Please explain amplifier output impedance

To bring this discussion back around to my original question, when a reviewer states that a particular loudspeaker would likely work best with a low output impedance amplifier, should one assume (based on comments made earlier in this thread), that a low output impedance measurement is .1 ohm? At what measurement (expressed in ohms) does the threshold cross from low output impedance to marginally high output impedance...1 ohm?

There is no hard and fast threshold but IMO, anything over 1 ohm is definately high output impedance. Many SS amps will be about 10x lower and tube amps are up to 10x higher so 1 ohm threshold seems to a decent rule of thumb.

Sean - My comment about amp impedance rising above 50kHz was based on at least a 3dB rise so it would visibly start to deviate around 20kHz. And you are definately correct in saying that many amps can't even do that well. I read all the Stereophile measurements several times each, no matter the piece of equipment. I am happy they are still around to do the good old fashioned measurements, even if I realize they aren't very applicable to musical reality. Nonetheless, I feel there is a lot to be said for an amplifer with an elegant set of curves.

Arthur

High damping factor is allways desirable. Since speakers are a reactive load they respond to the amplifier signal with a counter eletromotiv force that the amp must deal with, and with a low damping factor the amp will have more intermodulation distortion because of this fact. Low damping factor will cause the amp to have more problems in controlling the speakers drivers properlly.

Yes, it is impossible to make voltage without current. Power=Voltage X Current. Even in a preamp the voltage is there because current is there also. This fact is inescapable and is the result of Ohm's Law, the basic law of all electricity.

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.

Negative feedback runs counter to the rules our ears use and we're stuck with the ears we have. If we could eliminate the propagation delays inside audio amplifiers and gain stages, NF would work, but until then Negative Feedback is a failed concept.

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.