Class A power amplifiers are ideal for playback of music, as they provide a very smooth sound, with a warm, rounded tone and a transparent, open treble, which more than compensates for their shortcomings. Class A power amplifier heat generation is amazing, in order to effectively deal with the heat dissipation problem, class A amplifier must use a large radiator. Because of its low efficiency, the power supply must be able to provide sufficient current. A 25W Class A amplifier power supply is capable of at least enough for a 100-watt Class AB amplifier. So the size and weight of Class A machines are larger than Class A and B, which makes the manufacturing cost increase and the selling price more expensive. In general, Class A amplifiers sell for about twice or more the price of a Class A or B amplifier of the same power.
Class A amplifier sound has the advantage of full and transparent, transistor power amplifier is composed of transistors, and transistors are composed of multiple sets of matching pairs (N junction and P junction), the two structures into the when there is no external voltage is cut off, only in the top of the external bias voltage and higher than its threshold voltage, the N / P junction will conduct, there is a current through the transistor to start working.
Class A amplifier is the forward bias set at half of the maximum output power, so that the amplifier is not signal input is also in the full-load operating state, so that the amplifier in the entire signal cycle are conductive and have current output. Class A amplifier so that the triode is always working in the linear region, so Class A amplifier almost no distortion, listening to the texture is particularly good, especially when the small signal, the whole sound through the rich details. Pure Class A amplifier its cost is amazing, it is an air conditioning power consumption. Especially 100 percent of Class A amplifier is how the speaker impedance with frequency changes, the amplifier can maintain Class A work and output power is sufficient, a pair of speakers although its nominal impedance is 8 ohms, it will be working on its actual impedance factor will be with the frequency changes, high and low, sometimes as low as 1 ohm, which requires the amplifier's output power with the impedance reduced and multiplied, that is, we often see the giant! A number of amplifiers labeled output power indicators, such as Guifeng mono flagship amplifier Antillon; 175W (8Ω), 350W (4Ω), 700W (2Ω) 1400W (1Ω), which is 100 percent pure A amplifier. Only such an amplifier will enable you to hear the sound quality of pure Class A.
One, why the "hot" than "cold" sound
Amplifier just turned on the temperature rise or temperature rise is small, the temperature inside the machine and the ambient temperature is basically the same, this state of the amplifier is called a cold machine, this time, all levels of quiescent current is also small, the last level of current is only 20-30%. At this time, the quiescent current at all levels is still small, the last level of current is only 20 to 30 mA (slightly larger in summer), equivalent to low bias class A or B, the sound is naturally "good" up, but with the junction temperature slowly rising, every 1 ℃, β increased by about 1%, Vbe reduced by about 2.5mV, which both play a role at the same time, the static current of the transistor will be raised very quickly when the machine is baked to thermal equilibrium, all levels of the transistor can be used in the power supply, the power supply can be used in the power supply. When the machine baked to thermal equilibrium, all levels of work has long reached the Class A rated bias state, the sound at this time is also authentic "Class A sound", so it is relatively "good to listen to". And the amplifier up to thermal equilibrium, all levels of static operating point also tends to stabilize, but also conducive to improving the sense of hearing. For the use of BJT + MOSFET machine also follow the above rules, and the use of full MOS tube machine is different, its cold state at all levels of static current will be greater than the value of the hot state, but the cold state of all levels of the working point is not stable, and thus the sense of hearing is not as good as the hot state.
Two, why "thermal noise"
Some economic class A amplifier on the first boot, the speaker a quiet, and warming up after the heat, the noise came out, due to the "heat" has a relationship, people think this is the "thermal noise". "Thermal noise. Thermal noise is higher than the critical temperature, the amplifier resistive components of the background noise and transistor internal scattering noise, in fact, these "sounds" can not let the speaker work, then these audible noise is where it comes from? Amplifier cold state at all levels of static current is small, the power supply load is light, power supply ripple and noise is very small, and up to the thermal steady state, the static current can be increased by more than ten times, the power supply load is increased, the ripple and noise has become larger, the noise naturally comes out. To prove this point is very easy: 1. voltage amplification stage alone with a regulated power supply, can significantly reduce the "thermal noise", if you have used a regulated power supply alone, it may be worthwhile to change the capacity of a larger high-quality regulated power supply; 2. battery power supply, you will be surprised to find that at this time, "thermal noise! "There is no trace. In addition, the transformer in the load aggravation, its vibration and stray magnetic field will also increase, the increase in noise is also "helpful". Therefore, the so-called "hot" noise and temperature does have a relationship, but the essence of the main power ripple caused.
