Amp damping factor?

Interesting. I have never read anywhere before this about the relationship between loudness and harmonics 5,7,9, or about sensitivity to these harmonics. Please explain. How are these harmonics enhanced by loop feedback? What are the audible detrimental effects of feedback?

Deep loop feedback is equivalent to high gain before feedback. With such high gain (in order of thousands)any delay in the signal path (limited bandwidth) results in improper (late) summing of the input and the feedback signal causing TIM - tendency to overshoot, altering shape of the signal (exaggerate odd harmonics).

Sane designer would design amp as linear as possible without feedback and then would introduce just enough feedback to bring distortion below 1%. After that it would be necessary to reduce bandwidth at the input to one that amp had before feedback (to prevent TIM). At the end we would get nice sounding amp that has horrible spects - it wouldn't sell. There is probably much more to it but I wouldn't buy class AB amp with extremely low THD or extremely high Damping Factor (deep feedback).

Another aspect of loop feedback is that all amplifiers have a delay time- the time that it takes for the signal to propogate from input to output.

What this means is that the feedback signal will always be a little late getting back to the input of the amp. As frequency goes up, the problem gets worse as the propagation delay of the amplifier remains constant (in effect the feedback signal is progressively later). At very high frequencies this can cause the amplifier to oscillate if not treated properly in the design.

The result is a sort of ringing effect in the amp, which plays a role in odd-ordered harmonic enhancement. Keep in mind we are not talking about very much distortion; hundredths of a percent is all it takes to be audible.

??

An amp doesn't have a damping factor. Damping factor is the ratio of the rated impedance of the loudspeaker to the source impedance. In other words, you can only calculate a damping factor when you have a speaker amplifier combination.

What you are looking for is an amp with a low output impedance which then results in a good damping factor number.

Regards
Paul

http://homepage.mac.com/tlinespeakers/vaughn/downloads/Damping-Factor.pdf

An interesting article on damping factor.

John C.