How To Match The Power Of Amplifier And Speaker

- Jul 20, 2020-

When installing a sound system, it is inevitable to encounter problems with the connection between the power amplifier and the speakers. In terms of timbre, we will pay attention to whether the combination is suitable for cold and warm, soft and hard, and finally restore the whole set of equipment to a neutral tone. This is only an artistic consideration. From the technical point of view, the elements of 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, the performance of the equipment used can be maximized and maximized.


1. 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 this feeling: the volume is small, the sound is weak, thin, unable to come out dynamically, dull, the low frequency is significantly lacking, 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 loud, the sound is blunt, not soft, rough, and has an ear-piercing feeling. Therefore, the replay sound pressure level has a greater relationship with the sound quality. The sound pressure level in the listening area is better to be 80~85dB (A-weighted). 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.


The output power of a power amplifier circuit has many names, such as rated power (RMS), music power, peak music power (PMPO), etc. They have different meanings, but the most used and most important power is rated power. Merchants often produce power with other names. These are all for commercial propaganda, or to avoid weaknesses and propaganda advantages.


Strict power ratings should set strict regulations on frequency response range, harmonic distortion, load impedance, and signal-to-noise ratio. The power ratings without these restrictions are worthless. The rated power should be a comprehensive technical index.


The rated output power of the power 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 100W speaker. Practical speakers have a certain overload capacity, and the allowable value is about 1.5 times the rated power amplifier. Transistor power amplifier has strong overload capability, and its distortion changes little when overloaded.


In the actual use of power amplifiers and speakers, the rated power value is usually not reached, the actual average power used is relatively small, and the power used is only 1/3--1/5 of the rated power. The power should be adapted and matched. On the surface, the rated power of the two is similar. In fact, it means that the power reserve and the surplus are compatible; in other words, make the power amplifier and speakers work for a long time (for example, 8 hours) under the rated power state ( Within the specified frequency response range, distortion, signal-to-noise ratio and impedance, etc., various problems cannot occur.


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


In order to make the sound box not damaged or distorted by the impact of the strong burst of 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 a tube amplifier and a transistor amplifier, the required power reserve is different. This is because the overload curve of the tube amplifier is relatively flat. For the high quality of the over-loaded music signal, the tube power amplifier does not obviously produce clipping, but only rounds the high quality of the peak. 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 cuts 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 is phase shifted, 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: 10 times the high fidelity Power amplifier, 6~7 times the civilian high-quality power amplifier and 3~4 times the civilian mid-range power 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 as low as 10dB. For modern pop music, bungee music and other music, 20~25dB redundancy is required, so that the audio system can work safely and stably.


2. Impedance matching

Simply put, the rated output impedance of the power amplifier should be consistent with the rated impedance of the speaker. At this time, the power amplifier is in a better designed load line state, so larger 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.


The power amplifier and speakers must be matched, impedance matching is the most important. The speaker is the main body of the load of the power amplifier, and the nominal (or rated) impedance of the speaker should be equal to or similar to the custom output impedance of the power amplifier. How much rated load impedance the power amplifier circuit should be connected to is a basic parameter for the manufacturer to design the power amplifier.


The transistor power amplifier is a low impedance output circuit; while the tube power amplifier is a high impedance output circuit, it has very strict requirements on the impedance value of the speaker. 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 design power amplifier should be 8 ohms, which is an ideal power amplifier circuit. When a 16 ohm speaker is connected, its output power is reduced by about half, and when a 4 ohm speaker is connected, the output power is approximately doubled.


However, most power amplifiers are not ideal high-quality Suzhou. Its output internal resistance cannot be infinitely small, its amplifying loop cannot provide a large enough current gain, and a regulated power supply cannot provide a large enough working current. When the power amplifier is connected to a speaker with too low impedance, the transient characteristics will deteriorate and the degree of distortion will increase. There should be a greater power output, but the power value cannot be increased.


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


It should be noted that when the impedance is not matched, the damping coefficient of the power amplifier may change. The damping coefficient of the power amplifier is the ratio of the load resistance of the power amplifier (mainly the impedance value of the speaker) and the output internal resistance of the power amplifier. When the speaker impedance value changes, it can cause the damping coefficient of the power amplifier to change.


