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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.

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  • \$\begingroup\$ Could you add some details on the sound system (make & model, special features, etc). That might help someone diagnose the problem - from your description, it doesn't sound like anything I've experienced. \$\endgroup\$
    – Peter Bennett
    Commented Mar 7, 2016 at 17:11

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@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.

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  • \$\begingroup\$ I agree with this +1 \$\endgroup\$
    – Andy aka
    Commented Mar 7, 2016 at 11:24
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The signal output of the amplifier speakers goes back into the microphone, after that, it is amplified again, and goes out the amplifier, and the process repeats, in theory the signal grows and grows without limits, in practice, its amplitude is limited by the amplifier supply. To solve this you need to keep the microphone away from the amplifier. So when the signal goes back to the microphone its amplitude is much lower. If it does not do the trick you will have to lower the volume (i.e. gain) of the amplifier. The theory behind is that the "loop of sound" formed by sound going from the amplifier back to the microphone must have a gain lower than 1 to avoid oscillation. Gain greater than 1 creates the positive feedback oscillation. Here you can find more on the subject https://en.wikipedia.org/wiki/Barkhausen_stability_criterion

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  • \$\begingroup\$ The audio amplifier is responsible for having a gain greater than 1 in order to amplify the audio input. What is the circuit elements are responsible for keeping the gain lower than 1 in the feedback loop? \$\endgroup\$
    – Fraïssé
    Commented Mar 7, 2016 at 3:11
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    \$\begingroup\$ The element that couses attenuation (i.e. gain lower than 1) is the media in which sound propagates. If you scream, someone near to you will hear you quite good, but a person a few blocks away will hear you very softly. That is why putting the mic. away from the speakers will help. \$\endgroup\$
    – berto
    Commented Mar 7, 2016 at 3:19
  • \$\begingroup\$ The volume control on the amplifier, and the path from the speakers to the microphone together control the loop gain. If the amplifier has an equalizer or tone controls, you may be able to adjust them to reduce the gain at the offending frequencies. However, from your description, you seem to have something other than simple feedback. \$\endgroup\$
    – Peter Bennett
    Commented Mar 7, 2016 at 4:49

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