Matching of power amplifier and speaker
The connection of power amplifier and speaker, that is, power matching is a very examining problem, we must put "the faithful restoration of music" in the first place. When designing and installing a sound system, it is unavoidable to encounter the problem of power amplifier and speaker. In the aspect of timbre, we will pay attention to whether the matching is suitable for cold and warm, soft and hard, and eventually make the whole set of equipment restore the timbre to be neutral, which is only considered from the artistic aspect. From the technical point of view, the elements of power amplifier and speaker connection are as follows:
I. Power Matching
II. Impedance Matching
Matching of Damping Coefficient
IV. Sensitivity Matching
Fifth, timbre matching
If we recognize the above five points when matching, we can make the best use of the performance of the equipment used.
1. In order to meet the requirement of high fidelity listening, the rated power should be determined according to the optimum listening pressure. We all have this feeling: small volume, weak voice, thin, dynamic out, no luster, low-frequency significant lack, poor fullness, sound seems to shrink inside out. When the volume is appropriate, the sound is natural, clear, mellow, soft and full, powerful and dynamic. But when the volume is too high, the voice is stiff and soft, rough and has a deep-rooted feeling. Therefore, the sound pressure level of replay has a great relationship with the sound quality. It is stipulated that the sound pressure level of the listening area should be 80-85dB (A weighting). We can calculate the rated power of the speaker and the rated power of the amplifier from the distance from the listening area to the speaker and the characteristic sensitivity of the speaker.
The output power of power amplifier circuit has many names, such as rated power (RMS), music power, peak music power (PMPO), etc. They have different meanings, but rated power is the most widely used and most important power. Businessmen often produce power of other names, which are based on commercial propaganda, or to avoid weaknesses and publicity advantages. Strict rated power should be strictly regulated in frequency response range, harmonic distortion, load impedance and signal-to-noise ratio. Without these limitations, rated power values are of no value. Rated power should be a comprehensive technical index.
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, the 125 W power amplifier should drive about 100 W speakers. Practical speakers have a certain overload capacity, its allowable value is about 1.5 times the rated power amplifier. The transistor power amplifier has strong overload capability, and its distortion changes little when overloaded. In the actual use of power amplifiers and speakers, usually can not reach the rated power value, the actual average power used is relatively small, the practical power is only 1/3-1/5 of the rated power. In order to match and match the power, the nominal power of the two is similar on the surface, which means that the reserve and surplus of the power are compatible. In other words, the power amplifier and speaker can work at the rated power for a long time (for example, 8 hours) under the condition of the specified frequency response range, distortion, signal-to-noise ratio lattice impedance, etc. No problems can arise. Without lowering the restrictions, when the power value of the amplifier is increased, the price will also soar. Under the condition of ordinary small listening room (for example, below 20 square meters), no power amplifier with excessive output power is needed. The power amplifier with rated power of 60-80W (8 Euros) can complete the general playback task.
In order to keep the speaker from being damaged or distorted by the sudden strong pulse in the program signal. Here is an empirical value to refer to: the nominal rated power of the selected speaker should be three times the power calculated theoretically.
Compared with transistor power amplifier, the required power reserve is different. This is because the overload curve of the tube power amplifier is relatively flat. For the peak of the overloaded music signal, the clipping phenomenon of the tube amplifier is not obvious, but the tip of the peak is rounded. This is what we often call flexible shear peaks. When the transistor power is placed at the overload point, the non-linear distortion increases rapidly, which causes serious clipping of the signal. It does not rounden the peak but smoothes it neatly. A composite impedance analog loudspeaker composed of resistance, inductance and capacitance is used to test the actual output capability of several high quality transistor power amplifiers. The results show that when the load is shifted, there is a power amplifier named 100W, and the actual output power is only 5W when the distortion is 1%. The reserve of transistor power amplifier is selected as follows: high fidelity power amplifier: 10 times civil high-grade power amplifier; 6-7 times civil middle-grade power amplifier: 3-4 times and electronic tube power amplifier can be much smaller than the above ratio.
How much allowance should be left for the average sound pressure level and maximum sound pressure level of the system depends on the content of the program and the working environment. The minimum amount of redundancy is 10dB. For modern pop music, bungee jumping and other music, it is necessary to leave 20-25dB redundancy, so that the sound system can work safely and steadily.
