Please explain amplifier output impedance

10-10-06: Atmasphere
The reason Negative Feedback does not work is two-fold. Propagation delays inside all audio amplifiers insure that negative feedback arrives with a delay with respect to the input signal its supposed to provide correction for. At bass frequencies this problem is not profound, but at treble frequencies it is responsible for added odd-ordered harmonic content which (although in small levels) is something that the human ear uses as loudness cues- in effect a source of unnatural harshness to the human ear.

I believe it is this issue that Dakiom Feedback Stabilizers claim to correct. I have no idea if they work as advertised on amplifiers.

I've specifically mentioned the Td ( Time delay ) of a circuit that Ralph makes mention of several times in the past. Shorter signal paths with a consistent impedance will have a lower Td, which is a good thing. I only know of one manufacturer that has ever published this spec as standard procedure.

Speed and time delays are one of the reasons why local feedback works "better" than global feedback. That is, local feedback isn't as slow to respond and the correction factor is smaller in amplitude. On the other hand, global feedback is both slower with greater correction factors involved, making it less desirable. No matter how fast the error correction rate is, it is NOT "instantaneous". As such, it has the potential to introduce other distortions into the equation. Kinda funny how circuitry that is designed to cancel distortion can actually introduce distortion, isn't it???

As far as having maximum voltage with minimal current flow, this is definitely a reality. If such were not the case, we wouldn't have to worry about such things as arcing or corona. Yes, there is current flow involved, but it is minimal compared to the amount of voltage involved.

Try to measure the resistance of air by holding two test leads a few inches apart. With that high of a resistance ( next to infinity ), the current flow involved in arcing across that gap would be quite low even though the voltage required would be quite high. Kind of an extreme example, but i used this as i thought it would be easier to understand than trying to explain antenna theory : ) Sean
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I'm friends with a physicist who specializes in audio, and a couple of years ago he presented two papers on distortion perception at the Audio Engineering Society Convention. His study found that very high levels of low order distortion (30% second harmonic) were inaudible, but very low levels of the type of high order distortion produced by large amounts of negative feedback were quite audible and highly objectionable. Also, the type of distortion produced by amplifier crossover distortion and hard clipping were highly objectionable.

"Auditory Perception of Nonlinear Distortion", Earl Geddes and Lidia Lee, AES Preprint numbers 5890 and 5891. Earl and Lidia demonstrate that standard distortion metrics, THD and IMD, correlate poorly with distortion perception. In fact, THD actually has a slightly negative correlation to distortion perception! (Meaning that a signal with high THD is likely to be perceived as lower in distortion than a signal with low THD). They proposed an alternative distortion metric that correlates very well with distortion perception, but it has not caught on.

Earl has since made some very interesting discoveries about linear distortion too, but that hasn't been published yet.

Anyway, my point is that the type of distortion introduced by large amounts of negative feedback has been demonstrated to be both audible and objectionable.

Duke

Negative Feedback is a failed concept

I humbly beg to disagree. Done properly, negative feedback produces excellent accuracy, stability and linearity in electronic operational amplifiers. If this is not the case, then much of my first year electrical engineering and the techniques employed by hundreds of thousands of engineers is deluded. I doubt that the entire electrical engineering industry suffers such monumental delusions as to use failed concepts for most analog circuitry.