Types and characteristics of power amplifiers

Jacktang

Types and characteristics of power amplifiers [Copy link]

Power amplifiers are referred to as amplifiers. They can be said to be the largest family of all kinds of audio equipment. There are so many brands and models that it is really hard to list them all. Although they are all called amplifiers, they can be divided into two main categories based on their main uses, namely special amplifiers and civilian amplifiers. The amplifiers used in stadiums, theaters, dance halls, conference halls, public places for sound reinforcement, as well as recording and monitoring, generally have some unique requirements in their technical parameters. Such amplifiers are usually called special amplifiers or professional amplifiers. The amplifiers used for home Hi-Fi music appreciation, AV system playback, and karaoke entertainment are usually called civilian amplifiers or home amplifiers. Although there are some differences in the characteristics of dedicated amplifiers and civilian amplifiers, it is difficult to say that there is a clear-cut boundary. For example, an amplifier used for music recording monitoring may be an amplifier that can be used for home Hi-Fi or even Hi-end. Hi-Fi amplifiers and AV amplifiers Hi-Fi amplifiers and AV amplifiers are the two main categories of home amplifiers. These two types of amplifiers are used for different purposes and have different design emphases. Hi-Fi amplifiers are used to enjoy music, and users pursue as much "original flavor" as possible. Users of AV amplifiers pursue "live" effects that match the picture, or even exaggerated "live" effects. It is not easy to directly compare the two types of amplifiers. For example, the Hi-Fi amplifier and the AV amplifier are both priced at more than 3,000 yuan. The cost of the Hi-Fi amplifier is only invested in two channels, while the cost of the AV amplifier must take into account 5-6 channels and have certain effect processing functions. If we only look at the investment in the two main channels, it is definitely lower than the investment in the two channels of the Hi-Fi amplifier. The difference in the sound effects is obvious. However, whether it is a Hi-Fi amplifier or an AV amplifier, there are high-end boutique types and super value popular types. For example, when the Denon AVC-A1 AV amplifier is used for music playback, its sound effect is not inferior to that of a Hi-Fi amplifier of 4,000 or 5,000 yuan. Generally speaking, it is difficult to have an AV amplifier that is fully compatible with Hi-Fi and AV. There are conditions for AV amplifiers to take into account Hi-Fi music appreciation. This condition is the requirements and standards of users when appreciating music. If users only use it to appreciate some casual music, or only require to be able to hear the melody of the music, AV amplifiers are relatively easy to meet, but if there are higher requirements for music appreciation, general AV amplifiers are difficult to meet. Transistor amplifiers and tube amplifiers Amplifiers used for Hi-Fi appreciation can be divided into two categories: transistor amplifiers and tube amplifiers. In the past, there were Hi-Fi amplifiers that used integrated circuits or module circuits, but now they are rare. There is no difference in quality between transistor amplifiers and tube amplifiers, but the devices used are different (one is transistors, the other is tubes). Due to the differences between the two types of devices, their physical principles and circuit characteristics are also different. The current of the tube is formed by the attraction of electrons in a vacuum under the force of the electric field. The current of a transistor is formed when the outer electrons of a semiconductor are moved to different positions under the action of an electric field. This difference in physical principles results in different circuit characteristics in practical applications. Relatively speaking, the operating voltage of a tube amplifier is higher, but the operating current is relatively small, while the operating voltage of a transistor amplifier is lower and the operating current is relatively large. There is indeed a certain difference in the timbre of a tube amplifier and a transistor amplifier, and the two also respond differently to transient signals. This difference is adapted to different types of music and different music appreciators.Therefore, the current Hi-Fi amplifiers have formed a situation where transistor amplifiers and tube amplifiers coexist. However, in terms of brand, model, and quantity, the share of transistor amplifiers is still absolutely greater than that of tube amplifiers. Class A and Class B amplifiers The working state of transistors in the output stage of transistor amplifiers can be divided into Class A and Class B. The so-called Class A, simply put, is to make the output stage transistors work in the linear region during the positive and negative half cycles of the sinusoidal AC signal, while Class B is to make the output stage transistors work in the linear region only during the positive half cycle (or negative half cycle) of the sinusoidal AC signal. Due to the different working states of the output stage transistors, the power utilization efficiency of the output stage (that is, the ratio of output power amplifier to power consumption) is also different. In practical output circuits, the efficiency of Class B is 2-3 times higher than that of Class A. For example, the Marantz PM80 transistor amplifier has an output power of 100W when working in Class B under certain power supply conditions, but only 20W in Class A. Class A amplifiers do not have crossover distortion, and the internal resistance of the output stage transistors is constant regardless of the actual output power. Class B amplifiers always have a certain amount of crossover distortion (although this distortion may be very small). In addition, the internal