Simple discrete component 50-watt high-fidelity power amplifier

Audiophiles always try to use integrated power amplifier circuits when making their own audio power amplifiers of more than 30 watts . This does simplify the production process, but it makes it difficult for manufacturers to understand the circuit principles. Therefore, power amplifiers with discrete components are still necessary. The schematic diagram of the 50W amplifier introduced in this article is shown in Figure 1. There are only six triodes in the circuit, powered by a single power supply. When THD (Total Harmonic Distortion) is 1% and the power supply is unstable, the continuous output power is 50W: When THD is 5% and the power supply is stable, the dynamic output power is 60W. When THD is 1% and the power supply is stable, the dynamic output power is 60W. The output power is 60W. Within the rated continuous power range, regardless of whether the input end is short-circuited or open-circuited, the hum and noise are less than 82.3dB. At this time, the sensitivity is 100mV and the input impedance is 8.2 ohms. Figure 2 is the relationship between power and distortion, and Figure 3 is the relative frequency response curve.

The power amplifier stage of the amplifier circuit is composed of a complementary pair of tube emission limit followers, and the negative feedback of the large loop keeps the signal driving the complementary pair of tubes within a linear range. The structure of this circuit ensures that the two power amplifier tubes are not turned on at the same time, preventing short circuit to the power supply.

An ideal transistor should be able to turn on or off quickly, but in fact the switching speed of transistors is limited, especially for high-power transistors. When the changing signal input to the complementary pair of tubes flips rapidly, it is possible that both tubes will be turned on at the same time, causing excessive current . For this reason, when selecting a complementary power amplifier pair of tubes, a compromise between switching speed and transmission characteristics should be adopted. And add a high-frequency decoupling capacitor to its input end.

The final pre-stage transistor Q4 works in a Class A state, and its static collector current is equal to the power supply voltage minus the common terminal potential of the bases of Q5 and Q6 divided by the resistance (R13+R14). In order to make this Class A amplifier work at its best, the current in R14 should be kept constant, so a bootstrap capacitor C7 is added.

Due to the memory effect of the transistor, in the high audio range, the complementary pair of Q5 and Q6, which are Class B amplifiers, are no longer in a pure Class B state.

The DC negative feedback introduced from the common point of R15 and R16 establishes a bias voltage for the input stage, which causes a small current to flow through Q5 . The output voltages of Q5 and Q6 also establish a bias for the excitation stage. AC and DC negative feedback are added to Q3. The feedback depth is determined by the ratio of R9 and R10 and the Vbeo of Q3. Of course, the ratio of R9 and R10 also affects the static potential of the common output terminals of Q5 and Q6. AC negative feedback enables the amplifier to have a higher upper frequency limit, and the stability of the bandwidth is determined by Q1, which stabilizes the operating point through the introduced negative feedback.

The input circuit of Q1 is a common DC coupling circuit. Adjusting R4, R5 and R6 can make Q1 and Q2 work in the best state.

The negative feedback of the large loop is determined by the ratio of R15 and R16. The voltage gain of this circuit is 10 times. In order to enable negative feedback to work, an isolation resistor R1 is added to the input circuit.

Since the power amplifier stage is in Class B status and only consumes power when there is a signal, the heat sink of the power amplifier tube can be appropriately smaller. In addition, since the emitter resistor is omitted, power consumption is reduced, allowing the power amplifier tube to obtain a higher operating voltage.

To further increase the output power, the power amplifier stage can be changed to a Class A and B amplifier, or a more powerful triode can be used.

The models of each transistor in the electrical schematic diagram can be replaced by the following: Q1, Q2, Q3: 2SC2547E. Domestic low-power silicon NPN tubes, such as 3DG12, 3DK4, etc. can be used instead. Q4, Q5: 2N3055. Domestic high-power high-back-pressure silicon tube PNP tubes 3DD71, 3DD12, etc. can be used. Q6: For MJ2955, domestic high-power and high back-pressure silicon PNP tubes 3 CD 10D, etc. can be selected.