Amp damping factor?

Inductor in-series with the woofer has resistance approx. 0.08 ohm limiting DF to 100.

"Digital amps seem to be the exception which normally have a damping factor of up to 1000".

My Icepower amp has DF=4000 at low frequencies. It is inherent in class D amps since speaker is always shorted by Mosfets to power and GND (to very low source impedance). Mosfets have fraction of an ohm resistance while even small gain before feedback (I suspect around 100) will lower it a lot. Conventional class AB SS amp has gain before feedback of many thousands (lowers THD and IMD but increases TIM). Class A SS amps are better, with gain before feedback of only few hundred.

As Al mentioned, deep feedback brings negative effects (but improves spects). I wouldn't buy class AB SS amp that has very good spects (something has to give) or at least it wouldn't be my first choice to audit.

Here is the link: http://www.stereophile.com/reference/70/index.html to an article about feedback. Stereophile tests Cary amp with adjustable feedback and the sound is the best when feedback is set to the lowest.

Atmasphere - also speaker design can make for better quality of bass. As far as I know typical, used in 90% of cases, overhung motor speakers have a lot of distortions at lower frequencies. More expensive underhung speakers,used by some manufacturers (like Acoustic Zen) have much lower distortions (but are more expensive).

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).

"An amp doesn't have a damping factor"

Pauly - You'll often find DF in amps' specifications. "Having" DF simply means amp has that many times lower output impedance (at given frequency) than assumed 8 ohm speaker. DF is a measure of output impedance - hence amp "has" DF.

"What you are looking for is an amp with a low output impedance" - not likely to find in specifications, look for higher Damping Factor instead.

Pauly - I agree. Specifications are very flexible and often misleading. Amplifier (and any other) design is a series of compromises and when you see an amp with exceptionally high DF and extremely low THD something else has to give - the sound.

Joekapahulu - I wouldn't pay too much attention to DF. For instance Atmasphere MA-1 amps have DF<1 (tube amp without output transformer) but sound great according to 6Moons review ("gigantic imaging, excellent dynamics, frequency extension and potent music-making").

Bifwynne, It is a trade-off. Amplifiers with very high DF use a lot of negative feedback that enhances bright sounding odd harmonics.

How much of DF do you need? It depends on the speaker damping and room acoustics and perhaps many other factors. I would not even consider DF in my amp selection.