Class A power amplifier is an ideal choice for playing back music. Can provide a very smooth sound quality, mellow and warm tone, transparent treble. These advantages are enough to make up for its shortcomings. Class a power amplifier has an amazing calorific value. In order to effectively deal with the heat dissipation problem, class A power amplifier must adopt a large radiator. Because of its low efficiency, the power supply must be able to provide enough current. The capacity of a 25W Class A amplifier is at least enough for Class AB amplifier of 100W. Therefore, the size and weight of the A-class machine are larger than those of the B-class machine, which increases the manufacturing cost and makes the price more expensive. Generally speaking, the price of Class A power amplifier with the same power is twice or more than that of Class B power amplifier.
The advantage of Class A power amplifier is full and transparent sound. Transistor power amplifier consists of triodes, which are paired by multiple groups (N junction and P junction). When no voltage is applied, the two structures are closed. Only when a bias voltage is applied to it and it is higher than its mosfet, the N/P junction will be turned on, and the transistor will start to work when there is current.
Class A power amplifier sets the forward bias at half of the maximum output power, so that the power amplifier can work at full load even without signal input, and the power amplifier is turned on and has current output during the whole signal period. Class A power amplifier makes the transistor always work in the linear region, so the class A power amplifier has almost no distortion, and the hearing is particularly good, especially when the signal is small, the whole sound is transparent and the details are rich. The cost of pure class A power amplifier is also amazing, and the power consumption is equivalent to that of an air conditioner. In particular, the class A power amplifier with 100% refers to how the loudspeaker impedance changes with the frequency, and the power amplifier can maintain the class A work with sufficient output power. Although the nominal impedance of a pair of speakers is 8 ohms, the actual impedance factor will change with the frequency when working, sometimes as low as 1 ohm, which requires the output power of the power amplifier to double with the decrease of impedance, which is the output power index of the power amplifier with a large number of Class A, such as Guifeng. 175 w(8ω), 350 w(4ω), 700W(2ω) 1400 w( 1ω), which is a 100% pure A power amplifier. Only such a power amplifier can let you hear pure class A sound quality.
1. Why is a "hot engine" better than a "cold engine"?
The temperature inside the power amplifier is basically the same as the ambient temperature when it was just started, or the temperature rise is very small. In this state, the power amplifier is called a cooler. At this time, the static current at all levels is still small, and the last level current is only 20 or 30 mA (slightly larger in summer), which is equivalent to Class A or Class B with low bias. The sound is naturally not "pleasant", but with the slow increase of junction temperature, β increases by about 1℃. At the same time, the static current of the transistor will rise rapidly. When the machine is baked to thermal balance, the working points at all levels have reached the rated bias state of Class A, and the sound is also authentic "Class A sound", so it is relatively "pleasant to listen to". Moreover, after the power amplifier reaches thermal balance, the static working points at all levels tend to be stable, which is also conducive to improving hearing. Machines using BJT+MOSFET also follow the above rules, but machines using all-MOS transistors are different. The static current at all levels in the cold state will be greater than that in the hot state, but the working points at all levels in the cold state are unstable and the sense of hearing is not as good as that in the hot state.
Second, where is the "thermal noise"
When some economical class A amplifiers are turned on, the speakers are quiet, but after heating, the noise comes out. Because it is related to "heat", people think it is "thermal noise". Thermal noise refers to the background noise of the internal resistor and capacitor elements of the amplifier and the internal shot noise of the transistor when it exceeds the critical temperature. In fact, these "sounds" can't make the speakers work, so where do these audible noises come from? When the power amplifier is cold, the static current at all levels is small, the power load is light, and the power ripple and noise are very small. After reaching the thermal steady state, the static current can be increased by more than ten times, the power load is aggravated, the ripple and noise are increased, and the noise naturally comes out. This is easy to prove: 1. If the voltage amplification stage is only powered by a regulated power supply, the "thermal noise" can be significantly reduced. If the regulated power supply has been used alone, it may be better to change it to a high-quality regulated power supply with larger capacity; You will be surprised to find that "thermal noise" has disappeared without a trace. In addition, the vibration and stray magnetic field of the transformer will also increase after the load is heavy, which also contributes to the increase of noise. So the so-called "thermal" noise is really related to temperature, but its essence is mainly caused by power supply ripple.
Third, why is a pure class A amplifier much more expensive?
