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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
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
pdreher's avatar
pdreher
828 posts
 
I've driven the Harbeth M-40.1's with as little as 30 tube watts with reasonable success. IME, "watts" are over-rated.
elevick
2,439 posts
 
for 88db speakers at 8 ohms:
1 watt = 88db
2 watts = 91db
4 watts = 94db
8 watt = 97db and so on doubling your wattage to gain 3db.
For every 10db, you will hear a perceived doubling in wattage.
91db rated speakers should put out 3db more at the same volume.
88db speakers at 6ohms vs 91db speakers at 8ohms will be roughly 1/2 the volume (approximately).
roscoeiii's avatar
roscoeiii
1,155 posts
 
I am puzzled by the story of your 88dB speakers. What preamp are you using? This sounds like a gain problem, not a watts problem.

Taking gain into account is an important part of system building.
almarg's avatar
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almarg
8,673 posts
 
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 matching a particular amp with a particular speaker.
Bruce, as the other thread makes clear there are many variables and matters of degree that are involved. Therefore I don't think that a one-sentence or other kind of cogent guideline can be formulated, beyond Ralph's suggestion of trying to determine what the designer's intention was.

I think that your other comments above are well said, except that with respect to this statement ...
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.
... I would put it that in the case of a tube amp both current and voltage vary somewhat as a function of load impedance, with the net result being that power delivery varies significantly less than it would with a solid state amp that acts as a voltage source.

Also, the similarity of the MAXIMUM power ratings of a tube amplifier's 4 ohm and 8 ohm output taps involves different considerations, that are not directly related to the variation of power delivery as a function of load impedance WITHIN the amplifier's rated capability. The latter results from the interaction of amplifier output impedance and speaker impedance.

In considering all of this, it would probably be helpful to digest this Wikipedia writeup on the voltage divider effect. And to consider Z2 in Figure 1 as being the impedance of the speaker, which varies as a function of frequency, and Z1 as being the output impedance of the amplifier, which is essentially zero for a solid state amplifier, and typically one to several ohms for most (but not all) tube amplifiers. Then assume that in both cases Vin is being provided by an ideal voltage source (i.e., one having zero output impedance), and do a few calculations of power delivery for various combinations of Z1 and Z2. The power delivered to Z2 being equal to the square of the voltage across Z2, divided by Z2.

That is a bit oversimplified(!), because it neglects impedance phase angle, but a few such calculations should make clear what is happening with respect to the interaction between speaker impedance variation as a function of frequency and amplifier output impedance.

Best regards,
-- Al
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