Today, I am going to share on how you can make a low voltage audio amplifier. For this project, I am using LM 386 Audio Amplifier, which is commonly used among electronics hobbyist. I will break the article into a few sections. First, I will explain about the LM 386 chip itself. Then move on with some explanation on the parts used in this project. And finally, the design of audio amplifier.
Some of the advantages of using LM 386 Audio Amplifier is that you can power it with a 9V battery and enjoy the music. Besides that, the audio amplifier requires minimum amount of parts to operate as the gain has been set to 20 internally. However, further increasing in gain can be made externally. You can use it to amplify music from various devices such as mobile phones, laptop, or MP3 player.
First, let us look into detail on the chip, LM 386. LM 386 is available in Dual in-line Package (DIP8) and Small Outline Integrated Circuit (SOIC8) package where DIP is the through hole version while SOIC is the SMD version.
Some of the features of LM 386 Audio Power Amplifier:
- Battery operation
- Wide Supply Range: 4-12V or 5-18V
- Voltage Gain from 20 to 200
For more information, please refer to the LM 386 datasheet
I am using through hole version for my audio amplifier. I will briefly explain the pin functions of this IC.
- Pin 1 and 8 is used for gain setting. If both pin left opened, the gain would be equal to 20 which is set internally. Gain can be control by placing a resistor and a capacitor in between.
- Pin 2 and 3 is the input pin of the amplifier.
- Pin 4 has to be connected to circuit ground.
- Pin 5 is the output pin of amplifier which produce the amplified signal.
- Pin 6 is the power supply to the amplifier.
- Pin 7 is the bypass terminal. For stability, place a 0.1uF capacitor to GND at this terminal.
After reviewing the chip, now let me proceed with the typical application of LM 386. All the circuits shown are available on the datasheet.
Parts and components
Parts that you would need for this project:
- LM 386 Audio Amplifier IC x1
- 8 pin IC socket x1
- Loudspeaker x1
- 3mm green led x1
- 10k ohm potentiometer x1
- Male audio jack x1
- 10 ohm resistor x1
- 330 ohm resistor x1
- 10uF capacitor x2
- 47nF capacitor x1
- 220uF capacitor x1
- 0.1uF capacitor x2
- 100uF capacitor x1
- 9V battery holder x1
Most of the audio playing devices available are using 3.5mm audio jack. So I will be using a 3.5mm female jack connector on the audio amplifier board. A male to male audio connector will be used to connect the audio device to the amplifier. Image below show the connection on a male audio connector. Notice that there are two channel available which is the left channel and the right channel. In this project, I am using only one of the channel since the audio amplifier only able to amplify one signal at a time, which makes it a mono audio amplifier.
Note that some of the phone uses a different audio jack. The audio jack has an extra connection for mic. Therefore check the audio jack in your phone before using it as they might not be compatible.
The speaker that I am going to use in this project is a 4 ohm, 3W speaker. I got this speaker by taking apart an old speaker that I have. As compared to the speaker which I used for prototyping, where I used 8ohm, 0.25W speaker, this 4 ohm, 3W gives a better effect for the sound quality.
Schematic and Layout Design
The circuit itself consists of two part, one is the main amplifier circuit and the other is the power supply part. The power supply is pretty simple since LM 386 can be powered with a 9V Alkaline battery. Two filter capacitor, 100uF and 0.1uF is placed between the power supply to smooth the voltage supply in the circuit. A green LED is used as a power supply indicator.
For the main circuit, its almost similar to the typical application circuit shown in datasheet, except it was modified where the gain can be adjusted to either 50 or 200. This gain control is achieved by the jumper, JP1. Shorting the 1k ohm resistor will produce the maximum gain (200) while leaving the jumper open will produce gain of 50. A potentiometer is used to adjust the amount of input signal to be amplified.
One important point to take note is that you need to use a coupling capacitor between the input device to the amplifier. A coupling capacitor is used to block DC signal and pass AC signal. The theory behind this is that capacitor appears to be open circuit at low frequency (DC) and short circuit at higher frequency (AC). In the schematic above, a 10uF capacitor is used between the input source to the potentiometer to couple the audio signal.
For the layout design, I tried to make it as small as possible, squeezing everything into a 3.5cm x 2.5cm PCB.
Finally, a video demo of music played from LM386 audio amplifier. The song name played in this video is Spaceman by Hardwell.