Power matching
In order to achieve high fidelity listening requirements, the rated power should be determined based on the best listening sound pressure. We all have this feeling: when the volume is small, the voice is weak, thin and dynamic, the dullness, the low frequencies are significantly lacking, and the fullness is poor, and the sound seems to shrink in and out. When the volume is appropriate, the sound is natural, clear, rounded, soft, full, powerful and dynamic. However, when the volume is too loud, the sound is not soft, rough, and has a feeling of rooting. Therefore, the replay sound pressure level and the sound quality have a greater relationship, and the sound pressure level of the specified listening area is preferably 80~85dB (A weighting). We can calculate the distance from the listening area to the sound box and the characteristic sensitivity of the sound box. The rated power of the speaker and the power rating of the amplifier.
Power reserve matching
Speaker: In order to make it able to withstand the impact of intense bursts 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.
Amplifier: The required power reserve is different for the tube amplifier and the transistor amplifier. This is because the overcharge curve of the tube power amplifier is relatively flat. For the peak of the music signal that is overcharged, the tube amplifier does not obviously produce clipping, but only rounds the top of the peak. This is what we often call a soft cut. However, after the transistor power is placed at the overload point, the nonlinear distortion rapidly increases, causing severe clipping of the signal. Instead of rounding the peak, it cuts it neatly. Some people use resistors, inductors, capacitors, and composite impedance analog speakers to test the actual output capability of several high-quality transistor amplifiers. The results show that in the case of a phase shift in the load, there is a nominally 100 W power amplifier, and the actual output power is only 5 W at a distortion level of 1%! The selection of the transistor power amplifier reserves:
High fidelity amplifier: 10 times
Civil high grade power amplifier: 6~7 times
Civil mid-range amplifier: 3~4 times
The tube amplifier can be much smaller than the above ratio.
For the system's average sound pressure level and maximum sound pressure level should be left with the amount of margin, should be based on the content of the broadcast program, the work environment may be. The minimum amount of this redundancy is 10dB. For modern pop music, Judi and other music, 20~25dB redundancy is required, which can make the audio system work safely and stably.
Impedance matching
It refers to the amplifier's rated output impedance, which should be consistent with the speaker's rated impedance. At this time, the power amplifier is in the best design load line state, so it can give the maximum undistorted power. 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 amplifier, the audio system can work, but the amplifier has the danger of overload, requiring the amplifier to have a perfect over-current protection measures to solve, and the impedance matching requirements for thetube amplifier are more stringent.
Damping coefficient matching
The damping coefficient KD is defined as: KD = amplifier output impedance (equal to the speaker rated impedance) / amplifier output resistance.
Because the amplifier output internal resistance has actually become the speaker's resistance device, the KD value determines the resistance of the speaker. The larger the KD value is, the heavier the resistor is. Of course, the KD value of the power amplifier is not as large as possible. The KD value is too large to make the speaker resistance too heavy, and the pulse front establishment time increases, and the transient response index is reduced. Therefore, the KD value should not be unilaterally pursued when choosing the power amplifier. As an example, there is an empirical value for home-use high fidelity power amplifier damping coefficient. The minimum requirement is that 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.
To ensure the steady-state characteristics of the playback and the basic conditions for good transient performance, attention should be paid to the combination of the equivalent mechanical quality factor (Qm) of the speaker and the amplifier damping coefficient (KD). This coordination needs to use the speaker feeder as the overall sound system. Part to consider. The equivalent resistance of the speaker's feeder should be small enough to be negligible compared to the speaker's rated impedance. In fact, the power loss of the speaker feeder should be less than 0.5dB (about 12%) to achieve this cooperation.