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First, the power impedance problem: In order to achieve high fidelity listening requirements, the rated power should be determined according to the best listening sound pressure. We all have this feeling: the impedance problem of the amplifier and the speaker is low, the sound is thin, the dynamic is not coming out, the matt, the low frequency is obviously lacking, the fullness is poor, and the sound seems to shrink in the inside. Sound impedance problems, clear, round, soft, full, powerful, and dynamic when the volume is right. However, when the volume is too large, the sound is not soft, rough, and has a feeling of being rooted. Therefore, the sound pressure level of the playback has a large relationship with the sound quality. It is recommended that the sound pressure level of the listening area is preferably 80 to 85 db (a weighting). 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 amplifier.
Second, the power reserve matching: Speaker: In order to enable it to withstand the impact of the strong pulse in the program signal without damage or distortion. Here is an empirical value for reference: the nominal power rating of the selected speaker should be three times the theoretically calculated power.
Amplifier: Compared with transistor amps, the required power amplifier speaker impedance problem reserve is different. This is because the overcharge curve of the tube amp is relatively flat. For the peak of the over-charged music signal, the tube amp does not obviously cause clipping, but only rounds the tip of the speaker impedance problem. This is the flexible shear peak we often say. When the transistor power is placed at the overload point, the nonlinear distortion increases rapidly, and the signal is severely clipped. It does not round the peak but tidy it flat. Some people use a composite impedance analog loudspeaker consisting of resistors, inductors, and capacitors to test the actual output capability of several high-quality transistor amplifiers. The results show that in the case of phase shift of the load, there is a nominal 100w power amplifier, the actual output power is only 5w when the distortion is 1%! Therefore, the reserve of the transistor power amplifier is selected:
High-fidelity power amplifier: 10 times civilian high-end power amplifier: 6 to 7 times civilian mid-range power amplifier: 3 to 4 times and the tube power amplifier can be much smaller than the above ratio.
How much margin should be left for the average sound pressure level of the system and the maximum sound pressure level, depending on the content of the program and the working environment. This redundancy is at least 10db. For modern pop music, Judy and other music, you need to leave 20~25db redundancy, which can make the sound system work safely and stably.
Third, impedance matching: It refers to the rated output impedance of the power amplifier, should be consistent with the rated impedance of the speaker. At this point, the power is in the optimal 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 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 amplifier, the sound system can work, but the power amplifier has the danger of overload, and the power amplifier is required to have perfect over-current protection measures to solve the problem. For the tube amp, the impedance matching requirement is stricter. Home theater network welcome you Fourth, the damping coefficient matching: damping coefficient kd is defined as: kd = power amplifier rated output impedance (equal to the speaker rated impedance) / amplifier output internal resistance. Since the internal resistance of the amplifier output has actually become the resistor of the speaker, the kd value determines the amount of resistance the speaker receives. The larger the kd value is, the heavier the resistance is. Of course, the kd value of the power amplifier is not as large as possible. The kd value is too large, so that the speaker resistance is too heavy, so that the pulse front time is established and the transient response index is lowered. Therefore, when selecting an amplifier, one should not pursue a 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, the tube power amplifier kd value is greater than or equal to 6.
To ensure the basic conditions of the steady-state characteristics and transient characteristics of the playback, attention should be paid to 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 considered as part of the overall sound system. The equivalent resistance of the feeder'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 cooperation.
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( ×Hell Cat × ) I think that the box should not be measured by power. Or, power considerations are second to none.
The reason is that the box has no plug and does not directly energize, so no power is generated. Its current comes from the head of the box. When the head is energized, it generates power. The electric energy is transmitted to the box through the speaker wire, which pushes the speaker inside the box (the speaker itself is impedance). Then there is a concept that the power generated by the box head is like exerting a force on the box. Whether the box works normally depends on whether it can withstand this "force". So we see that on the foreign cabinet's website, the description of the power involved is "power handling" rather than "power".
This concept is clear, then what is the first consideration when we choose the box?
Yes, it is the impedance of the box and the impedance of the box head match.
The rule is that the impedance of the box head is not greater than the impedance of the box.
The impedance of the box head is the impedance output generated after the box head is energized, and will be indicated on the back of the box head. Many box heads are now designed for multi-impedance output.
It is to match the impedance of different boxes, because often the box is only designed as a single impedance.
We see that the relationship between the power and impedance output of many boxes is often described as 600W @ 4ohm, 350W @ 8ohm, which can be directly understood as the impedance of the box. That is to say, when the head of the box is connected to the box of this impedance, and the level of the master is turned to the maximum, several watts of power can be generated.
So what is the impedance of the box? It is the total impedance after the connection of multiple speakers in the box.
We know that the connection of the speakers is nothing more than series and parallel. For example, a 2X10 cabinet, the impedance of each 10-inch speaker is 8 ohms. If connected in series, the total impedance of the box is 8+8=16 ohms. If connected in parallel, the total impedance of the box is 1/(1/8+1/8)=4 ohm (according to Ohm's law).
After knowing the impedance of the box head and the impedance of the box, it will not be a problem to match the above rules. For example, if the cabinet is 8 ohms, then the box head can only be 8 ohms or less than 8 ohms.
As I said earlier, the box head is often designed to be multi-impedance choice, or just give an example. For example, a box head back plate says 600W @ 4ohm, 350W @ 8ohm, then when the box head is connected to the 4ohm box, the box The head operates at a resistance of 4 ohms, and the master is turned to maximum, producing 600 watts of power. If it is connected to a 8 ohm box, it produces 8 ohms and the Master's maximum power is 350 W. This is the case for normal work.
To talk about the opposite situation, if the head is biased, only one impedance is chosen to be 8 ohms. The impedance of the box is smaller than the box head, such as a 4 ohm box.
How about this?
Ohm's law is used here again. For the box head, the working voltage is constant, then the larger the impedance, the smaller the current, and the smaller the power generated (P=U*I). The 8 ohm cabinet produces power that is less than 4 ohms.
However, the impedance of the cabinet is 4 ohms, which requires the box head to transmit more power to it, and the box head can only produce 8 ohms of low power, so the box will require the box head to work overload to provide enough power (box The body is more "to". Over time, the head of the box may be overworked. . .
This is why the primary consideration when choosing a cabinet is the impedance. Please be sure to follow the above red rules to select the box head and the box.
The so-called power of the box, as mentioned at the beginning of the article, is just the power (force) that it can withstand from the box head. This factor is considered as a secondary factor.
For example, my own box head is Ampeg SVT-4PRO, its impedance output selection is more complicated, there are mono, two-channel distinction, divided into three grades, 2ohm, 4ohm and 8ohm. The single and double channel connection cabinets have different powers depending on the impedance of the cabinet.
Due to limited conditions, I also do not have the spirit of exploration. So I chose the simplest connection method: mono connected to a box: Ampeg SVT410HLF. This box is 4 ohms, so after the connection, the box head automatically selects the working mode of 4 ohms, that is, when the master is turned to the maximum, it generates 1200 W of power! ! ! Haha, but don't fear, power and volume are not linear, so the volume is not as big as horror!
And my box can withstand only 500W. Very simple, as long as I don't open the volume to the maximum, the box will not be bad.
In fact, when the Master opened to 12 o'clock, the volume was full of "shock". Therefore, the general use, it will not produce 1200W power, but also more power-saving, huh, huh.