Amp damping factor?

Magfan, I read the posts. I am not an electronics techie, so most of the discussion went over my head. Perhaps Kijanki will pick my post up and answer in layman's terms my question:

"I just happened to notice that most of the tube amps in the ARC line have pretty low DFs, e.g, less than 20. Nevertheless, most of the ARC tube amps sound great. So what's all the hubbabaloo about DF?? Does it matter that most if not all ARC tube amps use output trannys?"

Thanks

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.

If I manufactured an amp with low output impedance, I would certainly tout its high damping factor as a positive attribute. But, for anything other than really high output impedance amps, like some SET and triode pushpull tube amps, playing into speakers that present a particularly difficult load (low impedance at critical frequencies), the numbers are usually meaningless. Personally, I would not be that concerned unless the output impedance of the amp exceeds 2 ohms (nominal damping factor of 4) for any speaker that is reasonably rated at 6-8 ohms.

Bifwynne, most tube amps have much lower damping factors than most solid state amps. Some very highly regarded tube amps have damping factors in the vicinity of 2, although most tube designs are somewhat higher than that.

What is important is that damping factor and output impedance (which unless otherwise specified is equal to damping factor divided into 8 ohms) should be chosen to be suitable matches for the particular speaker. Mismatches can occur in two ways:

1)Tonal imbalances may result if the speaker design, particularly its variations of impedance as a function of frequency, reflects the expectation that it will be used with a solid state amp and it is used with a tube amp, or vice versa. See this Atma-Sphere white paper. What he refers to as a "power paradigm" amplifier would be one with high output impedance, and what he refers to as a "voltage paradigm" amplifier would be one with low output impedance.

2)Speakers rely to varying degrees on the amplifier for damping of "back emf" produced by the woofer. Inadequate damping will result in "loose" bass, because the woofer cone will tend to continue moving after the signal has stopped or changed. However, as indicated earlier in this thread, once a damping factor is provided that is adequate for the particular speaker, which is rarely if ever higher than a two-digit number, further increases in damping factor will not be helpful, and in some cases may be reflective of excessive feedback that would be sonically harmful.

The key thing is the matchup of speaker and amp. Some speakers are good matches for both tube and solid state amps, but some are not. A simple way to help narrow that down is to research what amps others have used successfully with the particular speaker.

Regards,
-- Al

Thanks Kijanki, Larryi and Almarg. The Atma-Sphere white paper was particularly interesting and helpful. Al's advice concerning matching amps with speakers now makes sense. In particular, I can see why DF may be a bit of a red herring. But I think there may be a practical problem with the other very good advice posted above. That is knowing how to research which amps and speakers match well.

For example, when I spoke with ARC a while back, the folks were not familiar with Paradigm Signature speakers. At the same time, when I spoke with the Paradigm tech folks some time ago, they were not very familiar with ARC tube gear.

Now to my ear, my Paradigm/ARC combo sounds ok, but is it really a good match? Am I really getting the best sound possible out of the combo? How can I really know? Let's face it, it's not practical to schlep a 62lb tube amp around with me when I audition speakers. At the same time, a dealer may not be driving his portfolio of speakers with a tube amp, but rather a SS amp -- which is often the case.

Can one say that if a speaker has a nominal impedance of 8 ohms, there should be a good speaker/tube-amp match? Is speaker efficiency a relevant factor? Any other good rules of thumb that might provide helpful guidance? Just thinking out loud, maybe the best thing to do is to call the speaker manufacturer and ask if a particular speaker matches well to a "power paradign" amp and/or a "voltage paradigm" amp.

Thanks again guys!