How to choose a power amplifier ?

                                                The choice of power amplifier!

1. Weight

Weight is also an important technical indicator of power amplifiers, do you believe this? This is one of the keys to choosing a good power amplifier. Generally speaking, if you weigh it with your hand, the heavier the power amplifier of the same kind and the same price, the more genuine it will be. Secondly, it should be checked whether the actual weight is consistent with the nominal value on the manual, whether there is any exaggeration or short "jin" and less "liang". If you think about the real power amplifier, which material can cut corners? If you want to get good sound quality, you can only use exquisite materials, which is bound to add weight.

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2. Rated output power and maximum undistorted power.

The rated output power of a power amplifier refers to the sinusoidal alternating power it can withstand for a long time, which is the knowledge that everyone must understand when purchasing a power amplifier. In actual work, the power amplifier is subjected to constantly changing power. Since the audio electrical signal changes in a large range, the peak value that appears at a certain moment may exceed or multiply the rated output power of the power amplifier. Therefore, unlike the speaker, the output power of the power amplifier is usually described by the maximum undistorted power instead of Its rated power, we must pay attention to this.

The definition of maximum undistorted power refers to the maximum power that can be output under the condition that the total harmonic distortion coefficient of the output signal is less than 1% in the frequency range of 20-20KHz when the power amplifier is connected to an 8 ohm load. Rated 

Power is generally taken as 50% of its maximum power output value. For stereo power amplifiers, the maximum undistorted power can be expressed by the sum of the power of all channels, for example, the maximum undistorted power of a power amplifier is 400W [3 channels]; but there are quite a few power amplifiers that use channel power + sound It is represented by channel power, for example, the maximum undistorted power of a power amplifier is 70W[L]+70W[R]. Since there is no unified standard in the world, everyone needs to pay attention to their differences when purchasing.

So how to consider the maximum undistorted power of the power amplifier when purchasing? For ordinary families in our country, due to the limitation of housing area, the sound power of medium volume when listening to music is generally about 0.2W[L]+0.2W[R]. 5%, so in order to obtain a sound power of 0.2W, the power amplifier needs to input 4W of electric power to the speaker. If it is calculated by 10 times the required power reserve, it is necessary to consider and choose a 2X40W power amplifier. Everyone should pay attention when purchasing.

Picture 3. Damping coefficient

The damping coefficient D.F is the braking coefficient. We know that the sound from the horn is produced by the vibration of the corresponding frequency audio and current flowing into the voice coil of the horn. When the audio current to the speaker is cut off, due to the inertia of the vibrating part and the energy stored, the speaker will continue to vibrate freely and emit a sound that was not there before, thereby reducing the clarity of the audio system and generating waveform distortion. Therefore, in order to prevent this phenomenon from happening, it is necessary to study the internal resistance of the power amplifier, that is, the relationship between the rated output impedance of the power amplifier and the rated impedance of the speaker, which is the birth of the concept of damping coefficient.

Damping coefficient = rated impedance of the speaker / rated output impedance of the power amplifier. The rated output impedance of the power amplifier is equivalent to a low-value resistor, which is connected in parallel with the speaker. When the speaker vibrates freely, the induced current will pass through the low-value resistor to damp the vibration. Therefore, when the rated impedance of the speaker is usually When it is a fixed value, the internal resistance of the output stage of the power amplifier should be reduced as much as possible. The test shows that the more suitable damping coefficient is between 10-100. For audio enthusiasts, practice has proved that when the very thin horn lead wire is lengthened, the damping coefficient will be reduced. Replacement, which will improve the sound quality to some extent.

4. Power reserve.

Speakers are mainly used for listening, and in symphony, the signal amplitude changes greatly, so there are many short-term peak powers, which may be several times to dozens of times higher than the average working power of general programs. If the maximum undistorted power of the power amplifier is not large enough, "clipping" will occur at these peaks, and the listener will feel that the sound is harsh and weak. In order to overcome clipping and HiFi replays the original signal with high fidelity, it is required that the power amplifier should have a certain amount of power reserve. The power reserve means that the maximum undistorted power of the power amplifier is greater than the multiple of its average working power. From the requirements of high-fidelity playback, it is of course hoped that the larger the power reserve, the better, but if the power reserve is too large, the volume and weight of the required power amplifier will be large, and the cost will also be expensive. Therefore, for a general professional or household high-end audio system, the power reserve of the power amplifier should be greater than 20 times; for a general audio system, a power reserve of 5-20 times is enough. You can have a little understanding of this aspect when purchasing a power amplifier.

5. Match the power amplifier and the speaker.

Nowadays, most audio enthusiasts buy speakers by themselves by purchasing various components or assembling them by stand-alone. In the process of audio combination, the matching of power amplifier and speakers is very important. In general, the following points should be paid attention to:

a. First of all, pay attention to the matching of impedance.