Pure class A power amplifier often works in the high temperature environment of 60℃ ~ 85℃, so the requirements for components and process level are very strict, and online calibration is cumbersome and time-consuming. For example, the matching of the final power amplifier tube is dynamic and static measurement and matching near the rated working temperature. Although the performance of the whole machine is guaranteed by using this standard, usually only one or two pairs of lamps can be selected from 100. However, some high-end class A power amplifiers generally have 2 ~ 12 pairs of transistors at the last stage of each channel. Imagine how much hundreds of pairs of high-quality genuine high-power transistors are worth and how long it will take to carefully select one or twenty pairs of lamps. For example, the power amplifier tube of JA- 100 in Zhongshen was selected from 260 pairs of genuine products.
4. Is the economical Class A amplifier suitable for you?
This refers to a class A power amplifier with a price not exceeding 2000 yuan. Compared with Class A and Class B power amplifiers or super Class A power amplifiers with the same price, the overall indicators are inferior, such as transient response, which should be better than Class B. However, Class A power amplifier itself consumes a lot of power and has relatively little power reserve, and soft feet often appear under large dynamic conditions. But with the same power supply capacity, the reserves of Class A and Class B are relatively large, and the transient response is better than that of Class A; Output power is one of the important parameters of matching equipment. Economical class A power amplifier generally has a power of about 20 watts or 30 watts. In order to increase the output power, the cost of transformer, radiator and power amplifier tube will exceed the design budget. Considering the cost, the final power supply voltage and output current of this kind of power amplifier are low, so it is difficult to match the speakers. However, the output power of Class A and Class B machines with the same price is often more than 3 times, so it is relatively easy to match the speakers. As mentioned above, the "thermal noise" of economical Class A machines can not be ignored, and the temperature difference between hot and cold States is also large, and the matching errors of transistors at all levels are also large. Moreover, due to the cost, it is impossible to do a good job in its adjustment, process, circuit optimization and many other aspects, which may be the space reserved by manufacturers for motorcycle operators. Of course, we should also see that the economical Class A power amplifiers introduced by some well-known domestic manufacturers have an ideal playback level in small-caliber units that match the parameters of the power amplifier.
Compared with Class B power amplifier, why does Class A power amplifier feel better than Class B power amplifier?
In the static state, Class A and Class B power amplifiers are connected with pure resistive loads, and the test indicators may be similar, even the thermal noise of Class A is greater. But in practical application, the speaker is connected with a real load (dynamic load), and the impedance of the speaker is different at different frequencies. At this time, the comprehensive electro-acoustic index will be worse than the pure resistive load, resulting in transient distortion. Because of negative feedback, it will be fed back to the previous stage. The key to this transient distortion is that the quality-related design of the loudspeaker system has been effectively and continuously damped (controlled). The higher the voltage rise rate of the signal, the more serious the distortion is. For high fidelity, it is very important whether the mass inertia of the loudspeaker system can be effectively damped (controlled) by the amplifier.
The damping of class B power amplifier can not effectively control the speaker. For any half cycle, only one arm output works, either pushing or pulling, but it can't work at the same time, so its damping is one-way, that is, no matter the positive half cycle or the negative half cycle, it only produces the power to push the speaker to work, but can't produce controlled tension. For omni-directional damping, the driving current must be reversed in time. This is the problem. Taking the input square wave as an example, the input signal may be more complicated than the waveform at work. When the signal rises, the speaker can work according to the signal waveform, but when the signal stops suddenly, the speaker will not stop immediately due to the inertia of mass. At this time, its voice coil generates back electromotive force, which leads to the turn-on A-arm output tube being turned off, while the originally turned-off B-arm is turned on. At the same time, this back electromotive force is sent back to the previous stage for amplification by negative feedback, thus stimulating the B-arm output tube to accelerate conduction. After this process is completed, the B-arm conduction is turned on and off, and the A-arm output tube which was originally turned on and reversed is turned on again, and the damping is restored after zero loss damping. Therefore, the damping of class B power amplifier is unidirectional at any time, and the damping of loudspeaker is realized by repeated zero-crossing commutation, which almost never produces distortion.
The positive and negative arms of Class A power amplifier are conductive, and the damping coefficient is bidirectional. When the voltage suddenly rises, the voice coil will move according to the waveform. When the signal stops, the back electromotive force completes the path through the conductive B arm, and the inertia is damped, so it is impossible to generate free oscillation, and the back electromotive force cannot be established. The omni-directional damping of class A power amplifier forces the vibration of the positive converter to always vibrate according to the waveform of the signal. It's like a prepared motorcycle. When it says start, it will stop.
Give me extra points if you are satisfied.
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