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          Power amplifier parameter index (below)

          Transient intermodulation distortion

          Transient intermodulation distortion (Transient IntermodulaTIon DistorTIon), was known as TIM distortion. It is when the author was found confused, but the methods of measurement TIM late 70s published. Since transient intermodulation distortion is closely related to negative feedback, it is necessary to start with negative feedback when discussing transient intermodulation distortion. Negative feedback (Negative Feedback) is a simple and practical control technology widely used in various engineering technology fields. Negative feedback is originally a part of the closed loop control (Close Loop Control) system in the control technology, but because widely used, it is often used as a synonym for closed-loop control. Negative feedback is actually a natural law that is ubiquitous in people a€?s daily lives. For example, when we drive a car, if we find that the car deviates from the driving route, we will twist the steering wheel in the opposite direction to make the car drove back the correct line. Here our eyes are acting as a negative feedback channel, which is responsible for feeding back the output value (the direction in which the car has to travel) to the excavator (brain), and then the controller compares the output value and the set value (correct direction) to each other (phase Save), then in accordance with the comparison error, the correction signal emitted (torsion of the steering wheel) to correct Thus, the negative feedback effect of the output value is inverted (becomes negative), then back to the input terminal, and provided subtraction value, deriving an error signal, then the controller will be amended according to the size of the error.

          In the electronic amplification circuit, due to the symmetry of the parts, the temperature change, the interference of noise and other various reasons, while reading the number is amplified, various distortions are inevitably added, and the negative feedback is can effectively reduce these distortions. To give a simple example, if the amplifier adds a distorted square wave signal when amplifying a sine wave signal, this sine plus square wave signal will be inverted by the negative return line, and then fed to the input, It is subtracted from the original sine wave to make the original signal amplitude smaller. It also contains an opposite square wave. This new signal will also be added to a distorted square wave signal again when passing through the amplifier. An opposite square wave, so that the positive and negative square waves will cancel each other, so that the output signal contains only a sine wave, which significantly reduces distortion. But the disadvantage of negative feedback is also apparent, as negative feedback so that output of the input signal and the feedback signal is subtracted signal level is reduced, if the output signal to make the phase put, reducing the signal level, if you want output When the signal is amplified to a sufficient strength, the amplification rate (gain) of the amplifier must be increased. Fortunately, this is not difficult, especially for the transistor machine. If we increase the amount of negative feedback, and to reduce the output signal level of the input signal is the same degree, i.e., no amplification, the amplifier circuit has such a special name called a buffer amplifier (Buffer Amplifier). Although the signal is not amplified, it is because amplifiers generally have high input impedance and low output impedance. Therefore, buffer amplifiers are often used for impedance matching.

          Since negative feedback can effectively reduce the distortion, why would it cause transient intermodulation distortion? It turned out that the problem was in time, among which the transistor machine was the most serious. Compared with the vacuum tube, the transistor has the advantages of durability, small size, light weight and high magnification. Its disadvantage is that the operating characteristics are unstable, and it is susceptible to distortion and even runaway due to temperature and other factors. One solution is to use up to a depth of about 50 to 60dB of negative feedback. Anyway transistor magnification is high, the expense of some does not matter, thanks to the great depth of the negative feedback, significantly reducing distortion, so the transistor machine is easy to get superb technical specifications. However, the trouble is also caused. In order to reduce the high-frequency parasitic oscillation caused by the deep negative feedback, the transistor amplifier generally needs to add a small capacitor between the base and collector of the pre-driver transistor to make the high frequency band slight lag phase, known as hysteresis, or said sub-premium price, but no matter how small capacitance, the total time required to charge a certain, when the input signal contains a high speed transient pulses, little time to charge the capacitor, that is to say in this For a moment, the line is in a state without negative feedback. Since the input signal is not subtracted from the negative return signal, the signal is too strong, and these excessive signals will cause an instantaneous overload of the line (Overload). Because the negative feedback of the transistor is large, the signal is too strong, and it often reaches tens or even hundreds of times. As a result, the output signal is clipped. This is transient intermodulation distortion, which is also called "atomic particle" sound because it occurs most in transistor lines.

