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Speaker choice: impedance, sensitivity, and tubes

Looking at most speakers' specifications I cannot help but notice that very few are rated 8 ohms, and most dip well below their nominal specifications at certain frequencies. This makes makes me wonder how audiophiles with tube amplifiers select their speakers. Most tube amps have 8 and 4 ohm taps only, and from what I understand tube amps don't take kindly to dips in impedance. Is there a rule to abide by when selecting a speaker to match well with a tube amp when it comes to impedance?

Same with sensitivity. Perhaps not as important as impedance, but a lot of popular brands out there have disappointingly low sensitivity (85-86 dB). Why is that? I never understood it since the higher sensitivity, the easier it is to drive a speaker without having to have a beast of an amp. Seems all manufacturers should be striving to design their speakers to have higher sensitivity. Is is more expensive to make speakers with higher sensitivity?

Case in point, I own two pairs of speakers, one rated 88 dB and the other 91 dB. The 91 dB pair has an impedance of 8 ohms flat (according to the manufacturer) while the 88 dB pair has a nominal impedance of 6 ohms (not sure about dips). The difference is quite dramatic in terms of volume on my 180 watt tube amps. I often have to crank the volume way up to get sufficient loudness level in my rather small listening room (11x12) with the less efficient pair. That to me is crazy. The speakers are my fall-back pair from my digital past, and knowing how they behave, I'd never purchase them for my all-analog system today.

So as I'm thinking of upgrading my speakers, I'm quite perplexed about finding a speaker that would match well with my tube monoblocks - provide an easy load and good loudness level without strain. I've been considering giving Harbeths a shot, but I'm really turned off by their low sensitivity of 85 dB (30.1) and impedance of 6 ohms. How big an amp would I need if speakers with a sensitivity of 88 dB barely generate sufficient volume with 180 watts per channel?!

Can anyone explain in technical terms how tube amps handle speaker impedance and, secondarily, sensitivity? And perhaps more important from the practical standpoint - how does one select a speaker to match a tube amp?
actusreus

Showing 6 responses by bifwynne

bifwynne
2,206 posts
 
Actusreus, I'm gonna jump in with my crude understanding of how the various electronic variables of tube vs SS amps interact with "conventional" dynamic cone speakers.

First, read Ralph Karsten's (Atmasphere's) white paper that discusses the so called Voltage vs Power Paradigms. In short, in the case of SS amps, voltage output remains somewhat constant, but current (amps) vary with the load. That's why SS amps can increase wattage when driving low impedance speakers, particulalrly in the low bass region. As impedance drops, power (watts) increase.

Not so for tube amps. In the case involving typical tube amps, current (amps) remains relatively constant regardless of impedance, but voltage varies. As a result, power (watts) doesn't change as much over the frequency range where impedance changes all over the place.

My ARC tube amp power rating is pretty constant regardless of impedance load, which is consistent with the Power Paradigm as described in Ralph's white paper.

Another difference is damping factor. SS amps generally have higher DFs than tube amps because SS amps generally have much lower output impedance factors than tube amps. I think DF is calculated as: speaker impedance/amp output impedance. Ralph and Al have written that much ado has been written about DF, and the bottom line is that it is a misunderstood and over-rated stat. The "lure of the law" so to speak is that the higher the DF, the better ontrol the amp will have over the woofers. The short answer is not necessarily.

My tube amp has a DF of 8. Yet it has great bass slam and extension. Why?? I have no idea.

I'm still waiting for a cogent response to a question that I posted in another thread which is how can one make an informed choice between mathcing a particular amp with a particular speaker. I still don't know. All I DO know is that I am satisfied with the sonic swill coming out of my speakers even even though I'm dumb as a stone.

Hope this helps.

Happy Holidays,

Bruce
bifwynne
2,206 posts
 
Xtil6 - Right and agreed. My Q is how many B&M stores really know how to mix and match, especially when there's an inherent bias (pun) that they will sell what they have.
bifwynne
2,206 posts
 
Ralph, Al and Actusreus -- if I reincarnate and if my I.Q. is high enough, I would love to be an EE designing speakers and amps. The problem is I have no idea where or what our hobby will be like in the future. In the meantime, I'll keep my day job and just try to pick up as much as I can on the web. Thanks Al for the hperlink -- I'll try to read and understand it.
bifwynne
2,206 posts
 
Ralph and Al, just reading over some of my older threads and re-read this one.

Just thinking out loud here. I wonder how many retail B&M sales people really grasp the electronic amp/speaker subject discussed in this and other similar OPs. I honestly am not making a statement one way or the other because I haven't stepped foot in a B&M store in decades to schmooze or listen to various combos of amps and speakers. I just raise the question.

Out of curiousity, I called Paradigm to ask whether the S8s (my speakers) were designed to work best with SS or tube amps. The tech service rep said the S8s soak up current, as much as the amp can push out. In fact, he said SS was the way to go.

Now, . . . my amp is tube. Would my rig sound even better with a high quality SS amp, I rhetorically ask?? Maybe, just maybe. In the meantime, the rig sounds pretty good as is. I have a great subwoofer, which back-stops the fronts, so if I have low quality bass output, it's pretty well masked.

I would be curious if any A'gon members who are or were on the retail B&M side of the hobby would chime in. I'd sure like to hear their views.
bifwynne
2,206 posts
 
Duke (Audiokinesis) - thanks for that cogent explanation. However, it riases a bunch of other questions for me.

First is, when a speaker manufacturer publishes frequency response specs, how can they possibly apply to BOTH tube and SS amps at the same time. Unless the amp in someway can sense the change in impedance and adjust its power output when encountering impedance curves, peaks and valleys, how is it possible for a speaker that is designed for a SS amp to have a flat FR if a tube amp is used, and vice versa of course?

Second, if the answer is "SS is SS and tube is tube, and never the twain shall meet (pun - LOL)," isn't it a bit of a misrepresenation on the speaker manufacturer's part not to mention that its published stats only apply in the case of [blank] type amps and that results can vary if [other blank] type amps are used?

I bet that most speakers on the market today have roller coaster impedance curves. I think I read somewher that Wilson speakers have pretty wild curves here and there too, but are also considered tube friendly. I understand that ARC uses Wilsons to check the sound of it amps.

So I guess the bottom line is what is the consumer supposed to do to make an informed decision when mixing and matching components??

Thanks for your cogent response. Hopefully we'll be hearing from some the EE/tech types too.
bifwynne
2,206 posts
 
Thanks Ngjockey -- that's exactly what I was looking for. In fact, I think it responds to the OP.
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