Introduction to the characteristics of common power amplifiers

Power amplifier is abbreviated as power amplifier. It can be said to be the largest family of various audio equipment. There are so many brands and models that it is really too numerous to mention. Although they are all called power amplifiers, they can be divided into two main categories based on their main uses, namely special power amplifiers and civilian power amplifiers. Power 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. This type of power amplifier is usually called a special power amplifier or a professional power amplifier.

The amplifiers used for home Hi-Fi music appreciation, AV system playback, and karaoke entertainment are usually called civilian amplifiers or home amplifiers.

Although dedicated amplifiers and civilian amplifiers differ in some characteristic parameters, it is difficult to say that there is a clear-cut boundary. For example, an amplifier used for music recording monitoring may very well be an amplifier that can be used for home Hi-Fi or even Hi-end.

Hi-Fi Amplifier vs AV Amplifier

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 price of Hi-Fi amplifiers and AV amplifiers is more than 3,000 yuan. The cost investment of Hi-Fi amplifiers is only on two channels, while the cost investment of AV amplifiers must take into account 5-6 channels and have certain effect processing functions. If you only look at the investment in its two main channels, it is definitely lower than the investment in the two channels of Hi-Fi amplifiers. The difference in their playback effects is obvious. However, whether it is a Hi-Fi amplifier or an AV amplifier, there are high-end boutique types and value-for-money popular types. For example, Denon's AVC-A1 AV amplifier, when used for music playback, its sound effect is no less than that of a Hi-Fi amplifier of four or five thousand yuan.

Generally speaking, it is difficult to find an AV amplifier that is fully compatible with Hi-Fi and AV. There are conditions for an AV amplifier to take into account Hi-Fi music appreciation. This condition is the requirements and standards of users when enjoying music. If users only use it to enjoy some casual music, or only require to hear the melody of the music, the AV amplifier is easier to meet. However, if there are higher requirements for music appreciation, ordinary AV amplifiers will be difficult to meet.

Transistor amplifier and tube amplifier

Amplifiers used for Hi-Fi listening 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. Audio Technology Super Forum 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 electron tube is formed by the movement of electrons attracted by the electric field in a vacuum. The current of the transistor is formed by the outer electrons of the semiconductor element shifting their positions under the action of the electric field. This difference in physical principles results in different circuit characteristics in practical applications. Relatively speaking, the working voltage of the electron tube amplifier is higher, but the working current is relatively small, while the working voltage of the transistor amplifier is lower and the working current is relatively large. There is indeed a certain difference in the timbre of the electron tube amplifier and the transistor amplifier, and the response of the two to transient signals is also different. This difference is adapted to different types of music and different music appreciators, so the current Hi-Fi amplifier has formed a situation where transistor amplifiers and electron tube amplifiers coexist. However, in terms of brand, model, and quantity, the share of transistor amplifiers is still absolutely greater than that of electron 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. Class A, in simple terms, makes the output stage transistors work in the linear region during both the positive and negative half cycles of the sinusoidal AC signal, while Class B makes 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 output stage power utilization efficiency (i.e. the ratio of output power 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, under certain power supply conditions, has an output power of 100W when working in Class B, 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, and the control of low frequencies is also stronger than that of Class B amplifiers, especially in the melody of the song, which is easier to be satisfied by AV amplifiers, but if there are higher requirements for music appreciation, it is difficult for ordinary AV amplifiers to satisfy them.

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 extremely 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 volumes, the bass texture is better. These characteristics of Class A amplifiers mean that Class A amplifiers do not need a large output power margin in actual applications. A 20W-30W Class A amplifier can already drive most speakers very well.

As mentioned above, the power efficiency of Class A amplifiers is low, which causes them to emit a lot of heat when working. In order to keep the operating temperature of the transistors below a certain limit, a heat sink with a larger volume and area is required, which makes the volume and weight of Class A amplifiers relatively large. For example, KRELL's KSD-50S Class A amplifier has an output of 50W+50W, but weighs nearly 30Kg. Marantz PM-80 has an output of 20W+20W in working state, but weighs 13Kg.

