Amp damping factor?

"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.

Most speakers do not need a great deal of 'damping' to achieve their design goals. A high damping factor is another way of saying 'low output impedance'.

However there are two schools of thought about what is meant by low output impedance! One school has it that you have to use a lot of loop feedback to achieve it. This is the path taken by most transistor amplifier manufacturers. Not all do though and they are noteworthy has having made some of the finest transistor amplifiers available: Nelson Pass, Ayre and Ridley Audio are the names that come to mind.

However, funny thing- there are a good number of tube amplifiers that IMO/IME play bass better than the transistor amps with high 'damping factor'. These amps do so with 'damping factors' between 2 and 20. Tubes are inherently lower distortion than transistors and so need far less feedback to make the resulting amplifier circuit linear- in fact a good number of tube amps do this with no feedback at all. As Kijanki points out, feedback has very audible detrimental effects.

Our ears use the 5th, 7th and 9th harmonics as a means of determining loudness. We are more sensitive to these harmonics than just about **anything else** in audio! So if these harmonics are emphasized by distortion at all- we will hear it. Audiophiles use the terms dry, hard, harsh, brittle, clinical, chalky, white and the like to describe very slight enhancement of these harmonics.

While loop feedback will reduce overall distortion (when listening to sine waves, the jury is out on whether this is true of a constantly changing waveform), these harmonics in particular are actually **enhanced** by loop feedback. The result is that lots of feedback in an amp will result in an amplifier that will never sound natural.

for more information see
http://www.atma-sphere.com/papers/paradigm_paper2.html

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).

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).