Damping Factor

Is "zero feedback" in an amplifier design referring to damping in any way?

Damping Factor is directly related to Amplifier Output Impedance. It is not possible to make an acceptable Solid State Amplifier without providing negative feedback. It is however possible to make a reasonably good Valve or Tube Amplifier without negative feedback, or "Zero Feedback". This amplifier, if correctly loaded, meaning the output transformer ratio is such that all available power is transferred to the speaker load will have a damping factor of 1. Adding negative feedback will lower the output impedance and increase the damping factor, it will also in most cases lower the distortion, but it will not increase the output power. Valve or Tube Amplifiers usually have only moderate amounts of negative feedback, resulting in a damping factor around 10. Solid State Amps on the other hand usually have massive amounts of negative feedback in order for them to work at all, giving a high damping factor.
Large amounts of negative feedback generally lower distortion at mid frequencies and tends to "Hide" crossover artifacts. In other words, it makes a fairly ordinary amplifier "Spec-Up Well" on test instruments, but unfortunately does not necessarily make an amplifier sound good driving real speakers playing real music. A high Damping Factor is something you get for free from most solid state amplifiers, not necessarily something you need.
A Damping factor of around 10 is sufficient.

Damping factor is the ratio of the load impedance to the amplifier's output impedance. But you also have to figure in the resistance of the speaker cable to get your "real world" damping factor. IMO, 20 or better is my minimum goal, but 50 or better is my target.

BTW, solid state amps don't use that much negative feedback anymore. At least those that are designed by competent engineers who've paid attention over the past 20 years or so.

Thanks Poulkirk, for an answer that I can mostly understand. I have always wondered this about tube and some SS designs, in regard to the bass response.

The Crown Macro Tech series has a DF of about 30,000.

Most tube amps can't get a low Df because of the output transformer being inherently "stuck" at one impedance. There are many tube amps with much more feedback than some solid state designs, but the SS designs I am thinking of still have a higher DF because the devices are an order of magnitude lower in impedance to start with (before feedback).

Depending upon things like the driver's Q and the "alignment" in the box (B3, C4, etc...) the effect of DF can be heard at frequencies where the driver may be operating in an area where it's damping (due to the in box alignment) may be significantly different than the damping *above* that point. For example, the driver at 150 hz. might act like it has a Qts of .7, whereas at the F3 point it may act like a Qts of 1.0 (or higher). (actually the alignment would be said to have the Qts of 1.0 in that case)

So, you might be able to hear a qualitative difference in the "fatness" or "heaviness", or the other way around,"thinness" or 'lean-ness" of the bass depending upon the DF of the amp used.

Of course, if the designer used a zero feedback amp in the design - and thought it sounded "right" putting on an amp with a 30k DF might very well make it sound a bit too thin
and lean...On the other hand, if the designer used a very high DF amp in the design, then with a low DF amp it might sound a bit bloated and fatter than desired...

Might.

_-_-bear