Four elements for connecting amplifier and speaker

When designing and installing a set of audio system, it is inevitable to encounter the problem of the connection between the power amplifier and the speaker. In terms of timbre, we will pay attention to whether the matching is suitable for cold and warm, moderate softness and hardness, and finally make the whole equipment restore the tone to be neutral, which is only considered from the artistic aspect. From the technical aspect, the factors for matching the power amplifier and the speaker are as follows: one, power matching, two, power reserve matching, three, impedance matching, and four, damping coefficient matching. If we realize the above four points when mating, the performance of the equipment used can be fully and maximized.

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 low, the sound is weak, thin, and the dynamics cannot come out, 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 ~ 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.

Power reserve matching

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 three times the theoretically calculated power.

Power amplifier: Compared with the transistor power amplifier, the power reserve of the tube power amplifier is different. This is because: the overload curve of the tube amplifier is gentler. 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 neatly cuts it flat. 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 there is a nominal 100W power amplifier with a phase shift of the load, and the actual output power is only 5W when the distortion is 1%! Therefore, the selection of the reserve of the transistor power amplifier:

Hi-Fi amplifier: 10 times

Civil high-end power amplifier: 6 ~ 7 times

Civil mid-range power amplifier: 3 ~ 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. It should depend on the content of the broadcast program and the working environment. The minimum redundancy is 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.

Impedance matching

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 amplifier is in the state of the optimal design load line, so it can give the maximum undistorted power. If the rated impedance of the speaker is greater than the rated output impedance of the amplifier, the actual output power of the 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 has the risk of overloading. The power amplifier needs to have perfect overcurrent protection measures to solve it. The impedance matching requirements for the tube power amplifier are stricter.

Damping coefficient matching

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 front settling time will increase, reducing the transient response index. 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.

The basic conditions to ensure good steady-state and transient characteristics of sound playback, attention should be paid to the matching of the equivalent mechanical quality factor (Qm) of the speaker and the damping coefficient (KD) of the amplifier. This cooperation requires the speaker feeder to be the overall sound system Part of it. The equivalent resistance of the speaker's feeder should be small enough to be negligible compared to 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.
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