2. speakers
  3. 1356668050

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
ngjockey
1,042 posts
 
Recently found this...

http://www.dhtrob.com/overige/tubefriendly_lsp_en.php
bifwynne
2,069 posts
 
Thanks Ngjockey -- that's exactly what I was looking for. In fact, I think it responds to the OP.
audiokinesis
1,872 posts
 
Bifwynne poses some excellent questions.

Bifwynne: When a speaker manufacturer publishes frequency response specs, how can they possibly apply to BOTH tube and SS amps at the same time?

Duke: If the impedance curve is fairly smooth, then there will be no significant change in frequency response when going from tubes to solid state. And the added power a tube amp delivers into the (virtually inevitable) bass impedance peaks usually only changes the shape of the curve in the low end ("fattening" it up a bit) without actually extending the -3 dB point significantly (UNLESS the port tuning is lowered). So while the frequency response curve is not going to be exactly the same, we can make an educated guess that we'll probably get a little bit more bass energy with a tube amp (and not to overly complicate things, but some tube amps moreso than others).

But if the impedance curve is a roller-coaster over most of the spectrum, then the speaker's tonal balance will change significantly with amplifier type, and the best bet is to follow the speaker manufacturer's recommendations on what sort of amps to use (if that information is hard to come by, you may be able to find out what amps he used at audio shows).

Bifwynne: 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?

Duke: Well maybe so, but there is enough... ah... "optimism"... already in the specs of many speaker manufacturers that making comparisions between speakers based on specs is somewhat "speculative" at best. Some manufacturers assume significant contribution from the room in calculating efficiency and/or bass extension, and some do not, and that will probably have a bigger effect on specs than whether a tube or solid state amp is used. But it's perfectly kosher to ask the manufacturer.

The one thing a manufacturer can't really "pad" with optimistic assumptions is the enclosure size. Assuming you're comparing the same type of enclosure (like vented boxes), if one box is significantly bigger than the other, that speaker will either go deeper or be more efficient (or maybe a little of both).

Bifwynne: 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.

Duke: If the Wilsons were "voiced" on "power paradigm" speakers (this paper should be required reading for anyone tube-curious and/or speaker geekish: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php), then they would work fine with ARC amps. Also if the speaker's impedance curve "zigs" where the frequency response curve "zags", a tube amp will tend to smooth out the frequency response.

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

Duke: The ideal would be to have the anechoic frequency response of the speaker (preferably out to at least 45 degrees off-axis) along with the impedance curve, but unless the speaker has been reviewed by Stereophile or SoundStage, that information is unlikely to be available.

If a tube amp is involved, try to find out what the output impedance (or damping factor) of the amp is, if possible. Then contact the speaker manufacturer, tell him the amp's output impedance, and ask him if that amp would be a good match with his speakers. Or tell the speaker manufacturer the brand and model of amp you're looking at, and he can probably probably still give you good guidance. Or tell the amp manufacturer what speakers you're considering, and get his input.

Don't be fooled into thinking "bigger is better" when it comes to damping factor - the woofer's cone is never unpowered as long as the amp is on, so it never has to rely on "damping factor" to stop its motion; the audible effect of "damping factor" is on the shape of the frequency response curve, so good speaker/amplifier/room synergy is the key. Those three form a system, in my opinion. Anyway damping factor is simply 8 ohms divided by the output impedance of the amp in ohms, and is not an indicator of quality in and of itself. The circuit techniques that result in a very high damping factor (very low output impedance) often tend to be detrimental to sound quality, but that's another topic, probably for another forum.
More to discover