Pure power amplifier and mono amplifier

Our common amplifiers are those that combine the preamplifier (front stage) that amplifies small signals with the power amplifier (rear stage) in one housing. This type of amplifier is often called a "combined amplifier". The combined amplifier is easy to use and has a relatively good performance-price ratio. However, this type of combined amplifier has some inherent disadvantages, the most difficult of which is the mutual interference between the front stage and the rear stage. In order to solve this problem, the front stage and the rear stage are respectively built in two housings, so that there is a pure rear stage amplifier. Most pure rear stage amplifiers are dual-channel structures, 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-channel pure rear stage amplifier that separates the two channels has appeared.

The main purpose of separating the power amplifier into pieces is to improve the quality of the 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, and this separation will still have limited effect on improving the overall quality of the amplifier.

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 characteristics, which gives users more room for choice.

The preamplifier matched with the pure power amplifier has a great influence on the quality of the entire sound system. First of all, it must have a certain quality, otherwise, the advantages of the pure power amplifier or monophonic amplifier will not be brought into play, and it may even highlight the "flaws" of a low-quality preamplifier, making the overall sound effect worse. In addition, different preamplifiers and power amplifiers have different timbre characteristics, and users can choose different combinations according to their personal preferences.

For example, many audio and music lovers like to use the combination of "tube preamplifier, transistor postamplifier" (i.e., tube preamplifier, transistor postamplifier), thinking that this combination not only takes advantage of the high power output and good transient response of the transistor postamplifier, but also enjoys the sweet and mellow "charm" of the tube preamplifier. However, this combination is not a "golden rule", because the specific preamplifier and postamplifier have their own characteristics, and the preference for tone varies from person to person. Users can find their favorite combination according to specific circumstances.

What is the output power of a Hi-Fi amplifier?

The output power of a Hi-Fi amplifier is affected by many factors. First of all, this output power is closely related to the connected speakers. Secondly, it is related to the quality of the amplifier itself. Thirdly, it is related to the environment in which it is used, that is, the spatial volume of the room.

There is a parameter called sensitivity for speakers, 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 1W of power is required to drive it to get a sound pressure of 90dB at 1m. To get a sound pressure of 100dB, 10W of power is required to drive it. But if the sensitivity of the speaker is 80dB (such as ATC's SCM-10), 100W of power is required 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 enough sound pressure.

The quality of the power amplifier itself is closely related to the output power that the power amplifier should have. One of the parameters of the power amplifier is called the damping coefficient, which is a parameter that indicates the control ability of the speaker, but 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 30 W. Then there is no need to require the power amplifier to have a higher power output. However, if the quality of the power amplifier is not very ideal, when the output power increases, it will cause its performance parameters to deteriorate, so the output power of the power amplifier should have a certain margin to ensure that there are still certain good parameters under practical output power. Under normal circumstances, when the power amplifier is a Class A output or a tube power amplifier, there is no need to have too much output power margin, and an output power of 20W-30W is enough. But if it is a Class B power amplifier or a power amplifier with poor quality, the output power of the power amplifier should have a larger margin. In addition, if the connected speaker is a large-scale inverted type, the power amplifier should also have a larger output power margin. When considering the output power that an amplifier should have based on its own qualities, choosing a larger power margin can indeed improve the compatibility between the amplifier and the speakers.

But we still need to make it clear here 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 adaptability of the amplifier to the speakers. If an amplifier with a moderate output power can already control the speakers well, then there is no need to require a higher output power for this amplifier.

The usage 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 we talked about above is based on the spatial volume of the room being less than 40. If the spatial volume of the room is larger, the output power of the amplifier should be increased.

Characteristics of the output stage of a 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 very little second harmonic distortion. 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 tube amplifier is very suitable. However, for this type of amplifier, the design and process of the output transformer are crucial. 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 restrictions 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 the power supply are also 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 speaker used has low sensitivity, it is more difficult to broadcast large orchestral music. This type of tube amplifier is very popular among some audio players. They often have this type of amplifier in addition to a high-power transistor amplifier, presumably to complement each other in tone. However, this also shows that the tone characteristics of this type of amplifier are indeed attractive.

Another type of output stage circuit of a tube amplifier is an OTL circuit, which is a circuit without an 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 tubes and transistors are different. Transistor amplifiers are easily suitable for speakers with an impedance of 4-8, while tube amplifiers have to go through some trouble 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 tube OTL amplifier are greatly improved compared to 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.