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Power amplifier and speaker matching


The matching of power amplifier and speaker, that is, power matching, is a very important problem. We must put "faithful restoration of music" in the first place. In the design, installation of a sound system, inevitably encountered amplifier and speaker matching problem. In terms of timbre, we will pay attention to whether it is matched with warm and cold, with moderate hardness and hardness, and finally make the whole set of equipment neutral in tone, which is only considered from the artistic aspect. From the technical aspect, the elements of power amplifier and speaker matching are:
1. power matching
2. impedance matching
Matching of 3. damping coefficients
4. sensitivity matching
5. timbre matching
If we recognize the above five points when matching, the performance of the equipment used can be maximized and fully utilized.
1. Power matching In order to meet the requirements of high-fidelity listening, the rated power should be determined according to the best listening sound pressure. We all have the feeling that the volume is small, the sound is weak, thin, the dynamic cannot come out, the dull, the low frequency is obviously lacking, the fullness is poor, and the sound seems to shrink inside and cannot come out. When the volume is right, the sound is natural, clear, round, soft and full, powerful and dynamic. However, when the volume is too high, the sound is stiff and soft, rough and has the feeling of pricked ears. Therefore, the playback sound pressure level has a great relationship with the sound quality. The sound pressure level of the listening area is preferably 80 ~ 85dB(A weight). We can calculate the rated power of the speaker and the rated power of the amplifier from the distance from the listening area to the speaker and the characteristic sensitivity of the speaker.
The output power of the power amplifier circuit has a variety of names, such as rated power (RMS), music power, peak music power (PMPO), etc., which have different meanings, but the most widely used and most important power is the rated power. Businesses also often produce the power of other names, which are out of commercial propaganda, or to avoid weaknesses and promote advantages. Strict power ratings should have strict requirements for frequency response range, harmonic distortion, load impedance, and signal-to-noise ratio. Rated power values without these constraints are of no value. Rated power should be a comprehensive technical indicators.
The rated output power of the amplifier and the rated input power of the speaker should be adapted to each other. The rated power of the power amplifier should be slightly greater than 1/4 of the rated power of the sound box. For example, a 125W power amplifier should drive a sound box of about 100W. Practical speakers have a certain overload capacity, and its allowable value is about 1.5 times of the rated power amplifier. The transistor power amplifier has a strong overload capacity, and its distortion changes less when overloaded. In the actual use of power amplifiers and speakers, usually do not reach the rated power value, the actual average power used is relatively small, the practical power is only 1/3-1/5 of the rated power. The power should be adapted and matched. On the surface, the rated power of the two is similar, which actually refers to the adaptation of the power reserve and surplus. In other words, all kinds of problems cannot occur when the power amplifier and sound box work at the rated power state for a long time (e.g. 8 hours) (under the specified frequency response range, distortion, signal-to-noise ratio grid impedance and other conditions). Without lowering the restrictions, when the power value of the amplifier in the world situation of the speaker is increased, the selling price will also be rapidly stingy. Under the condition of ordinary small listening room (for example, below 20 square meters), there is no need to choose a power amplifier with excessive output power, and a power amplifier with a rated power of 60-80W(8 ohms) can complete the general playback task.
In order to make the sound box in the program signal by the burst of strong pulse impact and not damage or distortion. Here is an empirical value for reference: the nominal rated power of the selected speaker should be three times the power calculated by theory.
 
The required power reserve of a tube amplifier is different from that of a transistor amplifier. This is because the overload curve of the electronic tube amplifier is relatively gentle. For the peak of the over-charged music signal, the tube amplifier does not obviously produce clipping, but only rounds the tip of the peak. This is what we often say flexible shear peak. However, after the transistor power amplifier is at the overload point, the nonlinear distortion increases rapidly, resulting in serious clipping of the signal. It does not round the peak but cuts it neatly. Some people use the compound impedance of resistance, inductance and capacitance to simulate the speaker, and test the actual output capability of several high-quality transistor power amplifiers. The results show that under the condition of load phase shift, one of the power amplifiers with a nominal scale of 100W has an actual output power of only 5W when the distortion is 1%! Therefore, the selection of the reserve amount of the transistor power amplifier: high-fidelity power amplifier: 10 times the civilian high-grade power amplifier: 6~7 times the civilian mid-grade power amplifier: 3~4 times and the electronic tube power amplifier can be much less than the above ratio.
How much margin should be left between the average sound pressure level and the maximum sound pressure level of the system depends on the content of the broadcast program and the working environment. The minimum amount of redundancy 10dB, for modern pop music, disco and other music, you need to leave 20 ~ 25dB redundancy, so that the sound system can be safe, stable work.
   