resistance of the output stage transistors is small at high output, but it is relatively large at low output. These differences also lead to different listening experiences. The sound of Class A amplifiers is softer than that of Class B amplifiers. In addition, the low-frequency control of the speakers is stronger than that of Class B amplifiers, especially at low volume. The texture of the bass is better. These characteristics of Class A amplifiers make it unnecessary for Class A amplifiers to have a large output power margin in actual applications. A 20W-30W Class A amplifier can already drive most speakers very well. As mentioned earlier, the power efficiency of Class A amplifiers is low, which causes Class A amplifiers to emit a lot of heat when working. In order to make the working temperature of the transistor not exceed a certain limit, a heat sink with a larger volume and area is required, which makes the volume and weight of the Class A amplifier relatively large. For example, KRELL's KSD-50S Class A amplifier has an output of 50W+50W, but weighs nearly 30Kg. The output of Marantz PM-80 in working state is 20W+20W, and the weight is also 13Kg. Pure post-stage power amplifier and mono power amplifier Our common power amplifiers are preamplifiers (pre-stage) that amplify small signals and power amplifiers (post-stage) built in one housing. This kind of power amplifier is often called a "combined power amplifier". The combined power amplifier is easy to use and has a relatively good performance-price ratio. However, this kind of combined power amplifier has some inherent disadvantages, among which the most difficult to overcome is the mutual interference between the pre-stage and the post-stage. In order to solve this problem, the pre-stage and the post-stage are built in two housings respectively, so that there is a pure post-stage power amplifier. Most pure power amplifiers are dual-channel, but this structure makes it difficult to solve the problem of mutual interference between the two channels. In order to solve the mutual interference between the two channels, a mono pure power amplifier with two channels separated has appeared. The main significance of separating the power amplifier piece by piece is to improve the quality of the power amplifier, rather than pursuing this form. If the separation is only achieved in form, although the mutual interference problem can be solved, other parameters are not significantly improved, so this separation is still limited in improving the overall quality of the power amplifier. For most pure power amplifiers and mono power amplifiers, a pre-stage is required. (For an introduction to the pre-stage, please refer to the article "The "Palette" of Audio Equipment" in the 4th issue of this year's magazine). Since pure power amplifiers and mono power amplifiers are designed to improve the quality of power amplifiers, the quality requirements for the pre-stage should also be adapted to them. Power amplifiers can be divided into transistors and tubes, and preamplifiers can also be divided into transistors and tubes. For audio enthusiasts and music lovers, there are many combinations of preamplifiers and postamplifiers, and different combinations have different sound effects, which gives users more room for choice. The preamplifier matched with a pure postamplifier has a great influence on the quality of the entire audio system. First of all, it must have a certain quality, otherwise, the advantages of a pure postamplifier or a monophonic amplifier will not be brought into play, and it may even highlight the "problems" of a low-quality preamplifier, and the overall sound effect will be worse. In addition, different preamplifiers and postamplifiers have different timbre characteristics, and users can choose different combinations according to their personal preferences. For example, many audio and music enthusiasts like to use the combination of "tube preamplifier, transistor postamplifier" (i.e., tube preamplifier, transistor postamplifier), thinking that this combination not only brings into play the characteristics of large power output and good transient response of the transistor postamplifier, but also appreciates the sweet and mellow "charm" of the tube preamplifier. However, this combination is not a "golden rule", because the specific pre-stage and post-stage have their own characteristics, and the preference for sound varies from person to person. Users can find their favorite combination according to the specific situation. How much output power should a Hi-Fi amplifier have? The output power of a Hi-Fi amplifier is affected by many factors. First of all, this output power is closely related to the speakers it is connected to. Secondly, it is related to the quality of the amplifier itself, and then it is related to the environment in which it is used, that is, the volume of the room. The speaker has a parameter called sensitivity, and its unit is dB/m·W, which means the sound pressure (dB) generated at a distance of 1m from the speaker when the speaker receives 1W of electrical power. If the sensitivity of a certain speaker is 90dB, then a 1W power is required to drive a sound pressure of 90dB at 1m. To get a sound pressure of 100dB, a 10W power is required to drive it. But if the sensitivity of the speaker is 80dB (such as ATC's SCM-10), it will take 100W of power to drive it to achieve a sound pressure of 100dB. The sensitivity of most speakers is about 85dB-90dB. For these speakers, 10W-30W of undistorted power is enough to produce sufficient sound pressure. The quality of the amplifier itself is closely related to the output power it should have. One of the parameters of the amplifier is called the damping coefficient, which is a parameter that indicates the control ability of the speaker. However, this parameter has a moderate range and is directly related to the specific speaker. Generally speaking, if the quality of an