Power Amplifier Basics

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Power amplifiers are usually divided into five categories according to their working status. They are Class A (A), Class B (B), Class AB (AB), and Class D

1. Class A power amplifier:

The output power is small and the power consumption is large, but the distortion is small, so it is rarely used.

The operating point of the output transistor (or electron tube) of the Class A amplifier is at the midpoint of its linear part. No matter how the signal level changes, the current it takes out from the power supply is always constant. It is inefficient. When used for audio amplification, due to the constant change of signal amplitude, its actual efficiency cannot exceed 25%. It can be operated by a single tube or push-pull. The advantages of Class A amplifiers are no crossover distortion and switching distortion, and the harmonic components are mainly even harmonics. In terms of listening, the bass is thick, the midrange is soft and warm, the treble is clear and neat, and the sense of layering is good, which is very pleasing. However, it has not been widely used in high-power amplifiers because of its high power consumption, low efficiency, easy heat generation and high requirements for heat dissipation. Because the device works at high current and high temperature for a long time, it is easy to cause problems in reliability and life, and the cost of the whole machine is high. Therefore, most manufacturers that are famous for manufacturing Class A power amplifiers have now stopped producing transistor Class A power amplifiers.

2. Category B (B) release:

The bias of the Class B (B) amplifier makes the push-pull transistor (or electron tube) in a low current state when there is no driving signal. When the driving signal is added, the current of one of the pair of tubes rises in half a cycle, while the other tube tends to be cut off. In the other half cycle, the situation is reversed. Since the two tubes work in turn, a push-pull circuit must be used to amplify the complete signal waveform. The advantage of the Class B amplifier is high efficiency, which can reach 78% in theory, and the disadvantage is large distortion.

3. Class AB (AB) Class AB amplifier:

The output power is large and the power consumption is moderate, but the distortion is greater than Class A. We currently use this type of amplifier in large quantities.

When the Class AB amplifier is driven at a low level, the amplifier works as Class A. When the driving level is increased, it changes to Class B. The advantage of Class AB amplifier is that it improves the efficiency of small signal input compared to Class A. As the output power increases, the efficiency also increases. Although the distortion is greater than Class A, it is still the most widely used transistor power amplifier program. The trend is that more and more Class AB with high bias current is used to reduce the distortion of low-level signals. Most car amplifiers now use this method.

4. Digital (D) Class D amplifier:

It is a new type of amplifier (digital amplifier) that has appeared in recent years. Its biggest advantage is extremely low power consumption. It usually uses a speaker effect tube that is resistant to secondary breakdown and has extremely high switching conversion efficiency. The efficiency can reach more than 90%. (The maximum efficiency of ordinary Class A and Class AB amplifiers does not exceed 50%) This means that it consumes less power, has low heat dissipation requirements, and is easy to integrate and mass produce. Another advantage is low distortion, fast conversion time, fast and accurate transient response, which is particularly suitable for heavy and low-power single amplifiers with high explosive power requirements. It has a tendency to replace other amplifiers in car audio.

Class D power amplifiers and traditional analog power amplifiers have two different working principles. They are switching audio power amplifiers. Class D uses PWM technology and is an amplifier whose switching frequency changes with the clock pulse cycle. In high-power applications, digital power amplifiers have wide frequency response, large dynamic range and good transient response. Its advantages are low distortion, strong anti-interference ability, small heat sink area, small size and light weight, low power consumption, high conversion efficiency, and Class A and B sound quality. At present, the main cost is high. Although the cost is reduced with the development of software and hardware technology, the application of digital power amplifiers will become more and more

5. Main performance indicators of power amplifier:

The main performance indicators of a power amplifier include output power, frequency response, distortion, signal-to-noise ratio, input sensitivity, input impedance, output impedance, damping factor, load capacity, operating voltage, etc.

1. Output power: The unit is W. Due to different measurement methods of different manufacturers, there are some different names, such as rated output power, maximum output power, music output power, and peak music output power.

Music power (PMPO): refers to the maximum instantaneous output power of the amplifier for music signals under the condition that the output distortion does not exceed the specified value. The output power calculated by the peak voltage that the music signal can reach instantly has more commercial significance than practical use. PMPO power can be 3 to 4 times higher than the internationally recognized effective value rated output power (RMS). For example, the RMS power of each channel of early portable radio recorders was only 4 or 5 watts, but with PMPO, the value can be increased to about 20W.

Peak power: refers to the maximum music power that an amplifier can output when the volume is adjusted to maximum without distortion.

