Amp damping factor?

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).

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.