amplifier is very good, it can still maintain its performance parameters at a certain level when it outputs 30W. Then there is no need to require the amplifier to have a higher power output. However, if the quality of the amplifier is not very ideal, when the output power increases, it will cause its performance parameters to deteriorate. In this case, the output power of the amplifier should have a certain margin to ensure that there are still certain good parameters under the practical output power. Normally, when the power amplifier is a Class A output or a tube amplifier,There is no need for too much output power margin, and 20W-30W output power is enough. But if it is a Class B amplifier or an amplifier of poor quality, the output power of the amplifier should have a larger margin. In addition, if the connected speaker is a large-scale phase-inverted type, the amplifier should also have a larger output power margin. When considering the output power that the amplifier should have from the quality of the amplifier itself, choosing a larger power margin can indeed improve the adaptation of the amplifier and the speaker. But here we still have to make it clear that the main significance of choosing an amplifier with a larger output power is not because we need such a large sound pressure, but to improve the adaptation of the amplifier to the speaker. If an amplifier with a moderate output power can already control the speaker handy, then there is no need to put forward a higher output power requirement for this amplifier. The use environment, that is, the spatial volume of the room, also has a certain relationship with the power that the amplifier should output. The output power size we talked about above is based on the spatial volume of the room below 40. If the spatial volume of the room is larger, then the output power of the amplifier should be increased. Characteristics of the output stage of the tube amplifier There are three types of circuits for the power output stage of the tube amplifier. One type is the push-pull output circuit with an output transformer. This type of output circuit accounts for the vast majority of tube amplifiers. The output transformer in the push-pull circuit has very little DC component and the second harmonic distortion is also very small. The output power of this type of circuit can be made relatively large, so the scope of application is also relatively large. Generally speaking, for audio enthusiasts who are interested in the sound of tube amplifiers, this type of output stage of the tube amplifier is very suitable. However, for this type of amplifier, the design and process of the output transformer are very important. If there are deficiencies in the design and process of the output transformer, the frequency response and transient response of this type of amplifier are often not ideal. In addition, due to the constraints of the output transformer, the adaptability range of the matching speakers is relatively small. Another type of power output stage circuit is the single-ended Class A circuit. This type of circuit also has a transformer, but the output transformer of this type of circuit has a large DC component, and the requirements for the output transformer are higher than the requirements for the output transformer in the push-pull output circuit. In addition, the requirements for power supply are relatively high. The characteristic of this type of output circuit is that there are more second harmonic components. Although this is a kind of harmonic distortion, for music signals, the second harmonic is a highly harmonious sound, so it sounds very pleasant to the ear. This feature makes the power amplifier of this output circuit very distinctive in the timbre of the sound, especially when the power amplifier stage uses triodes, the human voice sounds very sweet, and the string music in chamber music sounds very delicate, or in other words, the sound of this type of power amplifier is very tasteful. However, the output power of this type of power amplifier is not easy to make large, so if the sensitivity of the speakers used is low, it is more difficult to play large orchestral music. This type of electronic tube power amplifier is very popular with some audio players. Often, in addition to a high-power transistor power amplifier, there is also this type of power amplifier. I think it complements each other in terms of timbre. However, this also shows that the timbre characteristics of this type of power amplifier are indeed moving. Another type of output stage circuit of electronic tube power amplifier is OTL circuit. The so-called OTL circuit is a circuit without output transformer. The output stage of modern transistor amplifiers is almost all OTL circuits or improved versions of OTL circuits. The characteristic parameters and working conditions of electron tubes and transistors are different. Transistor amplifiers are easily suitable for speakers with an impedance of 4-8, while it takes some trouble for electron tube amplifiers to adapt to speakers with an impedance of 4-8 without an output transformer. Since the output transformer is removed, the technical parameters of the electron tube OTL amplifier are greatly improved compared with the two types of circuits mentioned above. The sound of the amplifier with this output circuit is very distinctive. Compared with the previous two types of output circuits, it has a magnificent momentum and a wide sound field. Compared with the transistor amplifier, its tone is warm and delicate. Since this type of amplifier has no output transformer, it can adapt to a wider range of speaker impedance. However, the power supply efficiency of this type of output stage amplifier is low, and the design, process, and debugging are relatively complicated. At present, amplifiers with this type of output circuit are only found in some high-end models, and it is difficult to see low-priced popular models.