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Schoolies Amp

This 20W audio amplifier module sounds great and is dead easy to build!

By Peter Smith

Building an audio amplifier is a popular choice when it comes to the hands-on part of electronics courses. We can well imagine the classroom question "Well, does it work?" answered in a flash with "Listen to this, disbeliever!"

That’s the best part of building an audio amp; you and your peers actually get to hear the final work punch out a favourite tune or two hundred!

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Fig.1: the circuit consists of little more than the LM1875, which contains a complete low-distortion 20W amplifier with overload protection in a 5-pin TO-220 package.

However, amplifiers that produce more than a few watts of power can be difficult to construct and expensive. This is where our new "Schoolies Amp" comes in.

It features a simple board layout for easy construction, is relatively inexpensive and even includes over-temperature and short-circuit protection.

As power amplifier modules go, this unit may not rank at the top for raw power but you’ll be hard pressed to find a simpler circuit.

It’s based on a single IC, the LM1875T 20W audio amplifier from National Semiconductor. This IC comes in a TO-220 package and, combined with a handful of other parts and a suitable power supply, delivers over 20W RMS into either a 4Ωor 8Ω loudspeaker.

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Shown here with its matching power supply, the Schoolies Amp (aka 20W amplifier module!) takes next to no time to build, costs very little - and will give a surprisingly good account for itself in a variety of audio projects.

What’s more, the specifications are quite impressive for such a bare-bones circuit. With a signal-to-noise (S/N) ratio of 105dB and a distortion figure of less than 0.04% for 1kHz at 20W (see graphs), it could well be used as the basis for a hifi stereo amplifier. The frequency response extends from 14Hz to beyond 100kHz when measured at 1W RMS.

The LM1875 includes an internal 4A current limit, preventing damage should the output be accidentally shorted to ground.

It also includes "safe operating area" (SOA) protection, meaning that the current limit is dynamically reduced according to the voltage present at the output.

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