If the damping coefficient becomes too small, the speaker's low-frequency characteristics, output sound pressure frequency characteristics, high-order harmonic distortion characteristics, etc. will deteriorate, and the output audio (especially low audio) will be bloated and muddy, 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 is rare and has little effect on the actual playback effect.


3. 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) / internal resistance of the power amplifier output.


Since 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 KD value of the power amplifier is not the better. Too much KD will make the speaker resistance too heavy, which will increase the settling time of the pulse front and reduce the transient response index.


Therefore, one should not one-sidedly pursue a large KD value when choosing a power amplifier. As a household high-fidelity power amplifier damping coefficient, there is an empirical value for reference. Lower requirements: the KD value of the transistor power amplifier is greater than or equal to 40, and the KD value of the tube power amplifier is greater than or equal to 6.


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


4. Sensitivity matching

The high output power of the power amplifier does not mean the strong driving force of the speakers. The powerful driving force is related to the output power of the power amplifier, and also related to many other factors, especially the sensitivity of the speaker. Speaker sensitivity is an important factor that determines the output power value of the power amplifier.


One definition of speaker sensitivity is: the sound pressure (dB) that can be obtained at 1 meter on the front axis of the speaker when 1W of electrical power is sent to the speaker, and the unit is dB/W/m. For example, the sensitivity of the speaker is 86dB/W/m, which means that the speaker input 1W electric power, and the sound pressure at 1m on the front axis of the speaker is 86dB. At present, high-sensitivity speakers are 95dB/W/m, even more than 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 (for example, 82-84dB/W/m); AV power amplifiers should be equipped with speakers with high sensitivity (about 90dB/W/m). But when the sensitivity is too high, the sound is thinner and brighter, and the details and charm of the music are not enough to reproduce.


The difference in speaker sensitivity has a significant impact on the speaker drive power requirements. 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 is reduced from 90dB/W/m to 87dB/W/m. The original 50W power amplifier was used, but now the power amplifier power should be increased to 100W.


Similarly, if the speaker produces the same sound pressure level, the drive power should be increased by 16 times; in other words, if a 160W power amplifier is used to drive a 83dB/W/m speaker, then a 10W power amplifier can 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 driving power is different; the speakers with high sensitivity can use a smaller driving power to achieve the required volume. When matching audio equipment, the problem of speaker sensitivity adaptation is very important.


5. Tone matching

Tone adaptation means that the tones of the power amplifier and the speakers must be properly matched with each other to obtain the replayed tone that users like. The timbre of the equipment is subjective. Different people love different timbres. Personality, hobbies, cultural accomplishments and experiences all affect the listener’s preference for timbre.


Due to the historical changes and national cultures of different countries, the audio equipment of different countries also has different timbre characteristics. For example, British speakers have warm and sweet sound, German speakers are cool and beautiful, French speakers are beautiful and luxurious, Danish speakers have strong music flavor, and American speakers are sharp and powerful. Even domestically produced equipment, after listening carefully, you will feel different characteristics. The tone of the tube amplifier is warm, but the transient response is poor; the tone of the transistor Class A and B amplifier is brighter, but a little bit stiff; and the transistor Class A amplifier is somewhere in between.


Since various power amplifiers and speakers have their own timbre characteristics, some are warm, some are cool, some are soft, and some are hard, so users can match them together according to the timbre characteristics to form a unique Audio equipment system with timbre characteristics. Through the reasonable matching of equipment, the strengths are avoided, the heating and cooling are complemented, the soft and the hard are matched, and the beautiful sound is obtained.


For example, use a bright and rugged power amplifier to match speakers with a soft tone, and use complementary effects to achieve a more neutral tone. If users have a clear preference for certain types of music, they may wish to choose power amplifiers and speakers with the same tone. Tone matching needs to consider many factors. It often requires careful listening and more taste to accurately appreciate the sound characteristics of the sound. If you want to combine Hi-Fi with home theater, it is recommended not to use equipment with a strong personality, and try to use equipment with easy-going temperament and neutral tone.