DIY LM1876 Dual 20W Audio Power Amplifier

We are building another enjoyable weekend DIY project. This is an audio power amplifier based on LM1876 which can deliver up to 20W per channel into 4 or 8 ohm load and guarantees less than 0.1% THD + N (Total Harmonic Distortion + Noise).

The amplifier is powered by -15 0 15 VAC symmetrical supply. The full bridge diode rectifier and the smoothing capacitors convert the AC input to ±21 VDC which is used to power LM1876. The inductors on the AC input line reduces the noise arising from the mains line.

DIY LM1876 Dual 20W Audio Power Amplifier

The audio input is connected to the board via 3.5mm stereo socket. The stereo potentiometer adjusts the amplitude of the audio signal. The potentiometer also includes a switch which pulls the Standby pins to logic high and makes the amplifiers go into the standby mode. In standby mode, LM1876 draws about 4.2mA and the power dissipation reduces significantly. The audio output of the amplifiers are connected to RCA connectors on the board.

LM1876 generates quite amount of heat so a heatsink is required to cool down the IC. A big heatsink is used for this purpose.

Circuit Design

The schematic of the project is drawn in SoloCapture, the schematic editor of SoloPCB tools. SoloCapture makes the schematic drawing process very easy and fast. You can download SoloPCB tools at for FREE.

You can download the SoloPCB design files of the project by using the link below.

LM1876 requires symmetric power supply. A special single supply configuration is also provided in the datasheet but it requires a high voltage single supply (about 40VDC for the same application) and there is a bit performance degradation. So the circuit is designed regarding the symmetrical supply operation. A transformer which converts the mains power to -15 0 15 (or 0 15 30) VAC levels. A 6A 1000V (Vishay KBU6M) single phase bridge rectifier,BR1, is used for full wave rectification of ±15 VAC. Then a pair of 3.3uH 5A inductors L1 and L2 (Coilcraft MSS1260-332NL) are connected in series to reduce the power line noise. Two 6800uF 50V electrolytic capacitors C7 and C8 smooth the rectified signal and outputs fixed ±21 VDC. R7 and R8 resistors are connected between the terminals of the capacitors to discharge the caps after power-down to prevent electrical shock. The positive 21 VDC is labelled as VCC and the negative 21VDC is labelled as VEE. D1 LED is placed between the VCC and the VEE lines to indicate the power-on status. ±21VDC are directly connected to LM1876 but 100uF and 100nF bypass capacitors are connected to the VCC and VEE pins of LM1876 as close as possible.

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