That "tube sound" and power ratings


This might be a newbie question since I've only begun researching tube technology. I understand to some degree the theory that tube sound is partly related to second harmonic distortion vs. the more prevalent odd order harmonic characteristics of SS. If "tubies" prefer that sound (I might be one of them), does it make sense to carefully match an amplifier's power rating such that it is NOT TOO HIGH for the speakers it's driving? If the rating is too high won't that mean lower distortion and hence less tube sound for a given volume for those speakers than a lower power tube amp (in general that is - I realize not all Watts are the same). So won't a high wattage tube amp have less of the special tube sound "tubies" like at their preferred listening volume?

I realize I'm likely missing something here. Set me straight!
hazyj
Power can be Electrical [Watts] or Mechanical [Joule per second]
In Both cases it's an amount of work delivered per given time or an amount of charge delivered to the load per given time.
The meaning of electrical power delivered to the load is absolutely the same no matter what source of power exists.
How things distort is one aspect that varies with different amplifiers but is only one aspect of performance of sound.

How much power is needed to drive any particular pair of speakers is mostly a function of recording quality and SPLs (how loud you listen).

For optimal performance, it is almost always a good idea to have more power than needed rather than less.

Clipping is always public enemy #1 when it comes to amps and speakers. you want to avoid clipping at all costs in that it will only compromise the sound quality to various degrees.

The only way to avoid clipping is to have more than enough power than will ever be needed to drive the speakers OPTIMALLY.

Having said that, optimal performance is a good first goal but alone does not assure "sound quality". Personal preferences come into play when talking about what sounds "good" . Optimal performance is always a good thing but alone does not assure you will like what you hear, though the chances are best that you will for sure.

So always strive toavoid clipping no matter what first. Then decide on your favorite flavor of sound from there.
I just went through this exact same question myself when trying to decide on amplification for a new system. In my case the speakers are rated for 100 watts. The amp I picked is tube and rated at 140 wpc. It sounds fine. Almost effortless. Would it sound even better with double the power? That would give me 3 db of additional headroom, a barely perceptible difference. As already explained clipping is bad, in addition to adding significant amounts of IM distortion, clipping can damage your speakers. Here is what I would do. Determine the sensitivity of your speakers. This is stated as so many db at 1 watt input, measured at 1 meter away. Then calculate how much power you need for the sound pressure level that you want to achieve. Every doubling of power adds 3 db. So for example: Assume the speaker sensitivity is 90 db at 1 watt. 2 watts = 93 db, 4 watts = 96 db, 8 watts = 99 db etc. Then add 3 db to your assumed max listening level for safety. Make sure your speakers can handle the power you calculate.
Further, the topology of the circuit says a lot about what sort of distortion it will make. For example, a push-pull amplifier with a single-ended input will make primarily 2nd ordered harmonic distortion, but will also have some of the 5th. If the amplifier is fully differential and balanced, the primary distortion product will be the 3rd harmonic.

If negative feedback is applied whether tube or solid state trace amounts of the 5th, 7th, 9th and beyond will be added.

At any rate loudspeaker efficiency is important when dealing with tubes, as tube power is more expensive than transistor power! Its important with any system to avoid distortion but in fact distortion is the name of the game as the human ear translates distortion into tonality- this is why transistor amps tend to sound bright, even though the distortion is very low, what distortion is present tends to be the odd orders, to which the human ear is particularly sensitive.

A lot of push-pull amplifiers will actually make more distortion below a certain power level, so for those amplifiers its helpful to have a speaker that is a little less efficient, so the distortion is happening at a lower level and is hopefully less noticeable. But this is why some amps seem to lack detail at lower power levels.

There is definitely more to this than meets the eye!
Clipping is the most common and pervasive form of distortion in most cases.

Unless you only listen at very low volumes, there is a very good chance that clipping and related forms of distortion will come into play and degrade sound quality in ways that might not be very apparent.

Its not that hard to avoid clipping. Just do not use an amp that is underpowered to drive the speakers. Use more power not less as an insurance policy. Most speaker manufacturers understate the power needed to drive their speakers optimally in order to keep markets open.

If your speakers are even moderately efficient, that helps a lot in terms of choosing an amp that is up to the task.

If you will only listening at low volumes, then none of this matters all that much. The world is your oyster and almost any amp/speaker combo might do.

