How to improve LM3886 Power Amplifier Circuit?

Introduction

In order to solve the problem of transient distortion, the typical application circuit of LM3886 power amplifier is changed to the current negative feedback type. The current sample flowing through the speaker voice coil is fed back to the power amplifier input terminal via resistance, and the loudspeaker system is also included in the feedback system.

Catalog

I Typical power amplifier circuit

The power amplifier circuit made of LM3886 consists of two parts: pre-amplifier and power amplifier. The preamplifier consists of an integrated operational amplifier NE5534, which provides a voltage gain of about 5 times. The power amplifier is composed of LM3886, which provides a voltage gain of about 10 times. Therefore, the full power output can be achieved by inputting about 0.5V signal at the input end. Figure 1 only shows the LM3886 dual power supply circuit, which can also work in a single power supply. Considering from the aspect of sound quality, the audio amplifier circuit generally does not use single power supply, but dual power supply.

Figure 1. typical power amplifier circuit

The typical application circuit of LM3886 adopts the traditional negative voltage feedback mode. Negative voltage feedback can improve the frequency characteristics of power amplifier and reduce nonlinear distortion, but the sound is lack of strength. As the volume increases, the low frequency will become tight, dry and hard, and the distortion will increase. At the same time, the high frequency becomes sharp and harsh, and the music and its definition are greatly reduced. This is the so-called Transient Intermodulation Distortion. The transient intermodulation distortion is mainly caused by the introduction of deep negative feedback. Voltage type negative feedback is effective to improve the nonlinear distortion of power amplifier, but it can not take into account the transient distortion at the same time.

II Improved Power Amplifier Circuit

The improved circuit is shown in Figure 2. The low-frequency spectral gain of power amplifier is determined by the ratio of R3 and R4. C3 and R5 determine the high-frequency gain of power amplifier. Due to the large capacitance reactance of C3 at low frequency, the current feedback is terminated at low frequency, while the high frequency is improved by current negative feedback. The result is that the total bandwidth is improved and the transient distortion is greatly reduced. The selection of feedback components should be based on the impedance and inductance of the loudspeaker, so that the low-frequency gain is 2-3 times of the high-frequency gain. Former power amplifiers often designed the frequency characteristics to be flat, which did not achieve good sound effects. In order to improve the sound effect of the power amplifier, we should use the negative feedback circuit to consciously increase the low frequency gain to achieve the best effect, which also meets the requirements of the large dynamic sound effect of today’s home theater.

Figure 2. schematic diagram of power amplifier circuit

The sound effect of this machine is excellent. The low frequency extension is increased and it is flexible. The high frequency is clear and smooth. The resolution is greatly improved, and the metallic sound is completely disappeared. Even if the volume was doubled, no significant distortion was heard.