          By the way, this kind of negative feedback time delay problem is also often encountered in industrial control systems, called the pure delay (Dead Time) problem, the cause of which is mostly because the installation location of the sensor (Sensor) is too far. For example, in a constant temperature water heater, the blast detection is installed at a location away from the heat. The result is that when the detector senses that the water temperature is sufficient, the water temperature near the heater has already been overheated. The result of such a control is necessarily a large swing of water temperature between overheating and subcooling, which is called control overshoot or system oscillation. Pure delay is still a big problem that plagues automatic control technology. There have been thousands of papers about solutions from the 1950s to the present, but there is still no simple and effective way.

          Although the current time delay of the negative return output is not easy to deal with, it is not impossible to solve it. We can simply let it appear, or even if it appears, it will not cause too much damage. There are many methods, for example, only a small amount of large Loop negative feedback, so that there is a negative feedback delay in the name, and the input signal is not too strong. The reduced only by the amount of the negative feedback amplification stage across a local negative feedback instead of negative feedback ,, local short path, time is fast, easy to induce transient intermodulation distortion. The vacuum tube works stably. It is not necessary to suppress the distortion with a large depth of negative feedback. Moreover, most of the distortion is even harmonic distortion that people love to listen to, so the amplifier does not have the so-called "atomic particle" sound. As for other methods used to prevent transient intermodulation distortion in line design, they will not be introduced one by one because they involve more boring theories.

          In addition to preventing transient intermodulation distortion in line design, audiophiles can also take another measure to reduce transient intermodulation distortion, that is, try to use various shielding and filtering measures to reduce various high-frequency interference signals into the amplifier. Although many of these signals are radio frequency interference inaudible to the human ear, because of their high frequency, transient intermodulation distortion is easily induced, overloading the input stage, and preventing the music signal from being amplified normally.

          Conversion rate

          Transient intermodulation distortion amplifier in addition to the large negative feedback loop caused by a time delay, the amplifier is not fast enough also an important reason, if the speed amplifier fast enough even in the same negative conditions under transfusion, transient intermodulation distortion can be reduced. The speed of the amplifier is a popular description. The correct statement should refer to the transient response capability of the amplifier (Transient Response). In control theory, the transient response and frequency response are two ways to measure system performance. Their advantage is that they do not require a detailed understanding of the detailed mathematical model of the entire system. They only need to estimate the system's response curve to a specific input signal based on the system's response curve to a specific input signal to estimate the system's characteristics, thereby making compensation Or improve. But the contrary, if we know the mathematical model of a system may not have been tested to estimate the response mode of the system.

          For systems with low accuracy requirements, we can selectively adopt the transient response method or frequency response method to evaluate system performance, and for systems with high requirements, both must be considered. The signals commonly used for transient testing are the unit step function (Step Signal) and the unit pulse function (Impulse). For convenience, the amplifier uses a special form of the former: square wave /. An ideal square wave contains a very high-speed voltage rising edge and falling edge, which is very suitable for testing the transient response of the amplifier.

          Measure the response speed of the amplifier is generally used slew rate (Slew Rate, Taiwan called "slew rate"). It is defined as the amplitude of voltage rise in 1 microsecond. If it is measured with a square wave, it is the time required for the voltage to rise from the valley to the peak. The unit is V / us. The larger the value, the higher the transient response. slew rate performance of the amplifier can generally be done more than 25V / us.

          Improve the transient response of the simplest way is to use high-frequency bonding characteristics of a good part. It can also be improved with proper loop negative feedback. This seems to be a contradictory approach, but it is not the case. Transient intermodulation distortion only occurs when the signal speed exceeds the amplifier's transient response capability.