2. Impedance matching
 
Simply put, the rated output impedance of the amplifier should be consistent with the rated impedance of the speaker. At this time, the power amplifier is in the best design load line state, so the maximum undistorted power can be given. If the rated impedance of the speaker is greater than the rated output impedance of the power amplifier, the actual output power of the power amplifier will be less than the rated output power. If the rated impedance of the speaker is less than the rated output impedance of the power amplifier, the sound system can work, but the power amplifier is in danger of overload. The power amplifier is required to have perfect overcurrent protection measures to solve the problem. The impedance matching requirements for the electronic tube power amplifier are more stringent. Power amplifier and speaker to adapt, impedance matching is the most important. The speaker is the load body of the power amplifier, and the nominal (or rated) impedance of the speaker should be equal to or similar to the customer output impedance of the power amplifier. How much rated load impedance should be matched with the power amplifier circuit is a basic parameter for manufacturers to design power amplifiers. The transistor power amplifier is a low impedance output circuit, while the electronic tube power amplifier is a high impedance output circuit, which requires very strict impedance value of the speaker. However, the transistor low impedance output power amplifier still puts forward certain requirements for the load impedance value. For example, the output load of the original design power amplifier should be 8 ohms, which is an ideal power amplifier circuit, and when equipped with a 16 ohm speaker, its output power is reduced by about half, while when equipped with a 4 ohm speaker, the output power is about doubled. However, most power amplifiers are not ideal top-level Suzhou, their output internal resistance cannot be infinitely small, their amplification loop cannot provide sufficient current gain, and regulated power supply cannot provide sufficient working current. When this power amplifier is connected to a speaker with too low impedance, the transient characteristics will deteriorate and the distortion degree will increase. It should have a larger power output, but the power value will not go up. Power amplifiers with external 4-16 ohm loads shall be connected to speakers with medium impedance range as far as possible. When the power amplifier is connected to a speaker higher than its rated load impedance, the rated output power decreases, which has little effect on other performance indicators. However, if the power supply voltage margin is not large, overload distortion may have occurred when the rated power of the above target is still reached. It should be seen that when the impedance is not matched, the damping coefficient of the power amplifier may change. The damping coefficient of the power amplifier is the ratio of the load resistance of the power amplifier (mainly the impedance value of the speaker) to the internal resistance of the power amplifier output. When the impedance value of the speaker changes, it can cause the damping coefficient of the power amplifier to change. If the damping coefficient becomes too small, the low frequency characteristics of the speaker, the output sound pressure frequency characteristics, and the higher harmonic distortion characteristics will all deteriorate, and the output audio (especially low audio) will be bloated and turbid, accompanied by innocence. If the damping coefficient is too large, it will weaken the sense of low frequency, and the sound will be dry and not rich. However, this situation is rare and has little effect on the actual playback effect.
 
3. Matching of damping coefficient
 
The damping coefficient KD is defined as: KD = rated output impedance of the power amplifier (equal to the rated impedance of the speaker)/internal resistance of the power amplifier output. Since the internal resistance of the power amplifier output has actually become the electrical damper of the speaker, the KD value determines the amount of electrical damping of the speaker. The larger the KD value, the heavier the electrical damping. Of course, the larger the KD value of the power amplifier, the better. Too large a KD value will make the electrical damping of the speaker too heavy, which will increase the pulse front establishment time and reduce the transient response index. Therefore, in the selection of power amplifier should not be one-sided pursuit of large KD value. As a household high-fidelity power amplifier damping coefficient has an empirical value for reference, the minimum requirements: transistor power amplifier KD value is greater than or equal to 40, tube power amplifier KD value is greater than or equal to 6. To ensure good basic conditions for the steady-state and transient characteristics of the sound, attention should be paid to the coordination of the equivalent mechanical quality factor (Qm) of the speaker and the damping coefficient (KD) of the amplifier. This coordination needs to consider the feeder of the speaker as an integral part of the sound system. The equivalent resistance of the feeder of the speaker should be small enough to be negligible compared with the rated impedance of the speaker. In fact, the power loss of the speaker feeder should be less than 0.5dB (about 12%) to achieve this coordination.
   
4. Sensitivity matching
 
The output power of the power amplifier is not equal to the driving force of the speaker. The powerful driving force is related to the output power of the power amplifier, and it is also related to many other factors, especially the sensitivity of the speaker. The sensitivity of the speaker is an important factor in determining the output power value of the power amplifier. One definition of speaker sensitivity is: 1W of electric power is fed to the speaker, and the sound pressure (dB) can be obtained at 1 meter on the front axis of the speaker, in dB/W/m. For example, the sensitivity of the speaker is 86dB/W/m, which means that the speaker inputs 1W of electric power, and the sound pressure at the front axis 1m of the speaker is 86dB. At present, high-sensitivity speakers stay capitalized 95dB/W/m, even more than 100dB/W/m, while low-sensitivity speakers are only 82-86dB/W/m. Many hI-Fi speakers used for listening to music have low sensitivity (e. g. 82-84dB/W/m);AV amplifiers should be matched with speakers with high sensitivity (about 90dB/W/m). However, when the sensitivity is too high, the timbre is thin and bright, and the details and charm of the music are not enough.