Design and production of 20W-50W single-ended FET pure class A power amplifier

The goal that audiophiles are tirelessly pursuing is to make their audio equipment have a purer and more natural sound quality. Single-ended pure Class A amplifiers have a mellow sound quality, rich even harmonics, a good sense of air, and a natural sense of hearing, which has become the goal pursued by many HI-FI enthusiasts. Although the efficiency of single-ended pure Class A amplifiers is low (theoretically it can reach 25%), in actual production, it can only reach about 20%, so the heat generation is large, requiring a large area of ​​​​heat sink, and the circuit design is also required to be stable and reliable. A dual 50W pure Class A amplifier is in the maximum power consumption state as soon as it is turned on. Regardless of the output power, the power supply consumes about 500W. Especially in the hot summer, listening to music with a 500W electric stove is also interesting.
The following introduces a single-ended pure Class A amplifier circuit with stable operation and simple debugging. The load can use 4-8 ohms, and the output power can be set at 20W-50W according to your needs.
1. The working principle diagram of the single-ended amplifier circuit is shown in the figure below:

The differential circuit composed of TR1 and TR2 uses a constant current source circuit to ensure stable operation and improve the common mode rejection ratio of the circuit. The output signal is a single-ended amplifier output stage composed of Q3, and the constant current source I4 is used as the DC load of Q3. R1 determines the input impedance of the amplifier, and R2 and R3 are negative feedback loops that determine the closed-loop gain of the amplifier.
2. Calculation of output power, supply voltage, and quiescent current:
Take the output power of 20W on a 4-ohm load as an example. Other output powers and load impedances can be calculated separately or refer to Table 1.
According to P=I*I*R, I*I=P/R=20/4=5, I=2.236A. Among them, I is the effective value of the output current, so the maximum value of the output current IM=1.414*I=1.414*2.236=3.16A. In actual production, 3.2A is taken. This current value is the quiescent current of the whole machine, which is I4 in Figure 1, provided by the constant current source of the circuit.
Since this circuit uses drain output and the final stage has voltage gain, the power supply voltage utilization rate is relatively high. For the convenience of calculation, 0.8 is taken, and the required power supply voltage is: VCC= (IM*R)/0.8=16V.
In order to reduce costs, this circuit adopts a power supply form of bridge rectification and large-capacity capacitor filtering. According to the current-voltage relationship of the circuit: U2=VCC/1.2, so U2=16/1.2=13.3V. In actual production, a transformer with a secondary voltage of dual 13.5V can be used.
Since this machine is a single-ended pure Class A power amplifier circuit, it works in the maximum power consumption state as soon as it is turned on, and the power consumption is: 2*16*3.2=102.4W. In order to make the circuit stable and reliable, the rated power of the power transformer should be 3-5 times the actual power consumption, so the rated power of the power transformer is 300-500W.
3. Selection of rectifier components and filter components:

Since the working current of this machine is relatively large, the actual working single channel is more than 3A. In order to reduce the internal resistance of the power supply, the rectifier element adopts a metal square bridge of more than 25A/200V.

5. Printed board drawings for reference