          In addition to transient intermodulation distortion, signals that are too fast will also produce another kind of distortion, called ringing, which is essentially the same. When the input signal speed is fast and the amplitude is small, the ringing phenomenon first appears. Only when the speed of this signal is fast to a certain degree will transient intermodulation distortion appear. However, when the signal speed is fast and the amplitude is large, the ringing If it does not happen, it will enter the state of transient intermodulation distortion. The signals that are most likely to cause ringing are all kinds of high-speed but low-frequency high-frequency interference noise, which is one of the reasons why sound equipment must have perfect anti-interference measures.

          Interface Intermodulation Distortion

          Interface intermodulation distortion is a relatively new and rarely mentioned amplifier specification. And damping will be mentioned below, except the amplifier circuit and related, speakers and have a great relationship. So before introducing these two specifications, let's briefly talk about the characteristics of the speaker in this regard.

          The current audio speaker is mostly adopt the principle of electrodynamic moving-coil loudspeaker, the structure includes a permanent magnet used to generate a magnetic field and one voice coil. In terms of structure, the dynamic coil speaker belongs to a special form of DC motor, because the voice coil only needs to move back and forth instead of rotating, so there is no need to use the carbon brush and commutator (commonly known as "copper head") common on DC motors.

          Whether it is the AC motor or DC motor, it is reversible, i.e., under certain conditions may be used as a generator. The structure of a DC motor is no different from that of a DC generator. In particular, a permanent-magnet DC motor can generate a certain voltage on its terminals as long as it can rotate its shaft. For a dynamic speaker, as long as we press the diaphragm with our hands, a voltage will be generated on the terminals, and the size depends on the speed and amplitude of the pressing.

          Due to the effects of loss and non-linearization, it is impossible for the speaker to utilize all the power output by the amplifier and there will be residual power generation. In addition, due to the mechanical inertia of the diaphragm, excess power will also be generated in the voice coil. Problems arising from the former stable for the interface intermodulation distortion, while the latter will make the speaker's low-frequency control deteriorates.

          Interface intermodulation distortion and speaker resistance and negative feedback about the line. When the power amplifier output can not change all of the mechanical energy, the excess energy will surely occur in the speaker coil in an additional BEMF (Back emf), the back EMF will be reserved by the speaker wire to the output terminal of the amplifier, and by the size of the amplifier resistance is formed a voltage, which is fed back to the negative feedback input line, and to mix the input signal. The mid-low frequency sound is turbid, and the analytical power and sense of hierarchy are greatly reduced.

          To reduce intermodulation distortion in the interface, the key point is to reduce the amount of negative feedback amplifier and a resistance (i.e., increase the damping coefficient). There are many Hi-End transistor amplifier using this principle is the design. In addition, two-lane feeder is also an alternative way to improve, because the line separating high and low frequency side of the back EMF does not affect the high-frequency signals, thereby improving sound quality.

          Damping (Damping Factor)

          The ratio of the damping coefficient of the speaker impedance to the amplifier output impedance. As the name suggests, it is a coefficient of resistance to be the degree of inhibition of a certain physical quantity change process. For the speaker, what is to be suppressed is the inertial vibration of the speaker diaphragm in the absence of electrical signal input, which is simply a braking action. Speaker diaphragm is not, can be used only by mechanical damping electromagnetically braking damping manner. This method requires the system to be in the generator state as much as possible.

