80W Class A and B power amplifier circuit

In the circuit, the power supply of NE5534 is provided by two integrated voltage regulators 78 18 and 79 18.
It can be seen from the circuit that the positive power supply is a positive 18v power supply through 7818, and the negative power supply is
provided through 7918. But here it is not 18V; it is 18.7V, that is, a diode
1N4001 is connected to the ground terminal of 7918. Using the diode conduction voltage drop of 0.7V to make the negative power supply 18.7Vo, the purpose of this asymmetric power supply
method is to put the output stage inside the integrated operational amplifier in a Class A working state, so that the output tube always has current flowing. This
improves the transient characteristics of the op amp and the inherent shortcomings of the quasi-complementary output, such as switching distortion, crossover distortion, etc. Since the output stage of the internal circuit of
NE5534 is in quasi-complementary form, it is necessary to use an asymmetric power supply method.
The voltage gain of this stage is 40dB, the maximum output voltage is 12Vo
    v, k, VTs are connected in parallel with VT6 and Vr7 Push-pull output, they form the amplifier output stage together with VTz and VT3 respectively
, and both use collector output. This collector output circuit has a certain voltage amplification capability.
The purpose of adopting this structure is to increase the output power and increase the dynamic range. The final voltage gain of this circuit is 9. 5dB. The output of the circuit
adopts Class A output working mode for low power and Class A and B output working mode for high power.
    VD2 and VD3 are the overload protection .
    VTs～VTio force speaker protection circuit .
    The adjustment of the circuit is relatively simple. You only need to adjust RPi so that the output current of the final stage is about 100mA (
the voltage drop on R23 can be measured to be 0.3~0.4V).