When designing and installing a sound system, it is inevitable to encounter problems with the connection between the power amplifier and the speakers. From the artistic point of view, the mating of the power amplifier and the speakers should be warm and cold, soft and hard, and ultimately make the whole set of equipment restore the tone to be neutral. From a technical point of view, the following points should be paid attention to when connecting power amplifiers and speakers: power matching; power reserve matching; impedance matching; damping coefficient matching. If we recognize the above four points when mating, the performance of the equipment used can be brought into full play.
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 this feeling: when the volume is small, the sound is weak, thin and dynamic, dull, the low frequency is significantly lacking, and the fullness is poor, and the sound seems to be shrinking inside and unable to come out. When the volume is right, the sound is natural, clear, round, soft, full, powerful, and dynamic. However, when the volume is too high, the sound is blunt, not soft, rough, and has the feeling of rooting in the ears. Therefore, the playback sound pressure level has a greater relationship with the sound quality. It is stipulated that the sound pressure level of the listening area should be 80～85dB (A weighting). We can calculate it from 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 amplifier.
Power reserve matching
Loudspeaker: In order to enable it to withstand the impact of strong bursts in the program signal without being damaged or distorted. Here is an empirical value for reference: the nominal rated power of the selected speaker should be 3 times the theoretically calculated power.
Power amplifier: Compared with the transistor power amplifier, the required regular reserve is different for the tube power amplifier. This is because the overload curve of the tube amplifier is relatively flat. For the overcharged music signal peaks, the tube power amplifier does not obviously produce clipping, but only rounds the peaks of the peaks. This is what we often call flexible shear peaks. After the transistor power is placed at the overload point, the non-linear distortion increases rapidly, causing serious clipping of the signal. It does not round the peak but flatten it neatly. Someone used a composite impedance composed of resistance, inductance, and capacitance to simulate speakers, and tested the actual output capabilities of several high-quality transistor power amplifiers. The results show that when the load has a phase shift, there is a nominal 100W power amplifier, and the actual output power is 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
High-end civil power amplifier: 6 to 7 times
Civil mid-range power amplifier: 3 to 4 times
The tube power 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 program and the working environment. This redundancy is at least 10dB. For modern pop music, bungee music and other music, you need to leave 20-25dB redundancy, so that the sound system can work safely and stably.
It refers to the rated output impedance of the power amplifier, which should be consistent with the rated impedance of the speaker. At this time, the power is in the best designed 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 audio system can work, but the power amplifier is in danger of overload. The power amplifier is required to have complete overcurrent protection measures to solve it. The impedance matching requirements for the tube power amplifier are more stringent.
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)/power amplifier output internal resistance. Since the internal resistance of the power amplifier output has actually become the resistance of the speaker, the KD value determines the amount of resistance experienced by the speaker. The larger the KD value, the heavier the resistance value. Of course, the KD value of the power amplifier is not the better. Too much KD value will make the speaker resistance too heavy, so that the pulse frontier settling time will increase and the transient response index will be reduced.