Electrical – What causes “positive feedback” in audio amplifier


I am currently using an audio amplifier to make a presentation.

However, sometimes mid presentation the audio amplifier will go into a "positive feedback" mode where the sound coming out of the amplifier gets increasingly louder and louder until it suddenly drops off.

At first I thought that my voice was too loud which causes the amplifier to hit the saturation limits. However, that does not explain why the sound volume gradually increases, also does not explain why it suddenly drops off.

Can someone explain to me what is causing this phenomenon and how do you prevent it?

Feel free to use circuit or control theory I have rudimentary understanding of it. A diagram will be most helpful.

Best Answer

@berto's answer and the comments on it refer to feedback, which normally results in a loud squeel which will continue until the amplfier gain is reduced.

Your problem appears to be something else. A possible cause could be a faulty automatic gain control or compressor/limiter in the amplifier.