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Protection Circuit for Power Amplifier Output and Loudspeaker
The circuit was designed to provide protection to the output of power amplifier thereby protecting the loudspeaker from unwanted noise and other elements.Power Amplifier – a type of amplifier built and designed for the purpose of delivering and supplying sufficient and maximum high output power to directly drive loudspeakers or loads, within a given percent of distortion LM393 – low power dual voltage comparator with features such as TTL, DTL, ECL, MOS, CMOS compatible outputs, differential input voltage range equal to the supply voltage, low output saturation voltage, input common-mode voltage range includes ground, low input offset voltage and current, very low supply current independent of supply voltage, and wide single supply voltage range or dual supplies BC337 – a small signal NPN Silicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group types BC639 – an NPN medium power transistor used in driver stages of video or audio amplifiers due to its high current with 1 A maximum, low voltage with 80 V maximum, and enclosed in a TO-92 SOT54 plastic package
The output of the power amplifier is isolated directly to drive the loudspeaker. The loudspeaker needs to be protected from any unwanted noise due to the factors like the presence of continuous voltage in the output of the power amplifier; the time delay in the connection to the loudspeaker and power amplifier, or the simultaneous rise of excessive temperature on the heatsink. These issues which contribute not only in producing noise can be prevented by the presence of capacitors in the supply as they perform the charge and discharge functions. These capacitors adds up to the binary comparator IC1 LM393, together with the LED indicators D5 & D6 and transistors BC337 Q1 & Q2. The LM393 consist of two independent low voltage comparators designed specifically to operate from a single supply or split power supplies over a wide range of voltages. They have a unique trait in that the input common-mode voltage range has ground even though operated from a single power supply voltage.
The positive supply of the circuit comes from point A of the main power supply. The positive 15 V is being stabilized by the presence of diode D3 and resistor R17. D3 is a zener diode that is rated with 1.5 W at 15 V. the main function of the zener diode is to permit the flow of current in just one or forward direction as a normal diode, but will also allow in the reverse direction if the voltage is above or larger than a certain value of the breakdown voltage. The secondary AC coil of the main transformer is connected to point B in which the AC voltage is being applied upon closing the transformer circuit. The AC voltage that the circuit receives is being rectified by diode D2 to produce a negative voltage through resistor R9. Rectification is the process of converting alternating current to direct current. Since R9 is connected to the base of Q3, the received negative voltage turns Q3 to its cutoff conduction that will cause the capacitor C4 to start charging through the resistors R10 and R11. During the charging of C4, the positive input of the IC1B will be at low stage with reference to the negative input. Since the output of IC2 be has low level, the transistor Q4 BC639 will stay in the cutoff stage which causes the 24V relay RL1 to remain deactivated while the LED D6 in turned ON. The relay will only be triggered and activated as soon as C4 gets charged. This will lead to the connection of loudspeaker to the output of power amplifier while LED D6 will turn OFF. When the supply is interrupted, the operation of the circuit will go the other way around where the loudspeaker will get disconnected without producing any annoying noise.
During the operation, the circuit might be subjected to problems like continuous voltage in the output of power amplifier. This would cause the relay RL1 to turn OFF to protect the loudspeaker. This is where BC337 Q1 and Q2 comes into action where the acoustic signal from the output of the amplifier is pointed towards the point D while the alternate voltage is connected towards the ground through the capacitors C1 and C2. The continuous voltage larger than 1.7 V will activate Q1 while continuous voltage higher than 4.8 V will activate Q2. This activation of both transistors will go down the positive input level of IC1B that will deactivate RL1.
Another protection part of the circuit is the thermic protection where the temperature is being monitored by RTH, which is a positive factor of temperature (PTC) type of resistor. It uses the model KTY81-122 which is a Silicon temperature sensor having a positive temperature coefficient of resistance and suitable for use in control systems and measurement. It is encapsulated in the SOD70 2 in-line leads plastic package. This device is highly sensitive to ElectroStatic Discharge (ESD) which requires proper handling and care. Nevertheless, it is stable for long term, reliable, and accurate. In the circuit, it is positioned on top of the heatsink. Its value would increase with the increase of temperature until the negative input voltage of IC1A reaches the level of positive input which is verified by voltage divider R2 and R3. When the negative input exceeds the positive, the output of IC1A will be low, forcing IC1B to change operation, thus, turning OFF RL1 while LED D5 turns ON which indicates thermic protection at about 70ºC.
The circuit will be mainly used in power amplifiers for protection of loudspeakers that will indicate, through the LEDs which aspect the loudspeaker is being protected. LED D6 will indicate the overvoltage protection while LED D5 will indicate the thermic protection of the circuit.