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.
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.
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.
There are two power amplifier inside LM1876 for right and left audio. Each amplifier has separate mute and standby pins. The audio signal coming from the stereo socket is connected to the non-inverting inputs of the amplifiers after the amplitude adjustment over the stereo potentiometer, VR1 (Bourns RK0971221Z05) which is used for volume control. Since it has two potentiometers built-in, the right and the left audio inputs are equally adjusted. The potentiometer also provides a single pole-single throw switch at the beginning of the rotation. This switch is used to put LM1876 into standby mode by pulling the standby pins of both amplifiers to logic high. Logic high is generated by using VCC and the voltage divider resistor pair R3 and R4. In standby mode, LM1876 draws at most 4.2mA which reduces the power consumption significantly.
The gains of the amplifiers should be calculated regarding the design guidelines provided in the LM1876 datasheet. Since the amplifiers operate in non-inverting configuration, gain of 21 is achieved by using 20K and 1K resistors in the feedback line. The feedback capacitors C5 and C6 ensures unity gain at DC. They also creates a highpass filter. The audio input capacitors C9 and C10 also blocks the DC from audio input. The audio outputs of each amplifier is connected to the RCA connectors J2 and J3.
The heat dissipation on LM1876 is quite high. When the output power of an amplifier reaches 20W, the power dissipation becomes about 42W when 4ohm speaker is used and 22W when 8ohm speaker is used. This values double when two amplifiers are considered. The maximum allowed die temperature is 165°C. So the heatsink should be chosen regarding the ambient temperature, die temperature, maximum heatsink temperature rise, die to case, case to heatsink and heatsink to ambient thermal resistance. Fortunately, LM1876 provides a thermal shutdown feature to reduce the stress imposed on the IC.
To decrease the overall thermal resistance, the metal case option for LM1876 should be chosen, thermal compound should be applied between the IC and heatsink, and a low thermal-resistance heatsink should be used.
The PCB design process is straightforward. It has two routing layers, BOTTOM and TOP. The power lines are drawn as thick as possible. The bypass capacitors are placed as close as possible to LM1876. The audio input and output connectors and the stereo potentiometer are placed carefully to make the user interface usable. LM1876 is placed to the edge of the board to make the heatsink attachment possible. There are mechanical holes at the corners at. 2cm threaded spacers are mounted to those holes with M3 screws.
Assembly And Test
The board includes SMD components. So you should have intermediate soldering skills. Through hole components are already easy to solder. The part list of the amplifier is given below.
|1||LM1876 Audio Power Amplifier||U1|
|1||KBU6M 6A 1000V Bridge Rectifier||BR1|
|2||Coilcraft MSS1260-332NL 3.3uH Inductor||L1 L2|
|2||6800uF 50V Electrolytic Capacitor||C7 C8|
|3||100uf 35V SMD Electrolytic Capacitor||C11 C12 C13|
|4||100nF 50V X7R SMD Ceramic Capacitor||C1 C2 C3 C4|
|4||10uf 25V 1206 SMD Ceramic Capacitor||C5 C6 C9 C10|
|4||4K7 5% 0805 SMD Resistor||R3 R7 R8 R9|
|2||1K 1% 0805 SMD Resistor||R5 R6|
|3||20K 1% 0805 SMD Resistor||R1 R2 R4|
|1||LED Red SMD 0805||D1|
|1||Bourns RK0971221Z05 Stereo Potentiometer with Switch||VR1|
|1||3.5mm Steroe Socket 5 Terminal||J4|
|2||RCA Socket 2 Terminal||J2 J3|
|1||1×3 5.2mm pitch Screw Terminal Blcok||J1|
After the circuit assembly, the amplifier is tested with two 4 Ohm Pioneer speakers. The sound quality and the loudness level is very pleasurable. As expected, the heat dissipation is quite high. The heatsink rapidly warms up and becomes hard to touch.
The test setup is shown in the photo below.
Please feel free to ask your questions and share your comments by using the comments field below.