Rated output power: the average output power when the harmonic distortion is 10%. Also known as the maximum useful power. For amplifiers, rated power generally refers to the effective value (RMS) power that can be continuously output; for speakers, rated power generally refers to the speaker's ability to withstand this value of power for a long time without being damaged. This does not mean that such a high-power amplifier is required to drive it. The difficulty of driving the speaker is mainly determined by its sensitivity and impedance characteristics. It does not mean that an amplifier with an output power greater than the rated power of the speaker cannot be used. Just like driving a car, driving a sports car at a speed of 300 kilometers per hour does not mean that a car accident will occur. You can drive at a low speed. Similarly, as long as the volume is not blindly increased, a high-power amplifier can also be equipped with a low-power speaker. Generally speaking, the peak power is greater than the music power, and the music power is greater than the rated power. Generally speaking, the peak power is 5-8 times the rated power.

2. Frequency response: Indicates the frequency range of the amplifier and the unevenness within the frequency range. The flatness of the frequency response curve is generally expressed in decibels [db]. The frequency response of a home HI-FI amplifier is generally 20HZ--20KHZ plus or minus 1db. The wider this range, the better. The frequency response of some top-quality amplifiers has reached 0--100KHZ.

Distortion: An ideal amplifier should amplify the input signal and faithfully restore it without any change. However, due to various reasons, the signal amplified by the amplifier is often distorted to varying degrees compared with the input signal. This distortion is distortion. Expressed as a percentage, the smaller the value, the better. The total distortion of a HI-FI amplifier is between 0.03% and 0.05%. The distortion of an amplifier includes harmonic distortion, intermodulation distortion, crossover distortion, clipping distortion, transient distortion, transient intermodulation distortion, etc.

3. Signal-to-noise ratio: refers to the ratio of various noise levels to signal levels output by the amplifier, expressed in dB, the larger the value, the better. Generally, the signal-to-noise ratio of a home HI-FI amplifier is above 60db.

Input sensitivity: This is the adjustment level set for hosts and pre-amplifier sound sources from different manufacturers and brands. The range is from 100mv to 4V or even higher, and it must be matched with the sound source when tuning.

Input impedance: Generally, the input impedance of the power amplifier should be high and the output impedance should be low. The higher the input impedance, the more effectively it can block various types of noise. Common values are 10KΩ or higher.

4. Output impedance: The equivalent internal resistance presented to the speaker is called output impedance.

Damping coefficient: This coefficient is calculated from the rated load (4Ω) output impedance. It is generally believed that the smaller the output impedance, the higher the damping coefficient, and the better the amplifier. In fact, for high-quality amplifiers, the ratio is mostly above 50, and some even exceed 500. Although some experts believe that a damping coefficient of around 50 is sufficient, based on my personal experience, the higher the coefficient, the more relaxed the wire requirements. Relatively speaking, a coefficient that is too high will affect the timbre, but it will help the bass performance.

Load capacity: Home amplifiers are generally 8Ω/4Ω; car amplifiers are 2Ω to 8Ω in stereo; 4Ω to 8Ω in bridge connection. However, some specially designed amplifiers can have impedance as low as 0.1Ω, which is extraordinary. At this time, an amplifier can connect dozens of woofers in parallel to create an ideal sound pressure level (sound pressure refers to the pressure of sound on the human ear. It is a standard for measuring the ability of the sound system, because the higher the sound pressure, the higher the requirements for the system. The highest record in China is 141.5dB and abroad is 176.5dB). Just like those vans with dozens of subwoofers in the car, which can break the glass, but this scene can only be seen in sound competitions.

5. Car Amplifier

Generally it works normally in the range of 10V to 15V.

①Basic settings of the amplifier:

From a functional perspective, most current power amplifier products already have the following functions.

Electronic crossover. This function has three options: FULL - full tone; HP: high pass - only allow frequencies above the crossover point (such as 80Hz) to pass, this setting is mainly used for mid-range and tweeter speakers; LP: low pass - only allow frequencies below the crossover point (such as 80Hz) to pass, this setting is mainly used for bass speakers. Some of them are also set to adjustable crossover points, which can be set according to different system designs, and some are fixed at 80Hz, 100Hz, 120Hz.

② Signal input (line input) selection: There are two input methods: RCA signal (low voltage) and host speaker line (high level). To obtain good sound quality, you can choose RCA signal input (ideal frequency response and excellent signal-to-noise ratio, but the premise is that the host has RCA output). If the host does not have RCA output or retains the original car host, you can only choose an amplifier with high-level input. In addition, many amplifier products have one or two groups of signal output (lineout) to transmit the signal to another amplifier. This can not only save the cost of the crossover, but also ensure excellent sound quality, because different merchants use different methods when the signal output is not enough to distribute, and some processing methods will deteriorate the sound quality.

③ Input gain adjustment (Gain): This knob is used to adjust the input voltage of the power amplifier and the signal voltage transmitted from the host to achieve the most ideal matching state to ensure that the sound will not be distorted.

④ Bridged output: When the power amplifier adopts the bridge connection, the output power can generally be increased by 2 times, so that it has another use when needed (such as driving a subwoofer).

