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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
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almarg
8,673 posts
 
Hi Marek,

I think that this recent thread answers a lot of your questions, and will be well worth reading.
Perhaps not as important as impedance, but a lot of popular brands out there have disappointingly low sensitivity (85-86 dB). Why is that?
One basic reason is that efficiency, deep bass extension, and physical size trade off with one another. If you want to increase efficiency you have to sacrifice deep bass extension and/or make the cabinet larger. Also, many speakers are designed with the expectation that they will be used with solid state amps, which can provide large amounts of power at much lower cost than a tube amp of comparable quality and power capability.

Regarding the lack of volume from your 88 db speakers, based on the observations you have described I suspect that the spec is inaccurate, and significantly overstates their sensitivity. 180 watts into 88 db speakers should easily produce higher than sensible volume levels in an 11 x 12 room.

Also, when you consider new speakers keep in mind that speaker sensitivity is often specified based on an input of 2.83 volts, rather than 1 watt. 2.83 volts corresponds to 1 watt into 8 ohms, so if the speaker's impedance is 8 ohms it makes no difference which way the number is defined. However, if the speaker's impedance is 4 ohms subtract 3 db to convert 2.83 volt sensitivity to 1 watt sensitivity (or more precisely, 1 watt efficiency). If the speaker's impedance is 6 ohms subtract 1.25 db to do that conversion.

Best regards,
-- Al
donjr
1,311 posts
 
I own the Rogue Cronus Magnum (90 tube watts) with Harbeth C7es3 and at half volume I can hear the music across the street and 3 doors down with all of my windows closed. Only tried it once because I don't enjoy that much volume. 180 tube watts is more than I've ever read anyone using with Harbeth.
actusreus OP
733 posts
 
Thanks Al. The linked thread was indeed very interesting and informational. I agree the specs on the speaker must be overstated.

Don,
Which tap do you use for your Harbeths, 4 or 8 ohm?
jperry
1,408 posts
 
Some additional information such as specific products might allow others to be more helpful.

I am using Quicksilver V-4 amps (120 wpc) to drive my Proac Response 2.5 (83 - 85 db efficient)in an 11x17 room and have no issue producing volume level louder than most people like.
bifwynne
2,069 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
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