1. Soft start protection value
An audio device with a large current draw can flow 4 to 10 times its average current value at the moment of power-on. This is an impact on the grid and the equipment itself, which can damage the equipment in severe cases. The soft-start circuit can suppress the inrush of the current at the moment of switching of the device, so that it can smoothly reach the normal function of protecting the device and not causing fluctuations in the power grid. This function is usually achieved with the negative temperature characteristic of the thermistor (NTC).
2. DC protection
It is possible to deliver a DC signal when the amplifier output stage is damaged or when the static offset is shifted. For the speaker, his working mode only produces impedance to the AC signal. For the DC signal, it does not produce any impedance (equal to zero impedance). At this time, the current is infinite, so the coil of the speaker is equivalent to one under the DC signal. The root heating wire will be burned quickly. Therefore, accurate and fast DC protection circuits are very important. The DC protection start value of the amplifier is usually set at 2V. When it is greater than or equal to this value, the amplifier will cut off the output and protect the speaker. There are also amplifiers that will cut off the output by blowing the built-in DC fuse.
If the DC protection circuit of a power amplifier is normal, but the coil of the speaker is burned out, there are only two reasons: the power input to the speaker is too large, or the signal output from the power amplifier is turned into a square wave.
3. Short circuit protection
When the output of the power amplifier is short-circuited for some reason, the current output by the power amplifier will circulate on its own line and become infinite. This situation is very dangerous, so there must be an accurate and fast short circuit protection circuit to protect the power amplifier equipment. Normally, when the power is placed in the short circuit, it first controls the input signal to reduce its amplitude even to zero. If the situation does not improve (the current flowing through the power amplifier still exceeds the safe value), it will suppress the output current. The current flowing inside the amplifier is always lower than the safe value of the output stage transistor.
4. Overcurrent protection
When the load of the power amplifier is too low but does not reach the short circuit state, the short circuit protection will not work at this time, but the output current will be very large than the safe use value of the power amplifier. At this time, the overcurrent protection circuit will intervene, the usual practice Yes: Control the input voltage and output current so that the amplifier is always working within safe limits.
5. Overheating protection
The well-designed power amplifier will not be protected from overheating under normal use. It only works when the external environment is bad or internal failure occurs. The hottest part of the whole amplifier is the C-pole (collector) of the output stage transistor, so the overheat-protected temperature sensor is typically mounted near the C-pole of the transistor or the hottest part of the heat sink. The threshold for overheat protection is typically 95 ° C, also 105 ° C, and the extreme temperature of the transistor is 105 ° C.
6. Distortion compressor
The input level value of the audio equipment has a specified range. If it exceeds this range, the signal will be topped, and in severe cases it will become a square wave. The purpose of the distortion limiter is to ensure that the level of the input signal is always within the linear operating range allowed by the audio equipment. The general standard is to start when THD is 1%.
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