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EE Design > 35W RMS x 3-Channel Power Amplifier System
Features 35W per channel into 8 ohms load at less than 0.5% THD+N from 20Hz-20KHz Signal-to-Noise Ratio >= 93dB Quiet fade-in/out mute mode Click and Pop supression SPiKe protection Compact 2.1 channel / 3-channel System Design Objectives This design is intended to drive the rear (stereo) and center channel speakers for surround-sound applications. This project combined with the 60W+60W power amplifier, which will drive the main stereo (front) speakers, gives a full 5 channel surround sound systems (Total power = 225W RMS). The power supply circuitry, PCB size and dimension should be designed to match nicely with the 60W+60W power amplifier so that later they may be nicely cased togather. System Operation & Description The LM4781 3-channel audio power amplifier are configured as non-inverting amplifier which has a gain of 20V/V per channel, which is high enough to maintain stability and not too high until increasing the noise and THD+N performance. The inputs are taken from the RCA jacks which then feed to three high quality 1uF metallized polyester capacifor with 63V rating. The outputs are wired to the speaker terminals directly.  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 LM4781 datasheet. I have also added the L//R network (L=0.7uH, R=10 ohms 2W) at all the outputs. Circuit Description Two 10,000uF capacitor provides sufficient buffer for the power amplifiers to pump and handles the bass response without distorting the output signal. A mica-sheet with heat sink compound is used in between the LM4781 and heatsink ensures proper heat transfer. Additional heat transfer compound are applied to both side of the mica sheet to ensure maximum 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 (just like 60W+60W power amp!), allows heat to dissipate naturally, without the need of a cooling fan.  Full view of the power amplifier The LM4781 pinouts are harder to solder compared to the LM4780, because most of the pins are used up. Solder the legs carefully and ensure the legs are not shorted. You may want to solder row by row by lifting one side of the row away first, then when one side is completed, you can then insert the other row for soldering. It will be very much easier!  Angle View You can see the snap-shot below I have added a standard 2.54mm pitch PCB header, give gives access to its internal pre-amp (just in case), if the RCA connectors are not being used.  Abit closer - You will see the high quality green input capacitor For all amplifier design, it is always advisible to return the grounding at a joining point, like the star grounding. It will reduce all eliminate all noise and distortion to the amplifier. And finally, joint the signal ground to the power ground thru a 2.7 ohm resistor!  Copper trace of the PCB - Star Grounding Upgrades & Comments Speaker protection circuitry are intended to be added later, in between the speaker output wires and the loudspeaker to ensure speaker protection. This power amplfier will be connected to a Hafler matrix surround-sound decoder, which will be posted later. The surround sound decoder will give the (L-R) signals for the rear channel and (L+R) signals for the center channel. 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. Related Design Files & Materials LM4781 Datasheet |