplifier, for example, an audio amplifier of total harmonic distortion THD indicator expressed as < 0.009% (1W). At first glance, it seems that the total harmonic distortion is very small, but it was only when the output power 1W of total harmonic distortion, this is not the same as in the standard conditions of measuring the total harmonic distortion values are different. Therefore, marked with the audio amplifier's total harmonic distortion indicators, usually indicate the measurement conditions.
It is well known that the human auditory system is extremely complex, sometimes harmonic distortion of small power amplifier as harmonic distortion of great resistance to listen to, the reasons are manifold. Among them, and the harmonic components on the sound quality of a direct relationship between the different levels. Although Stoner and steady-state test data danji, but people always think danji bass mellow baritone agitation, mellow, bright, fine clear, extremely high resistance to low frequency sound; machine is powerful, high frequency permeability in bright, high-frequency sound nervous, stiff, tone rather cold. The spectrum analysis found that machine contains a lot of odd harmonics, odd harmonics to the human ear cause unpleasant feeling awful; danji is rich in even harmonics, and the human ear is not sensitive dual harmonics. In addition, the dual-order harmonic distortion ear resolution is low, the high-order harmonics is very sensitive, this is the major cause of the phenomenon.
Reduce the harmonic distortion methods include:
1) impose an appropriate amount of voltage-feedback-feedback or current; 2) NF fT high, selection of small, linear good zoom components; 3) maximize the cell circuit on the tube of consistency; 4) using a mode a zoom mode, selection of excellent circuit program; 5) enhancing the power of power reserves, improved power supply filter performance.
2. The intermodulation distortion
Two or more different frequency signal through the amplifier or speaker at each other and are produced and frequency and difference frequency and harmonic combination produces and frequency and difference frequency signal, these additional frequency components of the nonlinear distortion called intermodulation distortion. Typically, two amplitude as a percentage (from 4: 1), high-frequency signal, mixed into the circuit, the new generation of nonlinear signal RMS value and the original high frequency signal amplitude ratio percentage to measure the intermodulation distortion, intermodulation distortion, intermodulation products available and rated voltage signal level of the percentage. The higher the value, the greater the intermodulation distortion. Obviously, the intermodulation distortion of size and output power. Since emerging these frequency components and the original signal no similarity, and therefore smaller intermodulation distortion is also very vulnerable to human ear perceives sounds felt was sharp, harsh, and accompanied by the "sound dyeing" phenomenon. In other words, the intermodulation distortion effects will make the entire Replay system sound field lacks a sense of clarity of hierarchies. In Hi-Fi amplifier, always want intermodulation distortion as small as possible, in order to do so is very difficult and, therefore, this value is less than the required gaobaozhengong placed 0.1%. Of course, the machine and compared to the former danji intermodulation distortion, which is why Stoner's timbre is not one of the reasons danji sweet.
Decrease the intermodulation distortion methods, common are:
1) electronic crossover, limiting amplifier or speaker of bandwidth; 2) audio power amplifier input DCLs, eliminating the time-frequency signal; 3) selection of linear good pipe or circuit structure.
3. transient distortion
Transient acoustic distortion is an important indicator, which reflects the amplifier circuit on transient jump ability to keep track of the signal, so it is also called a transient response. Transient distortion occurs, the output of high-fidelity music signal lack of sense of depth and transparency. Generally, the causes of transient distortion occurs are:
1) circuit function of reactance components too large frequency range is not wide enough; 2) speaker vibration system of actions to keep up with changes of the transient signals.
The main transient distortion, there are two forms of expression, i.e. transient intermodulation distortion and conversion rate (SR) is too low due to distortion.
A. transient intermodulation distortion
In the input pulses, transient signal circuit capacitance (such as delay compensation capacitance and the pipes interelectrode capacitance) exists toThe output will not be able to immediately get the recognition they deserve for output voltage (phase-lag), input level can not be timely access to their negative feedback, amplifier-opening in this moment is in a State that allows the input transient overload, in which case the input voltage is higher than the normal out dozens of times, causing serious input level moments, this clipping clipping distortion called transient intermodulation distortion. It is essentially an instantaneous overload.
