high power tube amps vs ss

I wholeheartedly agree with Charles1dad and others with like opinion who caution about amp/speaker compatibility. There's a thread running now about high current SS amps versus tube amps. Read the posts. There are some very excellent comments and much to learn. Suffice to say that the decision to invest in a quality tube amp is not just plug and play. Just read the OP. I don't have the energy to rehash all the issues again. But trust me, it's not just about tube warmth versus SS this or that. There are serious electrical issues at play.

Al . . . you're the best! Would you adopt me?? ;-')

Ralph, I tried your experiment of ramping up the gain. I was playing an EMI CD of Tchaikovsky (sp?) Nutcracker. The dynamic range is between 20 and 30 db. I never made it beyond 95db. At that point, external distortion made me turn the gain down. I'm referring to wifey. She threatened to throw a bucket of water on my gear. She introduced at least 90db of negative feedback and the odd ordered harmonics from her screaming really hurt my ears. Any suggestions?

Al, does the autoformer lower the output impedance of the amp, or increase the perceived impedance load of the speakers. I appreciate the DF math works the same way, but just wasn't sure what changes.

In other the DF of amp is 20:1 if speaker impedance at the test frequency is 20 ohms and the amp's output impedance is 1 ohm. DF is still 20:1 if speaker impedance is 10 ohms at the test frequency and the amp's output impedance is .5 ohms.

Cheers,

Bruce

Al, kinda curious about the variable negative feedback switch you mentioned. Out of curiosity, if you were driving a speaker with a bumpy impedance curve, how much "smoothing" would be achieved if you used NF?? For example, if your speaker had a 20 ohm bump at 2K Hz, what would happen if you used the maximum amount of NF?

Another question out of curiosity. What is considered a large amount of NF? For example, my amp puts out a pretty typical 26db or so of gain, and uses about 12 db of NF. What does that mean in terms of magnitude, output smoothing of speakers with bumpy impedance loads and the introduction of TIM?

Thanks as usual.

Bruce

But Ralph, I think you may have mentioned yourself that if a "pure" Power Paradigm tube amp that doesn't use negative feedback and that has a somewhat high output impedance is matched up with a speaker having a "bumpy" impedance curve, there could be coloration of the acoustic presentation. Ergo, the need and importance of matching this type of amp with a speaker having a relatively benign impedance curve. The problem, I think, is that those types of speakers are in the minority of what's out there in the market. As I may have mentioned, some of the "big name" speakers like Magico and the Revel Ultima Salon 2 have impedance curves that would make a "pure" Power Paradigm amp dizzy. Or am I missing something . . . again as usual??

Thanks again as usual Ralph and Al. Just want to re-post 2 links that answer in part how NF may work with tube amps driving “mountainous” impedance curve speakers.

The links show bench test measurements of 2 different ARC tube amps: (a) the new REF 150 as tested by John Atkinson of Stereophile in 2012; and (b) the VS-115 as tested by Soundstage in 2008. You'll note that both techs drove the amps into simulated speaker loads. Links of the simulated loads are also copied. While the simulated loads are not identical, they are both pretty steep in the same FR areas.

What I thought interesting is that both amps had roughly similar output impedance and NF specs. Also the output FR of both amps driving the simulated loads followed a similar pattern of reducing the impedance “mountains” into “small hills.”

I realize the only way to really know what’s happening is to mic an actual speaker’s FR output. But having said that, I suspect that an otherwise flat FR speaker that was designed to be driven by a low impedance SS amp, if driven by one of the ARC amps would be in the same actual "FR" neighborhood.

Acoustic colorations? Yes, some. Wild fluctuations – doubtful, given the bench test results. Possibly +/- 1 db or less, especially if the 4 ohm taps are used.

I realize that the “cost” of using the NF is TIM and odd ordered harmonic distortion as Ralph, Al and others have explained, but isn’t that part of the amp designer’s decision trade-offs??

http://www.stereophile.com/reference/60/index.html

http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements

http://www.soundstagenetwork.com/measurements/amplifiers/arc_vs115/

http://www.soundstagemagazine.com/measurements/test_amplifiers.htm

Great chart Mapman. The chart also provides insight into why a speaker may subjectively sound bright or flat (i.e., dull). The chart shows that our ears are very sensitive to midrange frequencies. So, if our audio rig emphasizes frequencies in the 2K to 3K range, the speaker may sound "bright." And I assume the opposite is also true.

@ Swampwalker +1

Thanks Jake. I think the impedance/phased angle plots are demanding as reported in Stereophile mainly because of the 2 ohm dip and 4-5 ohm range. But other than that, the impedance is quite flat between 4 and 5 ohms across the freq. band. The low bass peaks are typical. Nevertheless, it does demonstrate the VS-115's impressive performance capability. How many amps out there can push power into such low impedance loads??