Damping Factor and Overall Negative Feedback.

These specifications are assigned to some amplifiers but what is the difference between them? I had thought they were the same thing. I read the specs on a Karan Acoustics KSA 450 amp today with a high damping factor (?dumping factor?) and no overall negative feedback.
They are two different things. I suspect that what led you to think of them as the same thing is that negative feedback can have the effect of increasing damping factor.

Damping factor is generally defined as the amplifier's output impedance divided into 8 ohms. Sometimes a spec based on a 4 ohm load is also stated, which would represent the output impedance divided into 4 ohms. If the amplifier is a tube amp that has an output transformer providing both 4 ohm and 8 ohm taps, the output impedance of the 4 ohm tap will be approximately half of what it is on the 8 ohm tap, so the damping factor will be the same for a 4 ohm load connected to the 4 ohm tap as for an 8 ohm load connected to the 8 ohm tap. The output impedance of both taps is usually (but not always) considerably less than 4 ohms.

Negative feedback is a circuit design technique in which the output of an amplification stage, or a series of stages, is multiplied by some factor, and then subtracted from the signal that is present at some earlier point in the signal path. Among many other effects, that will reduce the overall gain of the stage or stages that are within the feedback loop. The amount of feedback is commonly expressed in db, and is equal to 20 x log(gain with feedback/gain without feedback). Since negative feedback reduces gain, the number of db will be a negative number (although the minus sign is often omitted if it is clear that the context is negative feedback).

The potentially beneficial effects of feedback include, among several others:

-- Reduction of the degree to which circuit gain and performance are affected by variations in tube or transistor parameters. The variations reflecting tolerances in the device specifications, or occurring as a result of temperature changes, aging, etc.

-- Reduced output impedance.

-- Improved linearity, and, related to that, reduction of total harmonic distortion (THD).

The main downsides of negative feedback result from the fact that due to the amount of time required for a signal to propagate through the circuitry, the signal that is fed back and subtracted lags the signal it is being subtracted from. That results in what is called transient intermodulation distortion (TIM), and enhancement of particularly objectionable odd-order distortion components.

A good design will require minimal or no feedback to provide good performance in the areas that feedback can help, and thereby avoid the downsides of excessive feedback.

BTW, the reference you cited to "no overall negative feedback" (which also may be referred to as "no global feedback") just means that there is not a feedback loop that encompasses all of the stages in the amplifier, and says nothing about whether or not feedback is applied around individual stages, or groups of stages that don't encompass the entire signal path.

Best regards,
-- Al
Really, really informative. Thanks!
That results in what is called transient intermodulation distortion (TIM), and enhancement of particularly objectionable odd-order distortion components.

It is this last bit wherein you see so much variance in amplifier design: odd orders as stated are very objectionable, in no small part because the human ear/brain system uses them to determine how loud a sound is. It is arguable that our ears are more sensitive to this than actual frequency response errors- so electronics that distort these harmonics will have a false loudness and brightness to them. Audiophiles use terms like 'hard', 'harsh', 'brittle', 'bright' to describe this phenomena.

This is why of two amps that have flat frequency response, one might sound bright and the other might not.