Please explain amplifier output impedance

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.

Hi Bob, in the case you mention, no work would be done. IOW this has no bearing on driving a speaker.

Shadorne, in fact we operate in a world of paradigms. If this was glossed over in school, paradigms are a set of rules that are accepted as fact until the flaws in the rules are perceived. Then a new paradigm emerges; the old paradigm comes to an end. We are living in an era of transistion (which has been going on for the last 10?-15? years) now: some of the stuff that you (and me, and thousands of others) were taught is now being found to be not so truthful. Take a look at Duke's post above- it points directly to the problem that negative feedback causes- in fact negative feedback is a failed concept (old paradigm) in audio. Astrology too was taught in the world's major universities as fact less than 400 years ago :)

What the theory of negative feedback overlooks is that propagation delay exists. Since propagation delay is a fact of the real world we are now witnessing the emergence of a new paradigm.

The alternative paradigm has a different set of rules. It too looks for low distortion, but achieved in a way that does not offend the human ear (i.e. no feedback).

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

I assume you are refering to Intermodulation Distortion or IMD distortion in an amp that is oscillating from large amounts of negative feedback. I agree that this is far more detrimental to the sound and something our ears seem quite sensitive to. Harmonic distortion is often indistingushable from the real sound of the instrument because the pitch of the note does not change (the note becomes fuller or leaner sounding). Even harmonic distortion is particularly hard to discern as your physicist friend points out. My understanding is that odd harmonics are more easily discerned, although still not nearly as easy to discern as IMD.

Hearing Harmonics

IMD distortion is one of the best arguments for promoting active speakers. Separate amplification for each driver over a limited bandwidth can only help to reduce IMD distortion significantly. Compare this to passive full range speakers where the amplifier must control the woofer at 40HZ and the tweeter at up to 20 Khz....hardly suprising that such a broadband system introduces audible IMD due to the combined interaction of all the drivers, the crossover and the amplifier fighting to control them.

Hi Shadorne, if you look at the quote you will see that it is in fact *harmonic distortion*, not IMD. Several studies now have shown the same thing: the human ear/brain system uses odd-order harmonics beyond the 7th harmonic or so as loudness cues. In nature these harmonics are quite low- and even very slight enhancement of them is easily detected by the human ear.

I learned this years ago while servicing an amplifier on the bench. The output of the amplifier was connected to a loudspeaker and a VU meter. The amp had a sine wave at the input. While malfunctioning and making less than 20db of its normal output, it still sounded louder than the normal undistorted output. Once you experience this you will not forget it!