Getting Started with Audio: Power Amplifiers

                                         Getting Started with Audio: Power Amplifiers

The working principle of the power amplifier is actually very simple, which is to amplify various sound signals played by the audio source to drive the speaker to emit sound. From a technical point of view, the power amplifier is like a current modulator, which converts alternating current to direct current, and then is controlled by the sound signal played by the audio source, and transmits currents of different sizes to the speakers according to different frequencies, so that the speakers will be the same. The size and the corresponding frequency of the sound. Due to the consideration of power, impedance, distortion, dynamics, and different use ranges and control adjustment functions, different power amplifiers are also different in internal signal processing, circuit design and production technology. According to the needs of current audio consumption, power amplifiers in civilian audio have basically been finalized into two categories, namely, pure music power amplifiers and home theater AV power amplifiers.

pure music power amplifier

The design of pure music power amplifiers emphasizes the lowest signal distortion, and faithfully shows the scenes, details, performance and recording skills of music to meet people's requirements for the best appreciation of music. This is what people often call HI-FI (hi-fi) fidelity, high fidelity). In terms of design and production, the requirements for pure music power amplifiers are extremely strict. The quality of a pure music power amplifier is not completely determined by its technical indicators. We should not simply look at how high its marked power is, how wide its frequency response is, and how low its distortion is. Instead, we should pay special attention to its design and production process and its music resolution. For example, tube amplifiers with low technical specifications are better than many transistor power amplifiers.

AV amplifier

Generally, it includes the power amplifier part and the signal processing part. The power amplifier part is no different from the traditional power amplifier in principle, but several channels are added, that is, several power amplifiers are combined together; the signal control processing part involves signal audio, video selection, signal decoding processing, and signal processing. Sound field processing and radio, monitoring and other functions.

Generally, a high-quality AV power amplifier should firstly have better sound field restoration in the signal processing of film and television programs, high channel isolation, and the atmosphere rendering should not be too exaggerated; secondly, in terms of the sound quality performance of the power amplifier part, especially the main The sound quality of the channel should be as close as possible to a better pure music power amplifier.

Classification of power amplifier

Power amplifiers are generally divided into front-level power amplifiers, rear-level power amplifiers and combined power amplifiers.

A combined machine is a machine that integrates the front stage and the back stage. The front stage is used to initially amplify the signal and adjust the volume; while the rear stage is to amplify the signal from the front stage to drive the speaker.

The front stage is also divided into two types: active and passive. The active front stage uses power to amplify the signal, while the passive front stage only has the effect of adjusting the volume. To be honest, there are not many successful passive pre-amplifiers today, because the internal resistance of the sound source and the post-stage is very different. Only a volume switch is used to connect the sound source and the post-stage. The difference in internal resistance will make the dynamics, details, frequency All should be lost! In addition to adjusting the volume, the active front stage can also be used as a buffer for the first part and reduce the internal resistance between the sound source and the rear stage.

How the amplifier works

1. Class A power amplifier (also known as Class A power amplifier)

The two (or two sets of) transistors in the output stage of the class A power amplifier are always in a conductive state, that is to say, they keep conducting current regardless of whether there is a signal input or not, and make the two currents equal to the peak value of the alternating current. At this time, the alternating current is at the maximum signal case of inflow load. When there is no signal, the two transistors each flow an equal amount of current, so there is no unbalanced current or voltage at the output center point, so there is no current input to the speaker. When the signal tends to be positive, the output transistor above the line allows more current to flow in, while the output transistor below the line relatively reduces the current. Since the current starts to be unbalanced, it flows into the speaker and drives the speaker to sound.

Class A power amplifier is the ideal choice for replaying music. It can provide very smooth sound quality, round and warm timbre, and transparent and open treble. These advantages are more than enough to make up for its shortcomings. Class A power amplifiers generate an astonishing amount of heat. In order to effectively deal with heat dissipation, Class A power amplifiers must use large radiators. Because of its low efficiency, the power supply must be able to provide sufficient current. A 25W Class A power amplifier power supply is at least enough for a 100W Class AB power amplifier. Therefore, the volume and weight of class A machines are larger than those of class AB, which increases the manufacturing cost and makes the price more expensive. Generally speaking, the price of a class A power amplifier is about twice or more than that of a class AB power amplifier of the same power.

2. Class B power amplifier (Class B power amplifier)

The working method of class B power amplifier amplification is that when there is no signal input, the output transistor is not conductive, so it does not consume power. When there is a signal, each pair of output transistors amplifies half of the waveform, and each other works in turn to complete a full-wave amplification. When the two output transistors work alternately, crossover distortion occurs, thus forming nonlinearity. There are fewer pure class B amplifiers, because the distortion is very serious when the signal is very low, so the crossover distortion makes the sound rough. Class B power amplifiers have an average efficiency of about 75%, generate less heat than Class A amplifiers, and allow the use of smaller radiators.

3. Class AB power amplifier

Compared with the first two types of power amplifiers, Class AB power amplifiers can be said to be a compromise in performance. Class AB power amplifiers usually have two bias voltages, and a small amount of current flows through the output transistors when there is no signal. It uses Class A working mode when the signal is small to obtain the best linearity, and automatically switches to Class B working mode to obtain higher efficiency when the signal increases to a certain level. The 10-watt class AB power amplifier of ordinary machines works with class A within about 5 watts. Since the power required for listening to music is only a few watts, class AB power amplifiers work in class A power amplifier mode most of the time, only when there is music It is converted to Class B only when there is a transient strong sound. This design can obtain good sound quality and improve efficiency to reduce heat, which is a quite logical design. Some Class AB power amplifiers adjust the bias current very high, so that they can work as Class A in a wider power range, making the sound close to pure Class A machines, but the heat generated is relatively increased.

4. Class D power amplifier (Class D power amplifier)

This design is also called digital power amplifier. Once the transistor amplified by the Class D power amplifier is turned on, its load is directly connected to the power supply, and the current flows but the transistor has no voltage, so there is no power consumption. When the output transistor is turned off, the full power supply voltage appears on the transistor, but no current flows, so no power is consumed, so the theoretical efficiency is 100%. The advantages of class D power amplifier amplification are the highest efficiency, the power supply can be reduced, and almost no heat is generated, so no large radiator is needed, the volume and weight of the body are significantly reduced, and theoretically, the distortion is low and the linearity is good. However, the work of this kind of power amplifier is complicated, and the added circuit itself will inevitably have deviations, so there are very few truly successful products, and the price is not cheap.

There are some Class D power amplifier integrated blocks with good sound quality, but they are only used in car audio now, and some interested DIY masters have remade them into home audio.

Many UK-made integrated amps aren't overly powerful, but have a very generous power supply and, with simple signal paths, can achieve excellent sound. Except for volume, balance, source selection and power switch on the panel of some products, all other controls are cancelled, so that the signal channel is shortened as much as possible. In pursuit of pure sound, control functions are sacrificed.