Three, why pure Class A amplifier is relatively expensive many
Pure Class A amplifier often work at 60 ℃ ~ 85 ℃ in a high-temperature environment, so the components and process level requirements are very demanding, on-line tuning is cumbersome and time-consuming, such as the end of the pair of amplifier tubes is in the rated operating temperature point near the dynamic and static measurements with this standard selection of components, despite the performance of the whole machine is guaranteed, but 100 pairs of tubes are usually also a good idea. But 100 pairs of tubes can usually only pick out one or two pairs, and some high-grade Class A amplifiers and their final stage per channel generally have 2 to 12 pairs of transistors, imagine, hundreds of thousands of high-quality genuine high-power transistors to how much money, from which carefully selected so one or two dozen pairs of tubes and how long it takes, such as the Jongshen JA-100 amplifier tube is selected from 260 pairs of genuine
Four.
Four, economic class A amplifier for you
This refers to the price is not more than two thousand dollars class A amplifier, it and the same price class A and B amplifiers or super class A amplifiers, compared to the overall indicators to be inferior, such as transient response, according to the reason that class A will be better than class B, but the class A amplifier itself is a large power consumption, power supply storage is relatively small, and thus the large dynamics of the phenomenon often appear soft feet! The same power capacity, class A and B storage is relatively large, transient response is better than class A; output power is one of the important parameters with the equipment, economic class A amplifier general power in the twenty or thirty watts or so, to increase the output power, transformers, heat sinks, amplifier tube cost will exceed the design budget, for cost considerations, these amplifiers are lower voltage supply at the end, the output current is also small, with speakers there is a certain degree of difficulty, but the power consumption of class A amplifier itself is large, the power supply storage is relatively small, thus often soft feet phenomenon in large dynamics. With the speaker there is a certain degree of difficulty, and the same price class A and B machine output power is often more than three times, with the speaker is relatively easy; as mentioned earlier, the economic class A machine "thermal noise" can not be ignored, while the temperature difference between its hot and cold state is also larger, the transistors at all levels of the pairing error is also larger, and because of the cost of the relationship between the tuning, Process, circuit optimization and many other aspects can not be done very well, this may also be the manufacturers dedicated to motorists to leave the space it. Of course, we also need to see, some well-known domestic manufacturers to launch the economic type of Class A amplifier and amplifier parameters matching the small-diameter unit on its playback level is also very ideal.
Class A amplifiers compared with class B amplifiers, why listening better than class B amplifiers?
In the static, Class A and Class B amplifiers connected to a purely resistive load, the test may be about the same indicators, and even thermal noise Class A larger. But the actual application, connected to the real load (dynamic load) - speakers, and different frequencies when the impedance of the speaker is not the same, then the integrated electro-acoustic indicators will be inferior to the pure resistive load indicators, resulting in transient distortion. Due to the existence of negative feedback will be fed back to the front stage, this transient distortion is the key to the quality of the speaker system off-type design by the effective, uninterrupted damping (control) caused by, and the higher the rate of rise in the voltage of the signal, the more serious this distortion. For high fidelity, it is important that the mass inertia of the speaker system can be effectively damped (controlled) by the PA.
The damping of class B amplifier can not effectively control the speakers, for any half-week only one arm output in the work, or push or pull, but can not work at the same time, so its damping is unidirectional, that is, regardless of the positive half-week or the negative half-week, he has only to produce to promote the speakers to work the power, but can not produce the control of the back of the pull, to all-around damping, the drive current must be reversed in time, and the problem is here. Input square wave, for example, may work when the input signal is more complex than a square wave, when the signal rises, the speaker can work in accordance with the signal waveform, but when the signal suddenly stops, the speaker due to the inertia of the quality of the role, but will not stop immediately, at this time it is the voice coil generates a reverse electromotive force caused by the conduction of the A arm of the output tube reverse bias and cut-off, while the original cut-off in the B arm is conduction, and at the same time, this reverse electromotive force And by the negative feedback back to the front stage is amplified so as to stimulate the B arm output tube to accelerate the conduction, *** with the completion of the class B amplifier this special damping, because the process to pass the zero point, there is a moment of loss of damping free oscillation. This process is completed, the B arm conduction into cutoff, the original conduction and was reverse bias A arm output tube to restore conduction, and experienced an instant of loss of damping over the zero point before restoring damping. Therefore, the damping of class B amplifier is unidirectional at any instant, and the damping of the speaker is realized by the repeated phase change of the zero point, which produces distortion almost all the time.
Class A amplifier positive and negative arms are on, damping coefficient of the two sides of the direction of the sudden rise in high voltage, the voice coil in accordance with the waveform to action, the signal stops, the counter-potential through the conduction of the B-arm to complete the path, the inertia is damped, can not produce free oscillation, the counter-potential can not be established, Class A amplifier of this omnidirectional damping, forcing the vibration of Young's change of generation is always in accordance with the waveform of the signal to vibrate. . This is like a motorcycle that is being pre-potentialized, saying go, say stop.
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