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Is feedback in audio ever a good thing?

We normally think of feedback in audio as being a totally bad thing. However, there is a type of feedback on which virtually the whole of audio technology depends. So why is it called 'negative feedback'?


Oh yes, yes, yes, yes, yes!

For many, feedback only has negative connotations, as in the feedback that occurs in a live sound system when the gain around the loop from microphone to loudspeaker and back to the microphone is too high (in fact, higher than 1).

But there is such a thing as negative feedback, and it is a concept on which virtually the whole of audio depends.

Electronic devices inherently produce distortion when they amplify a signal. Distortion is when the waveform changes shape from what it was originally. It leads to the generation of harmonics which, with the rare exception of the electric guitar amplifier, are wholly undesirable in audio.

Some devices, such as transistors, inherently produce massive distortion - so much that the signal they produce would be unlistenable. Without negative feedback that is.

Negative feedback was invented by audio hero Harold S. Black, way back in the 1920s. The concept is that a signal is amplified, then a proportion of the output is inverted and fed back to the input. Effectively this subtracts a little of the output signal from the input.

Black's colleagues thought he was, shall we say, not quite on the right track. Well it does sound crazy to feed the output back to the input. In fact if the output is not inverted, that's the way you make an oscillator!

But it does work. Think of it like this... the output isn't quite the same as the input. It is distorted. But if you subtract that distortion from the input signal, it will cancel out.

That works. The downside is that you can't separate out the distortion and subtract it from the input, you have to subtract a proportion of the whole output signal. That inevitably causes a loss in gain, which is why Black's colleagues thought it wouldn't work. Electronic designers of the day were struggling for all the gain they could get and could not afford to lose any.

However, within a short time developments had taken place to provide gain aplenty, so some of it could be spared. And now you can have as much gain as you like, so you can massively increase the amount of negative feedback applied.

Negative feedback has several knock-on benefits...

  • It reduces distortion (which we have noted already)
  • It flattens the frequency response by actually defining the gain, which otherwise would be different at different frequencies
  • It cancels out the characteristics of the amplifying device, and the characteristics of the amplifier become those of the feedback network, which is made from passive components that are much easier to manufacture accurately.

Negative feedback has done wonders for audio and Harold S. Black is a true audio hero.

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By David Mellor Wednesday July 6, 2005

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