Using an Op Amp to Drive a Simple and Practical Power Amplifier

I designed a simple and practical power amplifier driven by an op amp. The power amplifier driven by a traditional op amp 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 power on/off impact sound. The whole machine does not add any compensation capacitors, the original flavor, and the phase shift is small. Because 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.

The circuit is shown in Figure 1. IC1 and T1, IC2 and T2 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 input of the op amp 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 value of R3, R4 or R2, R6 is changed accordingly, the gain can be changed. The power amplifier protection circuit (Figure 2) adopts the method of cutting 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 attracted for self-protection, and the power remains connected. When the zero-point DC component 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 turned off, and relay J is released, cutting off the main 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, and then connect it to the speaker for a test. Subjective evaluation, the amplifier has a high signal-to-noise ratio, full and elastic bass, and delicate and smooth treble.

The operational amplifier uses dual operational amplifiers such as NE5532 or AD827, and the power tubes selected for T1 and T2 are 2SB647, 2SD667, etc. The final power amplifier tube can choose Sanken pair tubes 2SC2922, 2SA1216 or 2SC3281, 2SA1302 and other audio pair tubes, and the resistor uses 1/4W metal film resistor.