The output form of the power amplifier is generally divided into two types: constant impedance output and constant voltage output. Usually, the tube power amplifier adopts constant impedance output, while the transistor power amplifier adopts constant voltage output. For the currently widely used transistor power amplifier with constant voltage output, its main feature is to set a certain degree of negative feedback in the final circuit, so that even if the load of the amplifier changes within the rated power range, its output voltage and The change index of distortion is also very small, which makes it easier to match the amplifier and the speaker. The general method is to match through the line transformer.

Whether it is a power amplifier with constant voltage output or constant impedance output, it is required that the impedance of the speaker [speaker] as the load should not be less than the rated impedance of the power amplifier, and the impedance of the two should be the same. For example, the power amplifier is originally designed to be connected to an 8-ohm load. When it is connected to a 16-ohm speaker, it will not bring other effects except that the output power will be reduced by half; and when it is connected to a 4-ohm load speaker, its output power will increase by nearly 10%. Doubled, if the volume is turned up again, it is very likely that the high-power transistor will be damaged.

b. Power matching.

Power matching is also very important, it means that the rated power of the power amplifier and the rated power of the speaker should be compatible. Generally speaking, speakers with a rated power of 10-20W commonly used in households should choose a power amplifier with a rated power about three times larger than it, that is, 30-70W, which is more suitable, and the matching of power reserves is also considered here. Everyone must pay attention when purchasing, so as not to use "small horses to pull big carts" or "big horses to pull small carts", which are not good.

C. Damping coefficient matching.

When the damping coefficient is small, the low-frequency characteristics, output voltage characteristics, and high-order distortion characteristics of the speaker will all become worse; while the damping coefficient is too large, the impact on the actual performance is not significant. Therefore, the more consistent view is that the damping coefficient Should be between 10-100.

Picture 6. Frequency response.

Frequency response is frequency response. The frequency response of the power amplifier is an indicator of the ability of the amplifier system to reproduce all frequency components evenly. Frequency response can generally be divided into magnitude frequency response and phase frequency response. Its unit is dB. At present, the working frequency band of general household power amplifiers is 20Hz-20kHz; while the working frequency band of professional power amplifiers can reach 0Hz-40kHz. For HiFi power amplifiers, in order to obtain a good listening effect, the only way is to continuously widen its high-frequency response and low-frequency response. SNR.

The signal-to-noise ratio is the signal-to-noise ratio. It refers to the ratio of the useful signal output by the power amplifier to the output noise power, in dB. The larger the dB value of the signal-to-noise ratio, the smaller the impact of noise on the signal, and the higher the quality of the power amplifier. The signal-to-noise ratio of the current high-fidelity amplifiers must be at least 90dB.

The noise of the power amplifier mainly comes from the noise delivered by the preamplifier, which is amplified step by step to a larger noise level; the thermal noise generated by the components of the power amplifier circuit itself, which is manifested as continuous interference in a wide frequency range; the power amplifier The AC noise generated by power supply and electromagnetic induction has an interference frequency equal to the power supply frequency and its multiples. The key to improving the signal-to-noise ratio of the power amplifier is to reduce the noise from the front preamplifier circuit and the AC noise generated by the power amplifier itself and electromagnetic induction.

8. Dynamic range.

For advanced audio systems, the power dynamic range of the power amplifier [the change of the minimum volume and the maximum volume] is also an important technical indicator. We know that the dynamic range of the volume when we usually speak is about 20-40dB, the dynamic range of listening to music is generally about 40-60dB, and the dynamic range of symphony is about 80dB, sometimes even as high as 100dB. In order to meet the needs of the dynamic range of the above-mentioned sound source and achieve high-fidelity playback, the maximum undistorted power of the high-end audio power amplifier is often made very large to ensure that the dynamic range is around 100dB. When purchasing, we should try our best to choose a power amplifier with a wide dynamic range.

Nine. Distortion.

The distortion of the power amplifier mainly includes harmonic distortion, intermodulation distortion, transient intermodulation distortion, clipping distortion, crossover distortion and so on.

Harmonic distortion is also called harmonic distortion. It is caused by the nonlinear transfer characteristics in the amplifier circuit. It is specifically manifested as some new harmonic components in the output signal that are not in the original input signal in the original signal, which is reflected in the human body. The auditory sense will feel the sound of the instrument is out of shape. The distortion caused by each harmonic is superimposed to be the total harmonic distortion. The total harmonic distortion depends on the design and production quality of the amplifier circuit, but it is also related to the frequency and output power of the signal. The test shows that at the frequency of 1000Hz, the harmonic distortion is the smallest, and as the frequency increases, the harmonic distortion increases. It is wrong for some manufacturers to list the distortion degree of the power amplifier at 1000Hz as the factory standard. You should pay attention to this point when purchasing. In addition, the test also shows that when the output power of the power amplifier is close to the maximum undistorted output power, the total harmonic distortion will increase sharply. A good power amplifier should have at least less than 0.1% total harmonic distortion in the full audio range.

