The whole project started around one and a half – two years ago when I was surfing on Texas Instruments website in order to find interesting components. Actually, it was around my birthday when I have ordered them, lets say as a birthday gift for myself. I had no idea what I want to build or do, I just wanted to spend some money and finding new adventures. After some time, I found the LM2876 which is an AB class, 40 W power amplifier with some interesting features, so I fall in love by the first sight. It has that NDA0011 insulated package. Its big, a lot of space between its pins and so on. I got so exited about them so I have bought 10 pieces. Unfortunately, I had to wait around one painful week till them arrived.
But now lets see the most interesting features of this amplifier IC:
- 40W output power into 8 ohm, 75W peak output capability
- mute function
- output is protected from shorting to ground
- undervoltage protection under 12V
- wide supply range 20-70V
Recommended for use in:
- high-end TVs
- self-powered speakers
- surround-sound amplifiers
And now lets see the official application note for single voltage supply:
The whole schematic is quite straight forward. The Rin 10k potentiometer should be a logarithmic one, it helps to adjust the amplitude of the input signal which directly influences the output power. The C 10uF is an electrolytic type capacitor which decouples the DC offset of the input signal. CS is the supply voltage filter and bypass, it should be at least 1000uF or bigger, with a voltage rating greater than the supply voltage. If you plan to use this amplifier at higher power (around its limits) it is recommended to use a bigger (for example 4700uF) capacitor. RA1 91k, RA2 100k, CA1 0.1uF, RM 10Km CM 100uF together form the circuit that keeps the mute function off. The feedback from the output of the amplifier (from pin 3), together with RSN 2.7 ohm, CSN 0.1uF, RF1 20k, CF 50pF, RF2 20k, R1, CI 10uF and CC 220pF stabilizes the output and helps to maintain a unity gain, also it is used to bias the amplifier with a single supply.
Also, from pin 3 of the LM2876, the output COUT 4700uF capacitor, *L 0.7uH and with the inductor in parallel there is a 10 ohm resistor connected. These components are used in order to filter the output of the amp, not to let DC current onto the speakers, also cuts of too low and too high frequencies. The bigger the Cout is, than more low (in other words bass) is fed to the speaker, and the higher the *L inductor value is, the lower its maximum frequency will be. You can play around with these in order to get the best results, but it lowers the efficiency of the overall amplifier. It is more recommended to do these filtering at the input stage of an amplifier, because this way you do not dissipate in the form of heat the unwanted frequencies.
The LM2876 is represented in the form of an operational amplifier, which is quite common in audio amplifiers. Actually, if you take a look at the internal structure of the device, you will actually find a differential amplifier, mute function implementation and the final power stage. However, the over temperature and spike protection is not shown below. With green circle is marked the input, and the yellow circles highlight the AB class power stage, in other words the output part.
The actual building of these PCBs took me around an afternoon. Fortunately I had all the necessary components and tools so I did not have to take a visit to a local electronics shop and I could finish this project fast. If you wish to replicate this project, feel free – you can download the Eagle files and also in .pdf format.
Before the ending of this article I would like to mention that I have tested the over temperature feature, and it works quite well. I have heated up the heat sink of the amplifier simply with the tip of my soldering iron. The protection have kicked in multiple times, and the amplifier survived these brutal tests. Probably not this is the usual method for testing, but what can I say – this was the handiest solution for me.
As a future project I am going to install them into my current audio amplifier which I use for my PC because one of its channels have died a few years ago.
And now it is time to show the working prototype of this circuit built by me: