Gentleman AM50 amplifier circuit

The circuit design is unique, the process structure is reasonable, and it uses all discrete components and field effect transistor Class A output. As shown in Figure 2-31
. The whole circuit consists of input stage, push stage, power output stage and Gm volume control circuit.
    The input stage consists of field effect twin tubes K389 and Er09 to form a symmetrical complementary double differential circuit, and forms a common
source and common emitter circuit with the transistor. After two stages of symmetrical complementary push-pull amplification, the voltage is input to the amplifier stage. Adjusting RP3 can make the output tubes VTs and
VT6 work in Class A state.
    The voltage amplifier stage uses the same circuit structure as the input stage, but its output stage uses a pair of field effect tubes
2SK214 and 2SJ77. The quiescent current of tubes VT32 and VT33 is adjusted to Class A working status.
    The third part is the power output stage, which includes an output bias circuit and two pairs of parallel field effect tube complementary push-pull output circuits
. Capacitive coupling is used between the push stage and the power amplifier output stage. The coupling capacitor is a polypropylene non-polar capacitor (LW (P capacitor). The
high-quality coupling capacitor ensures pure sound quality.
    The input stage, push stage and power output stage are respectively Separate power supply ensures that the work of each stage does not affect each other. The input
stage and the push stage adopt a higher power supply voltage to ensure their high dynamics and high efficiency.
    The output stage and voltage amplification stage are not equipped with negative feedback of the large loop , which can reduce the transient distortion of the circuit and has a
certain effect on improving the sound quality. The input stage is equipped with a large loop of negative feedback (RP4, R23) to control the volume of the amplifier, which is called transconductance
(Gm) control technology .It is different from the usual negative feedback circuit in that it uses an adjustable potentiometer , and
the sliding arm of the potentiometer is grounded.
    We know that there are two traditional volume adjustment methods, one is at the input of the power amplifier Add a potentiometer to the ground and
control the voltage (flat) of the input signal through the voltage divider of the sliding arm to ground to achieve volume adjustment. The other is to use the
DC potential of the external connection control the volume output by changing the internal resistance of the circuit. , that is, DC volume control. Obviously, the former
will cause changes in input impedance during the adjustment process , making the circuit unable to work in the stable state of the best input, and the sliding noise will also
be amplified; although the latter does not have the above disadvantages, it requires the use of LC To realize it, its cost and performance should be considered. However,
the advantages of ‰ control technology are beyond the reach of the above two.