The driving ability of the power amplifier is not determined by a certain factor alone. It is the combined effect of many factors. According to the current research, it is related to at least the following factors: power supply, output power, damping coefficient, and ability to resist back EMF. Perhaps, if we look at the problem from the perspective of the speaker, it may be clearer. The difficulty of driving the speaker is related to the following factors: the trend of the impedance curve, the sensitivity, the shift of the phase angle, and the strength of the counter electromotive force.
Impedance curve
Let me talk about the impedance curve first. We know that the speakers have specifications of 4Ω, 6Ω, 8Ω, etc. In fact, these figures are only the nominal value of the impedance at a certain frequency point, and it does not mean that the impedance is constant 4Ω, 6Ω, 8Ω under any circumstances. Taking an 8Ω speaker as an example, it can be clearly seen from the impedance curve that no box's impedance can be kept at 8Ω from 20Hz to 20kHz, it will change with frequency, sometimes it will be high. Dozens of euros, sometimes as low as three or four euros.
What is the relationship between the impedance curve and the amplifier? Don't forget, the output power of the amplifier and the load impedance (that is, the impedance of the speaker) are directly related. If a power amplifier is nominally 100W output at 8Ω, then only 16W can be output at 16Ω, and even lower at 32Ω. , only 25W output. Conversely, when the power is placed at 4 Ω, the output may increase to 200 W, and at 2 Ω, it may increase to 400 W.
Please note that when the impedance of the speaker becomes higher, the load equivalent to the power amplifier becomes lighter. At this time, the output power is only proportionally reduced, which does not impose any burden on the power amplifier. However, when the speaker impedance is reduced, the output of the amplifier is not just as large as it becomes larger. The first question that will be encountered is the power required by the power section of the amplifier to provide that much output power. If not, the 200W output cannot be achieved at 4 Ω, not to mention the 400 W output at 2 Ω. Home theater network shopping guide network welcomes you!
If the power supply really has such a large power reserve that can meet the power requirement of 400W, we have to consider another question: Can the output transistor withstand such a large current? In general, manufacturers are unlikely to use the power transistors required for the 400W stage on a 100W machine, because the cost will increase significantly. Therefore, except for a few Hi-End products at any cost, most power amplifiers cannot achieve a proportional increase in output power when the load impedance is reduced.
How much can be increased depends on the power supply capacity of the amplifier and the specifications of the output power tube. The power supply design and output tube of different machines are different. When the load impedance changes (decreases), the ability to proportionally increase the output power according to the theoretical calculation results will be different, thus causing a sense of hearing. difference.
Sensitivity
The problem of sensitivity is superficially simple, and a 90dB sensitivity box should be better than a 86dB sensitivity box. The problem is that the sensitivity test tests the sound pressure that can be emitted from the entire speaker, rather than testing each unit individually. Therefore, when 100W of power is input to the speaker (assuming the speaker is a three-way), the frequency divider is first encountered, and after the divider has eaten some power, the remaining power is transferred to the three units.
At this time, the three units will react differently to the input power due to their different efficiencies and impedance curves. In other words, the volume of the high, medium, and woofer units will not be the same. The usual experience is that if the low-frequency sense is small, it will be difficult to push the horn, no matter how high the sensitivity is in the manual, it is difficult to push. Welcome to China Home Theater Network!
Phase angle shift
The shift of the phase angle is a relatively technical topic, and it is difficult to make it clear with a simple and common metaphor. We just need to remember the conclusion: the phase angle offset is also an aspect of the impedance characteristics of the speaker (the other aspect is the amplitude of the impedance, which we usually say is the amplitude of the impedance).
The larger the phase angle offset, the farther the speaker deviates from the pure resistance, the phase angle lead load characteristic is capacitive, and the phase angle lag load characteristic is inductive. For power amplifiers, purely resistive loads are the easiest to drive, and capacitive or inductive loads are difficult to drive. If a large phase angle shift occurs at a certain frequency, accompanied by a lower impedance amplitude, such a load is undoubtedly worse for the power amplifier and hard to drive.
Counter electromotive force
Finally talk about the counter electromotive force. We can see the speaker unit assembly as a generator with coils and magnets. When the current input of the amplifier comes in and drives the diaphragm to move the front and rear pistons, the coil cutting magnetic lines of the speaker unit also generate induced electromotive force and current. The machine works on this principle. This current is also returned to the input stage of the amplifier through the feedback circuit, interfering with the operation of the amplifier. The bigger the counter electromotive force, the harder it is to deal with the horn. Some transistor amplifiers use a loop-free negative feedback design that mitigates the effects of back EMF.
From the above points, it shows that the driving ability of the power amplifier is a very complicated problem, and the power output or power supply is not comprehensive. Therefore, our enthusiasts must first say "receive goods with their ears."
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