High-performance power amplifier driven by an op amp

The traditional op amp drives the power amplifier, but the power is difficult to increase due to the limitation of the op amp voltage. This power amplifier uses the voltage conversion current method to directly drive the power amplifier tube for power amplification, so the output power mainly depends on the final power amplifier tube and the power supply of the power amplifier, and the speaker has no on/off impact sound. The whole machine does not add any compensation capacitors, it is original and has small phase shift. Since the op amp is used for constant current amplification, it is very convenient to replace op amps with different performances, and there are more choices for tone. This paper designs a simple and practical power amplifier driven by an op amp.
The circuit is shown in Figure 1. IC1 and T1, IC2 and T2 respectively form a current negative feedback absorption constant current source, which is responsible for the voltage and current conversion amplification of the positive and negative half cycles of the audio signal, so that the base current of T3 and T4 is only controlled by the input voltage of IC1 and IC2. In other words, as long as the op amp input is a constant voltage value, the current flowing through the collector of the final tube T3 and T4 is also a constant value. W1 and W2 (multi-turn potentiometers) are used to adjust the static current and output zero point of T3 and T4 respectively. IC3 and R1, R2, and R5 form a quasi-current negative feedback circuit. As long as the resistance of R3, R4 or R2, R6 is changed accordingly, the gain can be changed. The power amplifier protection circuit (Figure 2) cuts off the total power supply. The advantage is that when the power amplifier fails, it not only protects the speaker, but also protects the final tube. The working principle is: when the power is turned on, T6 is energized and turned on, J is self-protected, and the power supply remains connected. When the DC component of the zero point of the power amplifier output is greater than ±0.24V, IC4 or IC5 outputs a high level, so T5 is saturated and turned on, T6 is cut off, and relay J is released, cutting off the total power supply, thereby realizing the protection function of the speaker and the power amplifier tube.

The machine is easy to debug. First, adjust W1 and W2 to the minimum position, then slowly adjust W1 and W2 to make the voltage drop on R8 and R7 40mV (200mA). Measure the output point, and then fine-tune W1 or W2 to control the output point voltage below 5mV. Then, after preheating for half an hour, readjust it again, and then connect it to the speaker for a test sound. Subjective evaluation, the amplifier has a high signal-to-noise ratio, full and elastic bass, and delicate and smooth treble.
The op amp uses dual op amps such as NE5532 or AD827, and power tubes such as 2SB647 and 2SD667 are selected for T1 and T2. The final power amplifier tube can be selected from Sanken pair tubes such as 2SC2922, 2SA1216 or 2SC3281, 2SA1302, and other audio pair tubes. The resistor uses a 1/4W metal film resistor.