Audio Power Amplifier

"An audio amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 - 20 000 Hz, the human range of hearing) to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain." Courtesy of Wikipedia.

The RnD team wanted to build an audio amplifier for an upcoming project and here are the results.

IC used - LM 1875 - Click here for Datasheet

The LM1875 is a monolithic power amplifier offering very low distortion and high quality performance for consumer audio applications. It is a 20 W audio power amplifier which can be used with a 4 ohm or 8 ohm speaker with a supply of ± 25 Volts. 

List of things used:-

1.) Metal Cabinet

2.) Speakers

3.) Audio power amplifier board

4.) 20-0-20 3A Transformer

Let us go step-by-step by first looking into the circuit:-

Here is the circuit:- Click here  to Download PDF

Here is the PCB layouts:- Click Here to Download PCB1 and PCB2

The speakers can be of 8 ohms or 4 ohms. But we found that the 4 ohm speaker gave the best performance. Hence we used two 8 ohm speakers in parallel to make it 4 ohm speaker configuration.

Now the transformer is 20-0-20 ratio with 3A configuration and soldered to the terminals is a Full wave rectifier. The capacitor is 2200uF/50V.

We have built three amplifier circuits on the general purpose board but have only two circuits are in operation. The idea was to provide 6 speakers with three circuits. But with just 4 speakers and 2 circuits the specification was met and done. Here is the pic of the audio power amplifier:

Now these above components were in-corp-orated  into a cabin box and the here are the pics.

The screen-shots below show the output from the speaker. After calculation, a 2W approx. output was derived from each speaker where the voltage from the transformer was 28 Volts.