⑤ Tone adjustment: A considerable number of products are equipped with bass (Bass) and treble (Tweeter) adjustments, which can be boosted or attenuated at the two frequencies of 45Hz and 10KHz respectively. The adjustment range is between 0dB-12dB, which can make the reproduced bass fuller and deeper, and the treble clearer and more transparent.

Amplifier and speaker matching

　　⑥The matching of power amplifier and speaker mainly includes power matching, impedance matching, frequency matching and style matching.

Power Matching:

People usually attribute the main factor that may cause the speaker coil to burn out to the fact that the power of the power amplifier is greater than that of the speaker. Therefore, when matching, the power of the speaker is greater than that of the power amplifier, which is a common misunderstanding. In fact, such a match is prone to burnout. The reason is: if the rated power of the power amplifier is 100W and the rated power of the speaker is 200W, when the system is connected, once the volume is increased to make the output power of the power amplifier reach about 100W, the power amplifier is already in a full load state. The speaker load is only half. If the volume continues to increase, the power amplifier will enter an overload state. In other words, the power amplifier begins to enter "clipping distortion". This distorted signal can generate an electrical signal similar to direct current, which can easily burn out the voice coil of the speaker. This is because the voice coil has a very small impedance and a large current when it is direct current. For a speaker with an input power of 200W, a 150W electrical signal with a distortion rate of 50% is more likely to burn out the voice coil of the speaker than a 300W electrical signal without distortion. Therefore, maintaining the margin of the power amplifier is an important condition for system matching. It is generally customary to select an amplifier whose power is slightly greater than or equal to the rated power of the speaker.

Impedance matching reflects the efficiency transmission relationship between the input circuit and the output circuit. When the circuit achieves impedance matching, the maximum power output will be obtained. Otherwise, the maximum power output cannot be obtained, and the circuit may be damaged. If the impedance of the speaker is much smaller than the output impedance of the amplifier, the amplifier will be overloaded and damaged. If the impedance of the speaker is much higher than the output impedance of the amplifier, the output voltage will increase, which is not conducive to the operation of the amplifier, causing distortion, and will also cause the amplifier to be damaged by overvoltage.

Under what circumstances can the maximum power be output? According to Ohm's law I=E/(r+R) and the electric power law P=IR, it can be deduced that P=IR=[E/(r+R)]R. After sorting, we get P=ER/(Rr), because E and R are constants. To get the maximum P, the denominator can only be minimized, that is, (Rr)=0, that is, R=r, so that the maximum power can be obtained. This is the reason why the output impedance of the power amplifier must match the input impedance of the speaker.

Dual outputs of amplifiers and single outputs for bridging

When the dual output of the amplifier is bridged, if the impedance remains unchanged, the output current doubles. According to the law of electrical power, P=I ×V. When the current doubles, the power increases four times, which will burn out the amplifier. Therefore, when bridging, the speaker impedance should be doubled. For example, if the dual output load is 4 ohms, it should be 8 ohms when bridging (unless the amplifier has special instructions).

Frequency Matching

Since the working frequencies of high, medium and low frequency speakers are different, the amplifiers are usually divided into full-band amplifiers and bass-only amplifiers according to this feature. Low-frequency amplifiers can only ensure that the distortion is less than 1% below 250HZ. When the frequency exceeds 250HZ, the distortion increases sharply. So don't try to use a bass-only amplifier to drive the mid-high speakers. Full-band amplifiers usually use class AB amplifiers, which have large power losses. So filter out the low-frequency signal, set it in high-pass mode, and only drive the mid-high speakers, saving power and ensuring the best choice for the sound quality of the mid-highs, such as the rear sound field. Therefore, the operating frequency of the amplifier is required to match the operating frequency of the speaker. The amplifier and the speaker can only play their best role when the impedance, power and frequency are matched between the two.

⑦ Style matching: Japanese and American equipment all consciously compensate for low and high frequencies, and the sound is often not as balanced as European products. Everyone has different tastes, so you can experience it for yourself.

　　⑧Installation of the amplifier in the car:

　　After reading this introduction, I believe you have realized the importance of the power amplifier to the entire audio system. Since the power amplifier plays a very important role in the audio system, it is very important to ensure the normal performance of the power amplifier during installation.

　　First of all, when installing the amplifier in the car, in addition to considering the matching with the speaker, the power supply, including the consideration of the wire material, and reasonable wiring, should all be paid attention to in advance. When installing, various wires should avoid short circuits, and it is best to install them with insulation, such as density board (this is a special wooden board, often used in speaker production, also known as sawdust board), and avoid nailing it directly on the iron sheet of the car body, which will cause noise.

　The current car audio installation technology, especially the Hi-end design, often pays great attention to the installation technology, and the system layout of the trunk is beautiful. The charm of the amplifier is artistically displayed through exquisite craftsmanship. This is not just a pleasant thing, the technical requirements are very rigorous.