Audio Power Matching

- Apr 21, 2020-

Explanation


When designing and installing a sound system, it is inevitable that the matching problem between the power amplifier and the speaker will be encountered. From an artistic point of view, the matching of the power amplifier and the speaker should be suitable for cold and warm sound, moderate hardness and softness, and finally make the whole equipment restore the sound neutral. From the technical point of view, the following points should be paid attention to when connecting the power amplifier and the speaker: power matching; impedance matching; damping coefficient matching. If we realize the above three points when mating, the performance of the equipment used can be fully exerted.


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 when the volume is small, the sound is weak, thin, and dynamic, and there is no gloss, the low frequency is significantly lacking, the fullness is poor, and the sound seems to shrink in. When the volume is appropriate, the sound is natural, clear, round, soft and full, powerful and dynamic. But when the volume is too loud, the sound is not soft, rough, and has a sense of ears. Therefore, the playback sound pressure level has a greater relationship with the sound quality. The sound pressure level in the listening area is preferably 80 to 85dB (A-weighted). We can calculate the distance from the listening area to the speaker and the characteristic sensitivity of the speaker The rated power of the speaker and the rated power of the power amplifier.


Speaker: In order to make it able to withstand the impact of burst strong pulse in the program signal without damage or distortion. Here is an empirical value for reference: the nominal rated power of the selected speaker should be more than twice the power calculated by theory.


Power amplifier: Compared with the transistor power amplifier, the required power reserve is different. This is because the overload curve of the tube amplifier is gentle. For the peak of the overloaded music signal, the tube amplifier does not obviously produce clipping, but only rounds the peak of the peak. This is what we often call flexible shear peaks. After the transistor power is placed at the overload point, the nonlinear distortion increases rapidly, which severely clips the signal. Instead of rounding the peak, it flattens it. Some people use a composite impedance composed of resistors, inductors, and capacitors to simulate speakers to test the actual output capability of several high-quality transistor power amplifiers. The results show that in the case of a phase shift in the load, there is a nominal 100W power amplifier, and the actual output power is only 5W at 1% distortion! Therefore, the selection of the reserve of the transistor power amplifier: high-fidelity power amplifier: 10 Times, civilian high-end power amplifier: 6 to 7 times, civilian mid-range power amplifier: 3 to 4 times. The tube amplifier can be much smaller than the above ratio. How much margin should be left for 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. This redundancy is at least 10dB. For modern pop music, pop music and other music, you need to leave 20-25dB redundancy, so that the sound system can work safely and stably.


2. impedance matching


The output of the power amplifier and the input impedance of the speaker should also be the same. Although there is a slight deviation between the two, it will not have a significant impact on the sound quality, but will only have an effect on the output power of the amplifier, but it must be reminded that if the speaker input impedance is much lower than the amplifier output impedance, it will cause a significant increase in distortion , In severe cases, the amplifier will be damaged. Therefore, everyone should try to choose the two consistent.


3. Matching of damping coefficient


The damping coefficient KD is defined as: KD = rated output impedance of the amplifier (equal to the rated impedance of the speaker) / internal resistance of the amplifier output. Because the internal resistance of the power amplifier output has actually become the resistance of the speaker, the KD value determines the amount of resistance the speaker receives. The larger the KD value, the heavier the resistance. Of course, the larger the KD value of the power amplifier is, the better. If the KD value is too large, the speaker resistance will be too heavy, so that the pulse leading time will increase, and the transient response index will be reduced. Therefore, when choosing the power amplifier, one should not unilaterally pursue a large KD value. As a household high-fidelity power amplifier damping coefficient, there is an empirical value for reference. The minimum requirements are: the transistor power amplifier KD value is greater than or equal to 40, and the tube power amplifier KD value is greater than or equal to 6.


What you need to know is that if the damping coefficient is too high, the sound from the speaker will be dry and not rounded; if it is low, the sound will stick together. Therefore, when you do the power amplifier configuration, you should pay attention to the damping coefficient of the stone machine between 30 and 40, and the amplifier has a high internal resistance, so it must not exceed 10.


Of these three principles, the most critical is the first point. Here we reiterate again that the matching of the amplifier and the speaker is better than the big horse-drawn carriage, rather than the last small horse-drawn carriage.