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Input stage works as a transconductance amplifier ( voltage in - current out ).
This stage consist of Long Tailed Pair (LTP) made of Q1 & Q5, working in differential mode. constant current flows from Constant Current Source (CCS) made of Q2, collector current of LTP is balanced by Current Mirror (CM) made of Q3 & Q4.
In simple words input signal is amplified by the input pair Q1 & Q5, current amplified input signal is sent to VA Stage for voltage amplification.
Voltage Amplification Stage ( VAS )
As the name suggests all the voltage GAIN of the amplifier is achieved in VA Stage.
VAS of this circuit is Darlington ( Beta Enhanced ) so low GAIN transistors can be used.
Signal received from input stage is amplified via Darlington pair Q6 & Q9. Constant current flows from CCS made of Q7.
Biasing circuit is made of Q8, preset RV1 is to adjust output stage biasing voltage.
it is important to note that the biasing transistor Q8 must be mounted on the same heatsink along with Output transistors to ensure thermal tracking.
In simple words signal received from input stage is amplified by Q6 & Q9. voltage amplified signal is sent to output stage.
Output Stage ( OPS )
This circuit has Class B output stage, so each half of the signal is amplified by each NPN output transistors.
Driver Transistors Q10 & Q11 work as buffer to prevent excessive loading of VAS as well as to provide input to the output transistors Q12 and Q13 respectively.
Output Topology is Quasi Complementary with Baxandall Diode, thus NPN transistors are used as power output devices.
Thermal feedback via transistor Q8 and correct bias voltage adjustment via preset RV1 is necessary to prevent output transistors from destruction by thermal runaway.
Negative feedback via R11 to the base of Q5 ensures overall amplifier performance and stability.
Zobel Network - made of R24 & C9 provides protection against high frequency oscillations.
In simple words signal received from VAS is given to the output transistors via driver transistors. output transistors conduct high current to drive the speakers in accordance with signal.
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Power output at various Supply Voltages -
Standard Test configuration for output power measurement -
PCB designed with thick copper tracks & large solder pads to facilitate straightforward placement and soldering of components.