[sdiy] How to explain how negative feedback lowers noise?

[Brian Willoughby]

On Mar 23, 2021, at 09:01, Richie Burnett wrote:
> Negative feedback reduces noise (and DC drift, and distortion,) that occur inside the feedback loop because all of these things represent sources of error at the output that are inverted and fed back in order to reduce the error.  All of these things happen in an audio amplifier.  The one that it harder to explain is why negative feedback increases bandwidth ;-)
> 
> Negative feedback doesn't remove things like noise or distortion present at the input because these aren't error as far as the control system is concerned.  So if the set-point input is noisy the control system will try it's best to get the output to follow it faithfully, noise and all.

Indeed. This can particularly be a problem in mixers, because a large bus bar can pick up all kinds of noise and feed it to the input of the mixer stage. No amount of feedback can correct that - you just end up with a lot of noise. You can't even expand the filter loop to include the individual channels because that would involve multiple tentacles of feedback connections. Feedback only works between one output and one input (at least I've never seen a circuit topology involving feedback across multiple unique inputs or multiple unique outputs).

Brian