          The previous discussion mentioned that the speaker can easily enter the state of the generator. When the input electric reading disappears, the speaker diaphragm is still not vibrating due to inertia. This vibration will produce an induced voltage in the voice coil. If the output resistance of the amplifier is low at this time, it is equivalent to connecting a small resistor in parallel to the speaker terminal. The induced voltage on the voice coil will drive a relatively high voltage. The large amount of current flowing through the internal resistance post office of the amplifier means that the speaker becomes the power supply at this moment, and the power output stage circuit of the amplifier becomes the load. According to the law of electromagnetic induction, this current is generated by the vibration of the voice coil in the magnetic field of the permanent magnet, so this voice coil current must generate a force opposite to the vibration direction to counteract the vibration. The smaller the internal resistance of the amplifier, the greater the current and the stronger the effect of counteracting inertial vibration. Because the energy of this current will be consumed as heat on the resistor, this braking method is called "dynamic braking" (Dynamic Bracking) in the motor control technology. Speakers at the maximum amplitude of the low frequency replay, inertial vibration caused by the most serious, if left unchecked will deteriorate the low-frequency control, lack of strength, flexibility and layering, but too much will make the sound suppression dry.

          Because the output train Danji coil resistance is present, a large damping very limited, conversely, the transistor using multi-tube unit and the like can easily contact the damping coefficient up to hundred and several tens, or even hundreds. However, it can be different from the requirement of a resistance factor, which results in a variety of different sound combinations between different speakers and amplifiers.

          For an amplifier that uses more than half of the negative feedback, the damping coefficient is not the only tool that will brake the speaker, because the speaker's inertial vibration current will generate a certain value of voltage when it flows through the amplifier's output internal resistance. , The negative return line feeds it back to the input immediately, so that the amplifier line thinks that a distortion voltage should not appear, and immediately generates an inverted signal to resist. This is one of the most powerful electric motor braking mode, referred to as "plug braking" (Plugging). But it is also the least used method, because a motor reversed suddenly will produce a huge mechanical impact and damage the machine, but the speaker is originally designed as a device that continuously moves back and forth, so this method is theoretically no problem, However, in practice, there are often problems, and the trouble comes from negative return.

          The speaker is not a microphone, and the voltage generated by the vibration of the diaphragm will not be as accurate as the microphone. Therefore, the cancellation voltage generated by the amplifier cannot be completely equal to the vibration and the direction is opposite. The result is that inhibit the process of instability, low frequency is not smooth and rapid reduction, this process is actually the process of intermodulation distortion and the interface is very similar. The low-frequency control power of some atomic particle amplifiers is not as good as that of amplifiers, which is why.

          There are some other specifications for measuring amplifier performance. This article only mentions the concerns of more enthusiasts, plus the frequently disputed specifications. The author is by no means an expert, just because I often need to take into account the technical principles of motors, electronics, and even machinery at the same time. When I have a headache, I find that there are many technologies or problems, phenomena, etc. in the field of fever. What has long been understood and recognized by people in the technical field is not esoteric and mysterious. It is just that different industries have different interpretation methods and are confusing. This article should try some specific metaphors to explain and distinguish some often confusing things. Specifications. The parties hope that some non-engineering enthusiasts can have a clearer concept.

          With the development of amplifier technology, it is believed that it is difficult to make special technological breakthroughs in circuit design and material application. High-quality equipment can only be relied on with a careful and conscientious attitude. It has been neglected in the past. A large number of trivial technical specifications have been improved bit by bit. Out of proportion. The so-called flat and pretty are just relatively speaking. The technology is built with money. There are many so-called high-tech military technologies that use only those scholars from universities and research institutions in various countries. In order to improve their academic status, they are openly The theoretical research results published on the channel have no secret at all, but the difficulty is only in the pre-research, design, test, production and quality assurance process technology. Like Hi-Dnd equipment, the cost invested is often astronomical. there may just have to come back to an old machine will be able to rely on the transformation process used.

          The world premiere of the technical zone! ROHM has developed the power supply IC "BD372xx series" for high-quality audio. A practical guide for the purchase of home wireless routers. Understanding the circuit diagram and working principle of the audio. Talking about the "frequency response curve" in the audio. Deep dismantling report of the M0pro speaker: both internal and external

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          Power amplifier parameter index (on)

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          Power amplifier parameter index (on)
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          亚洲 欧美 图片 自拍 视频
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