Sean's post above is right on but I would like to add a couple important technicalities.
First and most importantly, amplifiers are tested by STATIC means. This means two things: The test signal is continuous and repetitve, e.g., a sine wave or square wave. Secondly, the load is a resistance and not an IMPEDANCE. The difference between the two gets butchered all the time but basically, impedance takes capacitive and inductive effects into account whereas resistance does not. Many times, people use the word "impedance" but if you take a close look, they are actually using an "averaged resistance" at best. This is incorrect use of terminology but it runs rampant, especially here. :)
The sum total is that music is a very dynamic signal that is constantly changing. The speaker's impedance, in most cases, is a ridiculous mess of ups and downs. Combine the two and you get drastically different damping factors, reflected waves and varying slew rates at different points in time AND for different frequencies. None of this information is faithfully represented by the manufacturer's specs.
But as we already know, you have to listen to get a feel for how an amp/speaker combo works - and that some who measure great, fall short in reality due to poor handling of dynamics. Listening is the best feedback on performance you can get because only then are all the real variables taken into account.
The issue of negative global feedback is different than that of negative local feedback. The two are, again, not to be confused. Global feedback puts the entire amp in the loop whereas local feedback is only for the active devices. This latter one is always required for very good stability but the former is optional, depending on the quality of component matching, parasitic inductances, capacitive coupling, type of active devices and layout quality.
I have looked at the output impedance curves of a few amplifiers using an impedance analyzer. In the frequency domain, most of them are very flat but have an inductive rise at high frequencies (>50kHz or so).
Arthur
First and most importantly, amplifiers are tested by STATIC means. This means two things: The test signal is continuous and repetitve, e.g., a sine wave or square wave. Secondly, the load is a resistance and not an IMPEDANCE. The difference between the two gets butchered all the time but basically, impedance takes capacitive and inductive effects into account whereas resistance does not. Many times, people use the word "impedance" but if you take a close look, they are actually using an "averaged resistance" at best. This is incorrect use of terminology but it runs rampant, especially here. :)
The sum total is that music is a very dynamic signal that is constantly changing. The speaker's impedance, in most cases, is a ridiculous mess of ups and downs. Combine the two and you get drastically different damping factors, reflected waves and varying slew rates at different points in time AND for different frequencies. None of this information is faithfully represented by the manufacturer's specs.
But as we already know, you have to listen to get a feel for how an amp/speaker combo works - and that some who measure great, fall short in reality due to poor handling of dynamics. Listening is the best feedback on performance you can get because only then are all the real variables taken into account.
The issue of negative global feedback is different than that of negative local feedback. The two are, again, not to be confused. Global feedback puts the entire amp in the loop whereas local feedback is only for the active devices. This latter one is always required for very good stability but the former is optional, depending on the quality of component matching, parasitic inductances, capacitive coupling, type of active devices and layout quality.
I have looked at the output impedance curves of a few amplifiers using an impedance analyzer. In the frequency domain, most of them are very flat but have an inductive rise at high frequencies (>50kHz or so).
Arthur