With the continuous development of audio technology, power amplifiers are no longer available as only tube amplifiers as in the past. With the development of transistor machines (transistors, also known as "stone machines"), many types of amplification have been mentioned, because They represent a new amplification mode and show the sound characteristics of a new generation. The most representative one is naturally the European faction’s NAD. Of course, in addition to NAD, there are B&O, Lyngdorf, Trinnov Audio, etc., to focus on the research of Class D or digital amplifier circuits, and to take into account environmental protection and good sound design. In fact, there are many manufacturers focusing on this aspect, so I won’t list them all here.

Before introducing the new generation of amplifier types, let me first introduce our traditional amplifier types. According to the signal working state of the triode when amplifying the signal and the division of the quiescent current of the triode, there are three main types of amplifier circuits: one is a class A (class A) amplifier circuit, the other is a class B (class B) amplifier circuit, and the third is Class A and B (Class AB) amplifier circuit, Class D amplifier circuit, Class H amplifier circuit, etc.

Class A (Class A) amplifier is to add appropriate static bias current to the power tube, so that a triode is used to amplify the positive and negative half cycles of the signal at the same time. In the power amplifier circuit, the signal amplitude in the power output stage is already very large. If the positive and negative half cycles of the signal are still amplified by a triode at the same time, this circuit is called a Class A amplifier. This kind of circuit has a big feature. Because it belongs to linear amplification, the fidelity will be very high, almost reaching 100%, but at the same time, because of linear amplification, the amplification efficiency will not be very high, generally only 20-30%. There is also a problem. If the transistor is always charged, then when there is no audio signal, it will be like an oven. It will continue to heat up and generate heat seriously. This requires large-scale heat dissipation equipment to dissipate heat, and these heat dissipation devices make The power amplifier increases the weight, volume and price.

The second is the Class B amplifier, which is what we often call the Class B amplifier. The so-called Class B amplifier does not add a static bias current to the transistor, and uses two symmetrical transistors to amplify the positive half cycle and the negative half cycle of the signal. , The positive and negative half-cycle signals are combined into a complete periodic signal on the load of the amplifier. Since this amplifier does not add a quiescent current to the power amplifier output tube, it will produce crossover distortion, which is a kind of non-linear distortion and seriously damages the sound quality. Therefore, Class B amplifier circuits are rarely adopted in audio circuit design, but Class B still has particularly good advantages.

To understand the above statement in general, since the transistor of Class A is always in a charged state and emits a lot of heat during operation, let it not work when it is "resting", which can save a lot of heat dissipation costs, and because the transistor is not When working without electricity, the amplifier efficiency leaves more "space", and the amplification efficiency reaches 70-80%, which is really exciting in theory. But the sound quality is unbearable. It was originally a girl chanting poems, but now she has become a shrew cursing the street, becoming noisy and gaffes. In the final analysis, the switching speed of the current on the two transistors cannot keep up with the switching speed of the audio signal phase, resulting in distortion and severe crossover distortion. The sound quality is muddy, and no matter how high the efficiency of the power amplifier is, it is useless, so the Class B power amplifier is slowly abandoned.

The third type is a Class A and B amplifier, also known as a Class AB amplifier, which is a compromise combination of a Class A (Class A) and a Class B (Class B) amplifier. It also uses a bias current, but it is much smaller than a pure Class A amplifier. , The small bias current is enough to prevent crossover distortion, which can provide good sound quality. Its power consumption is between Class A and Class B amplifiers, but it is usually closer to Class B (Class B) amplifiers. Similar to Class B amplifier circuits, Class A and B amplifiers also need some control circuits to provide or absorb more The output current. However, well-designed Class A and B amplifiers also have a lot of power consumption, because the output voltage of the middle range is usually far away from the positive and negative power supplies. Since the voltage drop between the drain and the source is very large, a large instantaneous power LDS×VDS will be generated. Generally speaking, Type A and Type B have their own advantages. The former has good linearity but low power, and the latter is the opposite. Therefore, it is an inevitable trend to combine the advantages of the two technologies. In fact, there are many manufacturers that use Class A and B amplification types in the market. For example, the famous Krell in the United States also uses a Class A and B design, but with the patented iBias Class A technology, it can even increase the bias current characteristics while ensuring high power. , Make the sound linearity and simulation higher and better.

Digital amplifier circuit, PWM signal transmission

D-class design, many friends will say that you see it particularly on the subwoofer, yes! The characteristic of Class D is that it can produce extremely high output power with extremely low power consumption. The output stage of the Class D amplifier switches between the positive power supply and the negative power supply to generate a series of voltage pulses. This waveform is beneficial to reduce power consumption, because the output transistor has zero current when it is not conducting and has zero current when it is conducting. Very low VDS, resulting in smaller power consumption LDS×VDS. Since most audio signals are not pulse trains, a modulator must be included to convert the audio input into a pulse signal. The frequency components of the pulse include the required audio signal and important high-frequency energy related to the modulation process. A low-pass filter is often input between the output stage and the speaker to minimize electromagnetic interference (EMI) and avoid too much high-frequency energy to drive the speaker.

But when it comes to Class D, most of the current amplifiers use the famous Dutch Hypex Ncore amplifier components. This company specializes in the development of Class D amplifier modules, and its components are widely used in products of different brands. In addition, ICE Power in Denmark is also very famous. Many amplifiers and active speaker products in Northern Europe, such as B&O, are brands that use this component. However, today's Class D technology is different from the past. It is very advanced in terms of technological maturity and sound performance, even surpassing the traditional Class A/Class B sound quality effects. Products such as NAD amplifiers are deeply loved by a new generation of enthusiasts. Small size, extremely low energy consumption and excellent functional performance often make traditional amplifiers ashamed.

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