2. Impedance Matching
Simply put, the rated output impedance of the amplifier should be consistent with the rated impedance of the speaker. At this time, the amplifier 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 danger of overload of the amplifier requires that the amplifier have perfect overcurrent protection measures to solve, and the impedance matching requirement for the electronic tube amplifier is stricter. Impedance matching is the most important to match the amplifier with the speaker. The speaker is the main load of the amplifier. The nominal (or rated) impedance of the speaker should be equal to or close to the specific output impedance of the amplifier. How much rated load impedance should be connected to the power amplifier circuit is a basic parameter of the power amplifier design by the manufacturer. The transistor power amplifier is a low impedance output circuit, while the tube power amplifier is a high impedance output circuit, which requires very strict impedance value of the speaker. However, the low impedance output power amplifier of transistor still puts forward some requirements for load impedance value. For example, the output load of the original design power amplifier should be 8 ohms, which belongs to the ideal power amplifier circuit. When 16 ohm speakers are connected, the output power will be reduced by about half, while when 4 ohm speakers are connected, the output power will be doubled. But most of the power amplifiers are not ideal top-level Suzhou, their output internal resistance can not be infinitely small, their amplification loop can not provide enough current gain, and the regulated power supply can not provide enough current. When this power amplifier is connected to the speaker with too low impedance, the transient characteristics will deteriorate and the distortion will increase. There should be more power output, but the power value will not go up. For power amplifiers calibrated with external 4-16 ohm loads, they should be connected to speakers with median impedance range as far as possible. When the power amplifier connects the speaker with the rated load impedance higher than its rated load impedance, the rated output power decreases, which has little effect on other performance indicators. However, if the power supply voltage margin is not large, it may indicate that the rated power to achieve the purpose has already occurred overload distortion. It should be noted that when impedance mismatch occurs, the damping coefficient of the amplifier may change. The damping coefficient of the amplifier is the ratio of the load resistance (mainly the speaker impedance) of the amplifier to the output internal resistance of the amplifier. When the impedance value of the speaker changes, the damping coefficient of the amplifier changes. If the damping coefficient becomes too small, the speaker's low-frequency characteristics, output sound pressure frequency characteristics and high-order harmonic distortion characteristics will deteriorate, and the output audio (especially low-frequency) will be bloated and muddy, accompanied by naivety. If the damping coefficient is too large, the sense of low frequency will be weakened, the sound will be dry and not rich, but this kind of situation is rare, and has little effect on the actual replay effect.
3. Matching of Damping Coefficient
Damping coefficient KD is defined as: KD = rated output impedance of power amplifier (equal to rated impedance of speaker)/output internal resistance of power amplifier. Since the output internal resistance of power amplifier has actually become the electrical damper of the speaker, the KD value determines the resistivity of the speaker. The bigger the KD value is, the heavier the resistor is. Of course, the KD value of the amplifier is not the bigger the better. The excessive KD value will make the speaker resistor too heavy, so as to increase the setting time of the pulse front and reduce the transient response index. Therefore, when choosing power amplifier, we should not pursue large KD value unilaterally. As a household high fidelity power amplifier, there is an empirical value for reference. Minimum requirements: transistor power amplifier KD value is greater than or equal to 40, and tube power amplifier KD value is greater than or equal to 6. In order to ensure the stable and transient characteristics of sound playback, attention should be paid to the combination of the equivalent mechanical quality factor (Qm) and the amplifier damping factor (KD), which requires the feeder of the speaker to be considered as part of the overall sound system. The equivalent resistance of the feeder of the speaker should be small enough to be negligible in comparison 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 output power of power amplifier is not equal to the driving power of loudspeaker. The strong driving force is related to the output power of the amplifier, and to many other factors, especially the sensitivity of the speaker. Speaker sensitivity is an important factor to determine the output power value of power amplifier. One definition of speaker sensitivity is that the power of 1W is fed to the speaker, and the sound pressure (dB) can be obtained at 1m on the front axle of the speaker, in units of dB/W/m. For example, the sensitivity of the speaker is 86dB/W/m, which means that the speaker input 1W power, and the sound pressure at 1m of the front axis of the speaker is 86dB. At present, high-sensitivity speakers are capitalized at 95dB/W/m, even over 100dB/W/m, while low-sensitivity speakers are only 82-86dB/W/m. Many of the hI-Fi speakers used to listen to music have low sensitivity (e.g. 82-84 dB/W/m); AV power amplifiers should be matched with speakers with high sensitivity (about 90 dB/W/m). However, when the sensitivity is too high, the timbre is thin and bright, and the details and charm of the reproduction music are not enough.