Amp damping factor?

Bi,
Read the rest of this thread. Most of your questions answered.

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