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RE: Negative Feedback - violates a primary rule of human hearing

Posted by Ralph (M) on December 30, 2009 at 11:47:25

In Reply to: Negative Feedback - Call for Opinions posted by 6AS7_6SN7 on October 16, 2009 at 00:40:02:

While effective in reducing overall THD, loop feedback actually enhances the odd-ordered harmonics used by the human ear to detect the volume of a sound (the 5th, 7th and 9th). Our ears, IOW, are so sensitive to these harmonics that enhancement (distortion) of them by only 100ths or 1000ths of a percent is audible. This was proven by General Electric in the 1960s.

Additionally, there is a myth that negative feedback is supposed to stabilize amplifiers, but it is actually a destabilizing effect. In fact **positive** feedback has the stabilizing effect: once the amp is in oscillation, its behavior is considered stable (oscillating).

With negative feedback, the behavior of the amplifier is considered non-linear, when subjected to a constantly variable input signal and non-linear load (typical of most speakers). Chaotic behaviors can be observed: bifurcation of the input signal and other non-linear effects.

Additionally, each loudspeaker has its own critical damping value of output impedance in the amplifier that is not served by a set feedback value. In fact, there are no known examples of a speaker that benefits from damping factor values over 20:1, but there are speakers that benefit from damping values as little as 0.1:1. If the speaker is overdamped, it is much like trying to move the cone through fluid rather than air; the cone will see less excursion and bass response can be down by as much as 8 db as a result, in addition to loss of transients.

see:
"Critical Damping: Missing Link in Speaker Operation Parts 1 & 2"
at:
http://www.pearl-hifi.com/06_Lit_Archive/07_Misc_DownloadsMisc_Downloads.html

also:

• non-linear behavior in dynamic systems   (Open in New Window)

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