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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
9,670 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
atmasphere's avatar
atmasphere
11,753 posts
 
Something that has not been pointed out is the performance of the output transformer and the load that is used.

For example the 88db speaker is also 6 ohms. If used on the 8 ohm tap the impedance is transformed by the transformer to a much higher impedance that the power tubes are actually driving. However it will be too low as to perform correctly the 8 ohm tap should be loaded at 8 ohms. The result is that the amp can't make as much power and will have higher distortion.

On top of that the bigger the transformer, the more its going to have troubles making full bandwidth. These problems are compounded on the 4 ohm tap and especially if the tap is incorrectly loaded. You might see as much as an octave of bass response lost on this account!

However we see that transistors are not immune to the issue of impedance. While it is true that many transistor amps can drive lower impedances with more power, it is also true that those same amps will have more distortion while doing so. The ear translates many forms of distortion into tonality; IOW just because a transistor amp can drive the lower impedance is not at all the same as saying it sounds its best.

So here is the bottom line, tube or transistor:

"If sound quality is your goal, it will be best served by a speaker that is 8 ohms or more, all other things being equal." If you want greater **sound pressure** there is a slight argument for 4 ohms if you have a transistor amp. IOW, there is no argument in support of four ohm speakers in high end audio.
actusreus OP
733 posts
 
Thank you all for your valuable input. Really good stuff.

Roscoe-
I use Rogue Audio tube preamp (with gain adjustment) and monoblocks. I don't think it's a gain issue. My phono preamp also has adjustable gain and I can easily switch it with very little additional noise, but the sound becomes rather unpleasant - I'd describe it as shrill, especially the highs. So it's not just about getting it loud; it's about getting it loud enough while maintaining the right tonal and frequency balance. And to clarify, the loudness level from the 88 dB speakers (Totem Hawk) is sufficient, (about 85 dB) but it seems it's at the top of their capabilities and requires a lot of power from 180 WPC monoblocks, which seems just crazy. (As a side note, this also demonstrates to me how important gain matching for a cartridge is. My Delos sings at 58 dB setting, but shrieks at 63 dB. The gain values are converted from mV so may not be accurate to a decibel, but still I wonder how some people use phono preamps with fixed gain and multiple cartridges.)

Bruce-
Sounds like we are looking for an answer to the same question :) Plus the issue of why so many high-end speakers have low sensitivity and low impedance. I always thought high sensitivity, high impedance speakers would be better than low sensitivity, low impedance speakers even though I didn't have sufficient technical knowledge to fully understand it. So it's very interesting to read Atmashpere's post supported by actual technical understanding of the issue.

I would like to hear a speaker designer's point of view regarding this common tendency in speaker design. If designers such as John DeVore and Alan Yun (Silverline Audio) understand this quite well, why don't others?
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.
unsound
7,327 posts
 
...And if the speakers are measured at 2.83 Volts, as the impedance numbers go up, the power (Watts) of a ss amp output is reduced, as the efficiency of the speakers are correspondingly increased. All of which should work towards constant linear speaker output. Also consider that it's easier to make speakers with a more constant lower numerical impedance, than it is to make speakers with a more constant higher numerical impedance. Furthermore, wave form fidelity seems to be an elusive quality for speakers with higher numerical impedance/efficiency specs to achieve.
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