How should the power of the amplifier and the speaker match?

When installing a sound system, it is inevitable to encounter the problem of the connection between the amplifier and the speaker. In terms of sound, we will pay attention to whether it is suitable for warm and cold, moderate soft and hard, and finally make the whole set of equipment return to neutral tone. This is only from an artistic perspective. From the technical aspect, the factors for power amplifier and speaker matching are: 1. power matching, 2. impedance matching, 3. damping coefficient matching, 4. sensitivity matching, 5. tone matching. If we recognize the above five points when mating, we can make the performance of the equipment used to be larger and fuller.

Power matching

In order to meet the requirements of high-fidelity listening, the rated power should be determined according to the better listening sound pressure. We all have the feeling that the volume is small, the sound is weak, thin and dynamic, and there is no gloss, the low frequency is significantly lacking, and the fullness is poor. It seems that the sound cannot be contracted inside.

When the volume is appropriate, the sound is natural, clear, round, soft and plump, powerful and dynamic. But when the volume is too high, the sound is stiff and not soft, rough, and feels ear-piercing. 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 from the distance from the listening area to the speaker and the characteristic sensitivity of the speaker. Speaker's rated power and power amplifier's rated power.

The output power of the power amplifier circuit has various names, such as rated power (RMS), music power, peak music power (PMPO), etc., their meanings are different from each other, but the most applied and most important power is the rated power. Businesses also often create power under other names, which are all based on commercial propaganda, or avoiding weaknesses and propagating advantages.

Strict rated power should make strict regulations on frequency response range, harmonic distortion, load impedance and signal-to-noise ratio, etc. The rated power value without these restrictions is of no value. The rated power should be a comprehensive technical indicator.

The rated output power of the amplifier and the rated input power of the speaker should be compatible with each other. The rated power of the power amplifier should be slightly larger than 1/4 of the rated power of the speaker. For example, a 125W power amplifier should drive a speaker of about 100W. Practical speakers have a certain overload capacity, and its allowable value is about 1.5 times the rated power amplifier. Transistor power amplifier has a strong overload capacity, and its distortion changes less when overloaded.

In the actual use of power amplifiers and speakers, usually do not reach the rated power value, the actual average power used is relatively small, the power used is only 1 / 3--1 / 5 of the rated power. The power needs to be adapted and matched. On the surface, the rated powers of the two are similar. Actually, it means that the power reserve and surplus are adapted; in other words, the power amplifier and the speaker are operated at the rated power for a long time (for example, 8 hours) ( Within the limits of the specified frequency response range, distortion, and signal-to-noise ratio impedance), various problems cannot occur.

Without reducing the limiting conditions, when the speaker world power amplifier power value is increased, the price will also skyrocket. Under ordinary small listening room conditions (for example, less than 20 square meters), there is no need to choose an amplifier with an excessive output power. A power amplifier with a rated power of 60-80W (8 ohms) can already perform general playback tasks.

In order to make the loudspeaker not be damaged or distorted by the strong pulse of burst in the program signal. Here is an empirical value for reference: The nominal rated power of the selected speaker should be three times the theoretically calculated power.

Compared with transistor power amplifiers, tube power amplifiers require different power reserves. This is because: the overload curve of the tube amplifier is relatively smooth. For the high quality of overloaded music signals, the tube amplifier does not obviously produce clipping, but only makes the high quality of the peak round. This is what we often call flexible clipping.

After the transistor amplifier is at the overload point, the non-linear distortion increases rapidly, which severely clips the signal. It does not round the peak but neatly cuts it. Some people use resistors, inductors, and capacitors to compound speakers to simulate the actual output capability of several high-quality transistor amplifiers. The results show that under the condition of load phase shift, there is a nominal 100W power amplifier, and the actual output power is only 5W at 1% distortion! Therefore, the reserve of the transistor power amplifier is selected: 10 times higher fidelity Amplifier, 6 ~ 7 times the civilian high-quality amplifier and 3 ~ 4 times the civilian mid-range amplifier,

How much margin should be left for the average sound pressure level and the larger sound pressure level of the system depends on the content of the program and the working environment. This redundancy is 10dB lower. For modern pop music, dsico and other music, 20 ~ 25dB redundancy is required, so that the sound system can work safely and stably.

 Impedance matching

In short, the rated output impedance of the amplifier should be consistent with the rated impedance of the speaker. At this time, the power amplifier is in a better design load line state, so it can give a large 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 power amplifier, the audio system can work, but the power amplifier has the risk of overload. It requires the power amplifier to have perfect overcurrent protection measures to solve it. The impedance matching requirements for the tube power amplifier are more stringent.

To match the amplifier and the speaker, impedance matching is the most important. The loudspeaker is the load main body of the power amplifier. The nominal (or rated) impedance of the loudspeaker should be equal to or close to the custom output impedance of the power amplifier. How much rated load impedance should be connected to the power amplifier circuit. This is a basic parameter for manufacturers to design power amplifiers.

Transistor amplifiers are low-impedance output circuits; tube amplifiers are high-impedance output circuits, and they have very strict requirements on the impedance of the speakers. But the transistor low-impedance output power amplifier still puts forward certain requirements on the load impedance value. For example, the output load of the original designed power amplifier should be 8 ohms, which is an ideal power amplifier circuit. When it is connected to a 16 ohm speaker, its output power is reduced by about half. When it is connected to a 4 ohm speaker, the output power is approximately doubled.

However, most of the power amplifiers are not ideal high-quality Suzhou. The output internal resistance cannot be infinitely small, the amplification loop cannot provide sufficient current gain, and the regulated power supply cannot provide sufficient working current. When the power amplifier is connected to a speaker with low impedance, the transient characteristics are deteriorated, and the degree of distortion will increase. It should have a larger power output, but the power value will not go up.

