LM1875 120W(30W+30W+60W) 2.1 channel amplifier | Cheap and easy to build

Here’s a simple 2.1 channel amplifier for you home. it uses TI 20W LM1875 amplifier chip. It uses 4 of it so it can produce a total of 80W. It can output up to 30W with a bit higher THD. Maximum attainable power is 120W total.

About the Circuit

I keep the circuit as simple as possible because I want to make the board in small factor. The final board measures only 7cm x 9cm. I did avoid to add a high pass filter for the main speaker to fit the whole circuit on small board without using double sided board. I did limit the low frequency response of the main speaker by lowering the value of C5 and C13. If you want to have more low frequency sound on the main speaker then you can change value of C5 and C13 to 10uf instead.

Take note that R23, R24, R25, and R26 are just jumpers. They are just used because we are just using 1 layer PCB.

LM1875 can be able to deliver up to 20W into 4 ohms or 8 ohms load on +-25V supply. Using 8 ohms load and +-30V supplies, it can deliver up to 30W. I recommend using an 8 ohms speaker.

You can increase value of C1 and C2 for lower hum noise and better transient response at high volume. 1000uf should be better but I am not sure if it will fit. Capacitor voltage rating should 50V.

All resistors are 1/4W.


LM1875 is used on the amplifier is capable of delivering 20W output power. Subwoofer uses 2 configured as bridge so it can deliver up to 40W. The total combined power of this circuit is 80W. As I have pointed out, it can deliver up to 30W output power.

The biggest advantage of using an IC amplifier is that it is simple to build and has protections built in like short circuit and over temperature protections.


I simulated the circuit using LTSpice

Subwoofer output and main speaker frequency response graph

Crossover frequency is around 110Hz.

In this graph, the input is an 80Hz and 1kHz sinwave. The blue line is the output of the subwoofer and green is the main speaker. As you can see, the output of subwoofer have filtered out the 1kHz signal.

PCB Design

Here is my simple PCB design using kicad software. This software is free by the way.

Take note that J10 – JJ15 are only jumpers, they need to be connected. J14 must be shorted to J15, J13 must be shorted to J12 and J10 must be shorted to J10. Board size is 70mm x 90mm. Take note that I am not sure about the size of C1 and C2 though. It may be a bit bigger in actual, it will depend on the brand and voltage rating.

3D render using kicad


You can download this project on my download page


LM1875 Datasheet