Intermodulation distortion is also called intermodulation distortion. When the power amplifier inputs two or more frequency signals at the same time, due to the nonlinear transfer characteristics, each frequency and the sum frequency and difference frequency between frequencies will be generated at the output end, for example, the bass tube signal of 200Hz and the frequency of 600Hz The violin signals are combined to produce two weak intermodulation distortion signals at 400Hz [difference signal] and 800Hz [sum signal]. Since the intermodulation distortion signal has no similarity with the natural instrument signal, it is particularly easy to perceive, so reducing intermodulation is one of the keys to improving the sound quality.

Crossover distortion is caused by the Class B push-pull amplifier of the power amplifier excitation stage [driver stage] working in the nonlinear region. Using a bias voltage keeps the operating point of the amplifier in the linear region, but if the bias voltage is too low, slight crossover distortion may occur during small signal transitions from positive to negative.

Clipping distortion is when the excitation voltage of the power amplifier is increased, the output transistor may reach the peace point, and at this time the output voltage cannot be further increased, and the sine wave signal begins to clip. In addition, due to transient intermodulation distortion may also Causes crossover distortion in the input stage. Crossover distortion mainly occurs on small power amplifiers. It makes the sound with higher sound pressure level become blurred and flickering. At this time, the only thing to do is to reduce the volume to prevent the amplifier from overloading.

Finally, let's focus on the transient intermodulation [TIM] distortion of the power amplifier. Transient intermodulation distortion means that when the power amplifier suddenly inputs the pulse signal generated by the percussion instrument, because the compensation capacitor of the excitation stage of the negative feedback circuit needs charging time, the power amplifier has no output signal in an instant or the collector voltage of the excitation stage transistor has to go through a delay time. Only then can reach the maximum value, which causes transient intermodulation distortion. In addition, during this delay time, since the output voltage does not reach the desired voltage value, the input stage will not get the proper negative feedback voltage, and because of the deep negative feedback, the input stage will be seriously overloaded instantaneously. The voltage is often dozens of times higher than the rated voltage value, so the signal will be cut and flattened, which is transient intermodulation distortion.

Practice has proved that music with a large dynamic range is prone to sudden transient intermodulation distortion, especially when a series of percussion signals in the high frequency band appear, obvious transient intermodulation distortion will occur. Severe transient intermodulation distortion is reflected in hearing as high-frequency crossover distortion, and weaker transient intermodulation distortion gives people an uncomfortable feeling of "metal sound" or "transistor sound", unlike the tube power amplifier. The treble sound is thin and clear. There is a problem, is there a solution? Here are a few things to start with:

a. The power amplifier adopts the full complementary symmetrical way to directly integrate the power amplifier, and selects the circuit transistors of all levels with fast conversion speed and good linearity. The characteristic frequency value of the final high-power transistor should reach 60MHz or more, so as to improve the open-loop performance index of the amplifier, and at the same time Reduce deep negative feedback to small amount of negative feedback.

b. Change the lag compensation capacitor to an advance compensation capacitor to reduce transient intermodulation distortion. The specific method is to connect a large capacitor with a suitable capacity to the emitter resistor of the amplification stage of the amplifier voltage.

c. Under the premise of not affecting the signal-to-noise ratio, the quiescent current of the input stage can be appropriately increased, which can reduce the possibility of transistor overload.

d. Adopt high-voltage, low-voltage dual power supply mode, and the preamplifier is powered by high-voltage power supply to improve the overload capacity of the transistor and expand its linear area.

e. Each front stage of the circuit is best to use Class A amplification, and optimize the design to reduce the harmonic distortion of the whole system, especially to control the harmonic distortion of the amplifier in the mid-frequency band within 0.1%, which also greatly reduces intermodulation distortion.

f. Using a frequency division amplifier to reduce the operating frequency range of the amplifier unit. This kind of amplifier generally divides the audio frequency range into three sections of high, middle and low frequencies for amplification, and then directly matches with the corresponding speakers, so the damping of the power amplifier and the speaker is well matched, so it can also improve the system's new state characteristics and reduce transient interaction. tune distortion.

g. Increasing the output power reserve of the amplifier can also avoid the intermodulation distortion caused by the short-term overload of the signal to a certain extent.

Finally, it should be pointed out that, in addition to transient intermodulation distortion, transistor power amplifiers, such as harmonic distortion, intermodulation distortion, crossover distortion, clipping distortion, etc., will seriously affect the playback sound quality, and some of them are in the sense of hearing. It is also close to transient intermodulation distortion, so we should carefully distinguish the above distortions, comprehensively check and analyze the reasons, and don't prematurely attribute poor sound quality to transient intermodulation distortion.