Stereo 60W+60W RMS Power Amplifier using LM4780

Features
60W per channel into 8 ohms load at less than 0.5% THD+N from 20Hz-20KHz
Signal-to-Noise Ratio >= 97dB
Quiet fade-in/out mute mode
Click and Pop supression
SPiKe protection
Compact Stereo

Design Objectives

The idea is to design a compact power amplifier system. The design should be simple and cost effective while maintaining the sound quality at optimal performance. It should be built modular so that the same PCBs can be simply cascaded to provide additonal 4, 6 or more output channels. The selection of the components to be integrated into the final design are carefully picked to maintain the THD+N performance.

System Operation & Description

The LM4780 power stereo amplifier is configured as a non-inverting amplifier which has to a gain of 20V/V which is high enough to maintain stability and not too high untill increasing the noise and THD+N performance. The input is taken from the RCA jack which then feed to two high quality 1uF metallized polyester film capacifor with 100V rating. The power output socket provides easy push-release terminals for speaker cable connections (see snap-shots picture below).

Schematic diagram taken from NS referance PCB design schematic

The schematic diagram is taken straight out from the National Semiconductor's referance PCB design schematic. However I have omitted the muting switch and replaced it with the soft start circuit, as explaned in the LM4780 datasheet. I have also added the L//R network (L=0.7uH, R=10 ohms 2W) at both the outputs.

PCB Drawing (Not in Scale). The LM4870 power amp and the PSU (drawing left)

Circuit Description

Two 10,000uF capacitor provides sufficient buffer for the amplifier to pump and handles the bass response without distorting the output signal. A mica-sheet with heat sink compound is used in between the LM4780 and heatsink ensures proper heat transfer. A T0-3 type mica-sheet is being used because it is widely available. The heatsink is mounted vertically with its fins as shown on the snap shots below, allows heat to dissipate naturally, without the need of a cooling fan.

Full view of the power amplifier


Angle View


Abit closer


Complete side view of the power amplifier


Top view - A 125VA 24V-0-24V transformer is used


Two 10,000uF capacitor ensures there is enough buffer


Upgrades & Comments

A speaker protection circuit which detects faulty DC level at speaker terminals should be added to ensure speaker protection. This amplifier can be linked with the stereo pre-amplifier project for full-functional amplifier systems with digital tone and volume control. You can add an additional mono-channel 120W RMS power amplifier to drive a subwoofer, giving a 2.1 channel amplifier system.

Basically the output sound quality, channel cross-talk, stability, channel gain matching are much much better than those constructed using discrete components. The overall PCB size has been reduced as more function has been integrated into the LM4780 chip. The basic protection features are also integrated, ensures total protection of the chip.

With its high PSRR rating, a simple power supply design can be constructed thus reducing cost of components used in designing a well regulated power supply.