For power amplifiers that are calibrated with external 4-16 ohm loads, they should be connected to speakers with the median impedance range as much as possible. When the amplifier is connected to a speaker that is higher than its rated load impedance, the rated output power drops, which has little effect on other performance indicators; but if the power supply voltage margin is not large, it may still indicate that the intended rated power has been overloaded. distortion.

It must be seen that when the impedance does not match, it may cause the damping coefficient of the power amplifier to change. The damping coefficient of the power amplifier is the ratio of the load resistance of the power amplifier (mainly the speaker impedance) to the internal resistance of the power amplifier output. When the speaker impedance changes, it can cause the damping coefficient of the power amplifier to change.

If the damping coefficient becomes too small, the low-frequency characteristics of the speakers, the frequency characteristics of the output sound pressure, and the characteristics of higher-order harmonic distortion will deteriorate, and the output audio (especially low audio) will be bloated and turbid, accompanied by naivety. If the damping coefficient is too large, the low-frequency volume will be weakened, and the sound will be dry and not rich, but this situation is rare and has little effect on the actual playback effect.

Matching of damping coefficients

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 output of the amplifier.

Because the internal resistance of the power amplifier output has actually become the resistance of the speaker, the KD value determines the resistance of the speaker. The larger the KD value, the heavier the resistance, of course, the larger the KD value of the power amplifier, the better. If the KD value is too large, the resistance of the speaker 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 pursue a large KD value. As a household high-fidelity power amplifier damping coefficient, there is an empirical value for reference. The lower 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 for ensuring the steady and transient characteristics of playback are to pay attention to the coordination of the equivalent mechanical quality factor (Qm) of the speaker and the damping coefficient (KD) of the amplifier. This cooperation requires the speaker's feeder line as the overall sound system To consider. The speaker's equivalent resistance should be small enough to be negligible compared to the speaker's rated impedance. In fact, the power loss of the speaker feed line should be less than 0.5dB (about 12%) to achieve this coordination.

Sensitivity matching

The large output power of the amplifier does not mean that the speaker has a strong driving force. The strong driving force is related to the output power of the power amplifier, but also to many other factors, especially the sensitivity of the speaker. Speaker sensitivity is an important factor in determining the output power of the amplifier.

A definition of the sensitivity of the speaker is: 1W of electrical power is fed to the speaker, and the sound pressure (dB) can be obtained at 1 meter on the front axis of the speaker. The unit is dB / W / m. For example, the sensitivity of the speaker is 86dB / W / m, which means that the speaker inputs 1W of electric power, and the sound pressure at the 1m front axis of the speaker is 86dB. At present, high-sensitivity speakers stay capitalized at 95dB / W / m, or even exceed 100dB / W / m, while low-sensitivity speakers are only 82-86dB / W / m. Many hI-Fi speakers used for listening to music have low sensitivity (such as 82-84dB / W / m); AV amplifiers should be matched with speakers with higher sensitivity (about 90dB / W / m) as much as possible. However, when the sensitivity is too high, the sound is thin and bright, and the details and charm of reproduction are not enough.

The difference in speaker sensitivity has a significant impact on the requirements for speaker drive power. Every time the sensitivity of the speaker is reduced by 3dB, in order to achieve the same sound intensity, the output power of the amplifier needs to be doubled. For example, the sensitivity of the speaker has been reduced from 90dB / W / m to 87dB / W / m. Originally using a 50W power amplifier, the power of the amplifier should now be increased to 100W.

Similarly, if the speakers produce the same sound pressure level, the driving power should be increased by 16 times; in other words, if a 160W amplifier is used to drive a 83dB / W / m speaker, then a 10W amplifier can be used to drive 95dB / W / m speakers, they can produce the same sound pressure, it can be seen that when the sensitivity of the speakers is not the same, the required drive power is different; the speaker with high sensitivity can use a smaller driving power to achieve the required volume. When matching audio equipment, the issue of speaker sensitivity adaptation is very important.

Voice matching

Tone adaptation means that the tone of the amplifier and the speaker must be properly matched with each other to obtain the user's favorite playback tone. The tone of the equipment is subjective. Different people like different tones. Personality, hobbies, cultural accomplishments, and experiences all affect the listener's preference for tone.

Due to the historical changes and national cultures in different countries, audio equipment in different countries also have different timbre characteristics. For example, the British speakers sound warm and sweet, the German speakers are cool and gorgeous, the French speakers are beautiful and luxurious, the Danish speakers are rich in music, and the American speakers are loud and loud. Even domestic equipment will feel different after listening carefully. The sound of the tube amplifier is warm, but the transient response is poor; the tone of the transistor A and B amplifier is brighter, but slightly hard; the transistor A amplifier is somewhere in between.

Because various amplifiers and speakers have their own sound characteristics, some sounds are warmer, some are cooler, some are softer, and some are harder. Therefore, users can match them according to the characteristics of the sound to form a unique Tone-specific audio equipment system. Through reasonable matching of equipment, the strengths and weaknesses can be avoided, the heating and cooling can be complemented, and the soft and hard can be matched to obtain beautiful sound.

For example, using bright and rough amps with soft speakers, and using complementary effects, you can get a more neutral tone. If users have a clear preference for certain types of music, they may wish to choose amplifiers and speakers with the same tone color. Tonal matching requires multiple factors. Often, after careful listening and more taste, you can accurately experience the sound characteristics of the sound. If you want to take Hi-Fi into consideration with your home theater, it is recommended that you do not use equipment with too strong personality, and try to use equipment with easy-going temperament and neutral tone.

 

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