Editing Negative-feedback amplifier (section)

== Distortion ==
Simple amplifiers like the [[common emitter]] configuration have primarily low-order distortion, such as the 2nd and 3rd harmonics.  In audio systems, these can be minimally audible because musical signals are typically already a [[Harmonic series (music)|harmonic series]], and the low-order distortion products are hidden by the [[Simultaneous masking|masking]] effect of the [[Hearing|human hearing system]].<ref>{{Cite web|url=https://www.audioholics.com/loudspeaker-design/audibility-of-distortion-at-bass/nonlinear-distortion|title=Nonlinear Distortion and Perception at Low Frequencies|website=Audioholics Home Theater, HDTV, Receivers, Speakers, Blu-ray Reviews and News|date=13 August 2015 |access-date=2016-04-18|quote=most of the harmonic distortion has been masked, however, a couple of the high order harmonics were far enough away in frequency and loud enough to be heard. So in order to determine the audibility of harmonic distortion, we have to know how much masking is done by different tones at different loudness levels.}}</ref><ref>{{Cite web|url=http://projekter.aau.dk/projekter/files/9852082/07gr1061_Thesis.pdf|title=Perception & Thresholds of Nonlinear Distortion using Complex Signals|last1=de Santis|first1=Eric Mario|last2=Henin|first2=Simon|date=2007-06-07|quote=Masking is a principle concept in the perception of distortion, as distortion products will only contribute to the percept of distortion if they are not masked by the primary stimulus or other distortion products.}}</ref>

After applying moderate amounts of negative feedback (10–15&nbsp;dB), the low-order harmonics are reduced, but higher-order harmonics are introduced.<ref name=":1" />  Since these are not masked as well, the distortion becomes audibly worse, even though the overall [[Total harmonic distortion|THD]] may go down.<ref name=":1">{{Cite web|url=https://passlabs.com/articles/audio-distortion-and-feedback|title=Audio distortion and feedback - Passlabs|last=Pass|first=Nelson|date=2008-11-01|website=passlabs.com|access-date=2016-04-18|quote=Here we see that as low feedback figures are applied to a single gain stage the 2nd harmonic declines linearly with feedback, but increased amounts of higher order harmonics are created. As feedback increases above about 15 dB or so, all these forms of distortion [decline] in proportion to increased feedback.}}</ref>  This has led to a persistent myth that negative feedback is detrimental in audio amplifiers,<ref name=":2" /> leading [[audiophile]] manufacturers to market their amplifiers as "zero feedback" (even when they use local feedback to linearize each stage).<ref>{{Cite web|url=http://www.thetadigital.com/dreadnaught_iii_amplifier_info.shtml|title=Theta Digital – Dreadnaught III Amplifier|website=www.thetadigital.com|access-date=2016-04-18|quote=Dreadnaught III uses no global negative feedback.|archive-url=https://web.archive.org/web/20151121182517/http://www.thetadigital.com/dreadnaught_iii_amplifier_info.shtml|archive-date=2015-11-21|url-status=dead}}</ref><ref>{{cite magazine|url=http://www.hificritic.com/downloads/Archive_6.pdf|title=A Future Without Feedback?|date=January 1998|magazine=Stereophile|archive-url=https://web.archive.org/web/20130619065311/http://www.hificritic.com/downloads/Archive_6.pdf|archive-date=2013-06-19|author=Martin Colloms|access-date=9 May 2007}}</ref>

However, as the amount of negative feedback is increased further, ''all'' harmonics are reduced, returning the distortion to inaudibility, and then improving it beyond the original zero-feedback stage (provided the system is strictly stable).<ref>P. J. Baxandall, “''Audio power amplifier design''”, ''Wireless World'', 1978.</ref><ref name=":2">{{Cite web|url=http://www.edn.com/Home/PrintView?contentItemId=4420162|title=Negative feedback in audio amplifiers: Why there is no such thing as too much (Part 2)|last=Putzeys|first=Bruno|website=EDN|access-date=2016-04-18|quote=Of course this experiment gives the impression that more feedback is worse. You have to get past that bump. Hardly anybody who has ever tried it like this has actually heard the inevitable (and frankly magical) improvement that happens once you do get beyond, say 20 or 30dB. From there on you get an unambiguous net improvement that goes on forever.}}</ref><ref>{{cite web|url=http://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf|title=The 'F' word, or why there is no such thing as too much feedback|date=February 2011|publisher=Linear Audio|author=Bruno Putzeys|access-date=19 March 2013}}</ref>  So the problem is not negative feedback, but insufficient amounts of it.