Because of the strong resistance to overload danji, magnification low, there is no depth-level only negative feedback, among some local negative feedback, so difficult to produce transient intermodulation distortion. General machine are large loop depth negative feedback network to meet low distortion, wide-band requests. Visible, transient intermodulation distortion occurs mainly in the machine. In addition, in volume, frequency is high, wide dynamic range of the program source most easily produce transient intermodulation distortion. The reason is: the music in the zero signal level near the maximum time rate of change, will make the sound becomes not entirely clear, in particular the lower machine, often appear in the high frequency part produces sharp, harsh feelings, the so-called "transistor sound" and "metal sound".
Transient intermodulation distortion is in the 1970s by a dynamic index, mainly by audio amplifier internal depth negative feedback. Is acknowledged as the impact sound quality, "Stoner transistors sound" and "metal sound," the culprit, this attaches the utmost importance. Improve TIM mechanism from its start, often using methods are:
1) will open-loop gain of amplifier and feedback control in volume and around 20dB 50dB; 2) choosing high fT tube, fore use pipes greater than 100 fT, last stage power tube fT should be greater than try to broaden the circuit 20MHz-opening frequency response, and to increase the levels of their current negative feedback, cancel the negative feedback loop. There are some power amplifier (such as Bell JA-100) last stage expansion flow circuit does not intervene in the negative feedback loop, the purpose of this one; 3) adopts all complementary symmetric circuit, increase the power output stage of the work of current and output level before the additional buffer amplifier, improve circuit of transient response; 4) cancel the phase lag Capacitance to delay compensation for advance compensation, i.e. without delay compensation capacitance, while in the loop feedback resistors connected in parallel on a small capacitance appropriate capacity; 5) appropriate to increase the input of quiescent current, increasing its dynamic range and input circuit set low-pass filter, high above the elimination 80kHz clutter and prevent high-frequency interference signal input level instantaneous overload.
B. conversion rate too low distortion caused by
Conversion rate means the audio device on quenching audible signal or pulse signal tracking or response capacity, is reflected in the amplifier's transient response capabilities of important parameters. Conversion rate is too low distortion is caused by a transient because the amplifier output signal changes to keep up with rapid changes in the input signal. If you enter an amplifier with an adequate pulse signal, the voltage of the maximum rate of change should be the voltage on the appreciation and the ratio of the required time, the unit is how many volts per second increases, written as a numeric expression for SR = V/μ s. SR on gaobaozhengong drop, it directly affected the amplifier's transient response and reaction speed, and SR values high power amplifier, analytical ability and sense of depth and orientation are all good, feeling good, replay music. SR numerical size and power amplifier output voltage and output high-frequency cut-off frequency, output power, SR value is very large, high-frequency cut-off frequency, SR value also large, high-quality amplifier of SR value up to 100 V/μ s. In order to improve the power amplifier of SR value, usually uses the ultra high speed, low noise of the tubes, but SR value too high, and easy to make the circuit of self-excited, stability of variation. In addition, the front-stage circuit of SR value should not be higher than level circuit after, otherwise easily lead to transient intermodulation distortion. By the way, say a few words, a power amplifier to SR available oscilloscope by first estimating, to audio amplifier feed a square wave signal, as the input signal, the output signal waveform edge up to the time required for the rated value, the result v/μ s indicates that it is the size of the conversion rate. Obviously, if an audio amplifier to handle square wave signal, which indicates that it has very good conversion rate and wide frequency response.
4. the communication interface distortion
AC connector distortion by speakers of back-EMF through line feedback to the circuit.Improve this distortion methods are: 1) to reduce the progression of the appropriate increased circuit, circuit of static current; 2) select the appropriate speakers, the rationalization of damping coefficient; 3) the use of large-capacity high-quality power supply transformer, and increase the capacity of the filter capacitor, in filter capacitor parallel capacitance small capacity CBB.
In addition, because the circuit DC operating point select inappropriate or component quality, and also appears in other nonlinear distortion such as crossover distortion and clipping distortion, they can cause harmonic distortion, intermodulation distortion. Crossover distortion is also called crossover distortion, it is a push-pull amplifier, mainly by CPI push-pull amplifier power tubes starting on non-linear, especially in the case of small current, its output current generated at the junction of nonlinear distortion and signal amplitude is smaller, the more serious distortion. Clipping distortion is the amplifier tube dynamic range, caused by saturated via large-signal is limiting clipping, clipping distortion produced a large number of ultrasonic, make sound become blurred and jitter, listen to the long time that headache. Reduce cross distortion common methods, it is appropriate to raise the push-pull output tube DC operating point; and improve the clipping distortion measures is generally appropriate to increase the scope of the work of linear circuit.