high power tube amps vs ss

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??

05-21-13: Bifwynne
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?

Hi Bruce,

I don't know. It would depend on the particular amplifier, and what its output impedance is with and without that amount of feedback. Also, keep in mind that while what would be smoothed as feedback is increased is the variation of output voltage as a function of variations in speaker impedance over the frequency range, the variation of how much power is delivered to the speaker as a function of speaker impedance variation would probably be worsened. And the net sonic effects of all of that would be dependent on the design of the particular speaker.

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?

Ralph would most likely have a better quantitative feel for that than I do. But my general impression is that 12 db of feedback is probably a bit more than average for a high-end tube amp.

Re TIM, variables other than the amount of feedback, such as propagation delay through the particular amplifier (i.e., how much time is required for an input signal to be propagated to the output, which is a parameter that is rarely if ever specified) will have major effects on the answer.

Best,
-- Al

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??

The ear has a tipping point where it will favor colorations due to distortion **over** actual frequency response errors. On top of that you can't count on flat frequency response from any speaker. The Power Paradigm favors the idea that the equipment be designed to obey human hearing perceptual rules (as opposed to arbitrary specs on paper- the Voltage Paradigm); with this in mind, it can be seen that it might be more important to keep distortion down over trying for a perfectly flat frequency response.

Of course, if the frequency response is really messed up that won't do either, so you are correct that some care must be taken. In general though, if the speaker is a Power Paradigm device as is the amp, there are no worries. The problem comes in when you try to mix the two technologies- in that case you can always count on a tonal aberration.

Just to be clear- its not important that the speaker have a flat impedance curve- it just has to be designed to work with the power response of the amp. If I can quote Duke Lejeurne of Audiokinesis. Duke starts out his comments by quoting another poster on that thread:

Drew Eckert wrote:

"Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.

"For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.

"Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.

"With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way...."

In reply, I'd like to point out that, with the same 4 to 16 ohm impedance difference described here, the power that a transistor amp puts out varies by 6 dB, because it is putting out constant voltage rather than constant wattage. Why does the audio world accept this without a blink, and yet think there's a problem when a tube amp exhibits less variance in power output into the same load??

It is because the audio world is accustomed to the way transistor amps behave, and most speakers are designed to work well with transistor amps. The designer designs the speaker to sound right when driven by an amplifier that puts out 1 watt into the 8-ohm portion of its curve, 2 watts into the 4-ohm portion of its curve, and 1/2 watt into the 16-ohm portion of its curve, all at the same time (2.83 volts).

Now, what if the designer's goal was a speaker that works great with an amplifier that puts out approximately constant wattage, regardless of the impedance curve (within reason)? Well that can be done just as easily, but there are fewer amplifiers designed that way out there, so his potential market is smaller.

These two approaches to amplifier and speaker design have a name: Voltage paradigm, and power paradigm. You can read more about the subject here:

www.atma-sphere

At this point Duke put a link to the Power Paradigm article. Continuing:

Okay, what kind of amplifier sounds best? Boy that's a long debate for another day, but a lot of people familiar with many types of amps prefer the sound of a good OTL or SET amp, assuming a good speaker pairing, and that includes yours truly. Some speaker manufacturers give priority to building speakers that will work best with power paradigm amps because they believe that combination sounds best, and just accept that they are fishing in a smaller pond.

Is it possible to build a speaker that works well with both types of amps?

Yes, by keeping the impedance curve as smooth as possible, the speaker will work well with both types of amps. And with such a speaker, you can really make an apples-to-apples comparison of the different amplifier types, rather than actually evaluating whether a (typically roller-coaster impedance) speaker synergizes best with a voltage paradigm amp or a power paradigm amp.

Drew's numbers above illustrate an argument in favor of using a high impedance speaker with a low-output-impedance amp: The amp's output is approximately constant-power when the speaker's impedance is varying between 16 and 64 ohms, but the amp's power output changes significantly when the speaker's impedance varies between 4 and 16 ohms (same 4-to-1 variance in both cases). Also note, in both cases the amp's power output change vs speaker impedance is in the opposite direction of what happens with a voltage-paradigm (solid state) amp, and this is probably the main reason why simply dropping a specialty tube amp into your current system is a roll of the dice (and the odds are against you) unless you already know your current speaker is a good match.

Back to the original question, what's with 4 ohm speakers if they're harder to drive, well in general they can play louder with a solid state amp, and most people have solid state amps, so they get more sound per dollar with 4 ohm speakers (quantity outsells quality). Also, most woofers are 8 ohms, so if the designer wants to use two such woofers, he has to choose between series connection (16 ohms) and parallel connection (4 ohms). Most choose parallel connection because 4 ohm speakers outsell 16 ohm speakers. I believe that most amps - tube or solid state - sound better into a 16 ohm load, so my home audio two-woofer designs are 16 ohm loads (whereas my prosound two-woofer designs are 4 ohm loads, because there we're trying to maximize available SPL with solid state amps). What do I do about the typical halving of maxium power from solid state amps when driving a 16 ohm load? I start out with speakers that are about 3 dB more efficient, and of course pay a corresponding price in box size vs bass extension. However there is one "free lunch" to using power paradign amps: They generate equal or even increased, rather than reduced, power into the speaker's virtually inevitable bass impedance peaks, and if we keep this in mind and design our box accordingly, we get back most of the bass extension we otherwise would have lost when we traded off in the direction of higher efficiency.

So I take the position that, all else being equal, 16 ohm speakers sound better than 4 or 8 ohm speakers, and that power-paradigm amps are well worth seeking out matching speakers for.

Duke
dealer/manufacturer/power paradigm groupie

this is taken from the speakers forum: http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1368259003&openflup&13&4#13

Harking back to Mapman quoting Einstein... the only way to really get a handle on this is to get yourself to a show or an audio club and listen. See what floats your boat. Right now I am listening to an 18 wpc SET amp driving 97 dB high impedance speakers in a 14 x 23 x 8' room. I have to use 10 db attenuators to get the volume control up to 10:30 and it's about as loud as I ever want it. Plenty of bass. Vocals/midrange sound RIGHT. Source is 16/44.1 Pandora, so it's not the ultimate in detail, but it sounds pretty darn good to me. Go to RMAF and hear Ralph's 60 watt OTLs, driving the Audio Classic horns to near rock concert levels in one of the very big exhibition (not sleeping) rooms will show you that you do not need a mammoth amp (ss or tube)to provide mammoth sound w minimal distortion. OK, the cost is a little heat. Maybe you live in Phoenix and that is a game changer for you. That's cool (pun intended). Give a Berning ZOTL amp a try. Long tube life, auto-bias, compact, pretty cool running- certainly cooler than many ss amps biased into class A.

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