Protection of power amplifier
The power tube is the most easily damaged device in the power amplifier circuit. Most of the reason for the damage is that the actual power dissipation of the tube exceeds the rated value. In addition, if the power amplifier is mismatched with the speaker or the speaker is overloaded during long-term use, it is also very easy to damage the speaker (or speaker). Therefore, in audio equipment, the purpose of protection is to protect expensive power amplifiers and speakers. The overload and short circuit protection of the speakers is absolutely necessary.
(1) Power supply protection: discrete component voltage stabilizing circuit. Ri in the circuit is an overload current sampling resistor. When its voltage is greater than 0.7V, V13 is turned on, the collector potential drops, and the regulator V11 is turned off to limit the power output current.
Adjustable output voltage module, power consumption up to 70W, current up to 10A, voltage regulation rate of 20.8%, output voltage of 1.25 to 15V, and short circuit protection.
When using a switching power supply (such as the chip CWl225), there is a special protection control terminal ⑩ pin, as long as the input over-current or over-voltage signal, the protection purpose can be achieved.
(2) Power amplifier-level transistor protection: In addition to paying attention to the ambient temperature and selecting a suitable radiator in use, the power amplifier transistor mainly considers over-current and over-voltage protection. The current applied integrated circuits are equipped with current limiting protection and thermal protection. Cut off the protection function (such as HAl350, HA2211, LM2879, etc.), so you must pay attention to overvoltage protection when making your own power amplifier. Relying on the breakdown of R (power supply internal resistance) and Vl and V2, the over-voltage cannot be increased and Vl and V2 are protected.
(3) Speaker speaker system protection: The protection of the audio system has two meanings: one is the overload of the audio speaker; the other is not the excessive audio power, but the offset of the DC potential, resulting in OCL or non-capacitor isolation The BTL circuit speaker burned out. In case of overload, the power amplifier circuit is already protected and no additional consideration is needed. Here we only introduce the protection circuit of the DC offset sound system.
It can be seen that when the sound signal sent from the left and right channels to the speaker is bypassed by capacitors C2 and C3 through R1 and R2 without DC offset, the rectifier bridge has no DC output, V11 is cut off, and V12 and V13 are on. Relay K pulls in, and left and right audio signals are output through fuse F; when there is a DC offset, the rectifier bridge output turns on V11, V12 and V13 are off, and the release of relay K cuts off the audio signal to protect the speaker.
In the circuit, C2 and C3 are filter capacitors. C4 has the function of delaying turning on the speaker when the machine is turned on and off to avoid impulsive noise when starting up. V has the effect of short-circuiting K off back-EMF to protect the V12 and V13 transistors.
The understanding or evaluation of the power amplifier circuit is mainly considered from the three aspects of output power, efficiency and distortion.
1. In order to obtain the required output power, the circuit must select a transistor with a large enough collector power consumption, and the working current and collector voltage of the power amplifier tube are also high. In the circuit design and use, we must first consider how to give full play to the function of the triode without damaging the triode. Since the working state of the circuit ** amplifier tube is often close to the limit value, be careful when adjusting and using the power amplifier current, and should not exceed the limit.
2. In terms of energy consumption, the power output of the power amplifier is ultimately provided by the power supply. For example, the power consumption of the radio amplifier accounts for 2/3 of the whole machine, so we must pay great attention to improving the circuit efficiency, that is, the output power and power consumption. The ratio of power.
3. The input signal of the power amplifier circuit has been amplified in several stages with sufficient strength, which will cause the working point of the power amplifier tube to move greatly, so the power amplifier circuit is required to have a larger dynamic range. If the working point of the power amplifier tube is not properly selected, the output will be severely distorted.