Audio Signal Testing amplifier

Posted on Apr 11, 2012

This mini audio amplifier will test the audio stages in amplifiers such as the front end of FM bugs. You can also use it on lots of our other projects as well as the output stages of radios. It is a very handy piece of test equipment and will make a perfect addition to your collection. The LM 386 is a high power op-amp. It is used to drive a speaker. The inverting input has been connected to the negative rail and the non-inverting input has been connected to a capacitor. Resistors inside the chip bias this input so that it only requires about 7mV before the chip turns on. This voltage is called the off-set voltage.

Audio Signal Testing amplifier
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As we said, there can be lots of problems with audio stages. Sometimes they develop over a period of time, or they can be present from the moment the amplifier is assembled. You can also get intermittent faults, drop outs, squealing, motor-boating, distortion or simply lack of gain. The amplifier presented in this article is used as an audio probe by listening to the loudness and quality of the audio going into the stage under test and comparing it to the audio emerging from the output of the stage. You do this by putting the earpiece near your ear and listening carefully - that's why the probe has two long leads. We could write a book on fixing audio stages but to reduce it to one page, here is the approach: The simplest situation is a single transistor amplifier with a distorted output. The first thing to do is replace the transistor. Even though transistors are very reliable devices, you may have overheated it and this will cause the device to have a different gain and thus the voltages on the stage will not allow the signal to be processed correctly (you can put it back later if it is not the cause of the problem). The next thing is measure the voltage on the collector. Ideally it should be half-rail. This allows the transistor to turn on and off THE MAXIMUM AMOUNT and thus give the stage the maximum gain. To carry out any more investigation on the stage you will need a copy of the circuit diagram. This...

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