Once teh amp is up to the task of driving the speakers OPTIMALLY, then you are in a position to determine what other remaining forms of distortion to choose among based on what you hear.
Public Enemys (of good sound):

1) room acoustics
2) amp clipping
3) choose your poison
Note that most tube amps (and some SS amps) soft clip which is why fewer watts seem to go further. But the clipping still occurs, though it will take more volume before the results become clearly unpleasant. But any clipping is distortion. You loose dynamics whic is what adds the excitement to a lot of music. If you don't care about that and only listen to small ensemble chamber music, you might be fine, but eventually you may realize what you are missing and look elsewhere.
Look, solid state uses electron junctions that are squished inside doped silicon. In tubes, electrons are free and happy. All Watts are the same ;-)
Wow. Quick first responses! But before this gets too far in, I better qualify that I didn't mean that I'm new to EE or Physics or high-end audio listening. However, I am new to tubes as they're used in audio equipment. So my question is only partially about the physics/engineering behind tube implementation in audio. I realize that theory and circuitry may very well be part of any good explanation, so I don't want to discourage it. But the main question is about what tube enthusiasts like to hear and how they achieve that sound at their preferred listening levels.

I think Atmasphere understands where I'm coming from. In fact he/she may have answered part of my question, but I'd like to hear other viewpoints.

Mapman, I'm sure you have some good insight now that (hopefully) I've explained a bit better. I'm sure you know more about the subject than me and I look forward to more input from you. However my question is more subjective than came across. How does a tube enthusiast choose the right power for their system given that they actually may prefer more distortion of the right harmonics? If they choose a higher power amp I'm assuming (and yes I might be wrong!) that there will be less of the DESIRED distortion that adds to the fun of tubes.

Czarivey, I appreciate the input but you misunderstood the point that not all watts are created equal. They most certainly are not, or we'd all have $200 Bose equipment and stop there.
OK. Let's please get off my "not all Watts are the same" comment. I didn't realize it would be so badly misunderstood - my mistake. Just so it's understood - I'm perfectly aware what a watt is and is not, so let's move on ...
The tube amps that have the most unique sound compared to the norm are probably those made well and with little or no Negative feedback applied, like Atmaspheres.

Hi-fi amplification is not the same as with guitars. You want want your amps to be running in their lowest-distortion range, every time (or as close to that as possible given your SPL needs) -- whether tubes or SS. And you can still clearly hear the effects of tube rolling even when amps are perfectly well within their "clean" range.

Too much power doesn't cause problems unless you're careless and let a very high-power signal through to your speakers -- besides that scenario, it's generally safer to have more power, to avoid clipping (which can be problematic for tweeters). When it comes to protecting your gear, there's no silver bullet -- some care and education will be required by yourself, whether you choose tubes or SS, high power or low power.

I run 250 Watt tube amps on 96db/Watt. So I'm definitely on the "too much power" side, and it works extremely well; I love it. The system gain structure is a little suboptimal, but that's a (kinda) different issue.
Thanks Bill, but your response answers the opposite of what I'm asking. I'm not trying to find out if tube enthusiasts like the sound of less clipping from more powerful amps. Rather, I'm asking if they prefer the sound from less powerful amps, and if so how to decide which power rating provides the best tube sound given an amp's characteristics together with the speaker sensitivity and the preferred listening level.

Maybe I need to explain how "tube sound" is partially related to distortion? I didn't want to do that here, and I'm not really the person to do it. An earlier post from Atmasphere explains the sound charateristics of push-pull circuitry. While that may be the most important influence on "tubiness", it's only part of the story. One also needs to consider harmonics that occur when an amp is almost or even softly clipping. These harmonics can be pleasing to the ear at certain levels.

Tube people, please feel free to add or subtract to my knowledge here. As stated I'm a tube audio newbie hence this post.

Anyone else want to take a stab at asking my question in an easier-to-understand way? Atmasphere?
Mulveling - thank you!

"You want your amps to be running in their lowest-distortion range, every time (or as close to that as possible given your SPL needs) -- whether tubes or SS. And you can still clearly hear the effects of tube rolling even when amps are perfectly well within their "clean" range."

Great answer. Does every tube enthusiast agree with this? I know I shouldn't believe everything I read, but it sure seems there are a lot of "audiophiles" who believe a great amp is a great amp tube or SS, and unless you're nearing or at clipping you probably won't hear the tube characteristics that drove your tube enthusiasm to begin with.

So ... if you never operate in this range are you happy with your tubiness?
Hazy,

I don't understand why you are tying this question to optimal power levels. I don't think it has anything to do with that.

If it is the purest tube sound that you seek, that's probably the best tube amp design with the fewest tubes, like some very expensive sets. The best tube amp design is the one with the most linear gain, lowest associated distortion levels, and lowest noise floor. The more tubes involved, most likely the harder it will be to achieve the absolute best performance in this regard. Decide how you will determine those things, then after that, factor in how it will perform with given pair of speakers, which does have everything to do with power levels, and I think that's about it.
Thanks Mapman. Great response, but is it really true that "that's about it"? Maybe it is. I'm not so certain which is why I posted the question, and I think the only way to know is to hear your same opinion echoed by enough other tube enthusiasts to make it convincing.

But I wouldn't say it's about "the purest tube sound" that I or anyone else seeks. It's about what makes tube listeners happiest. I'm not sure the purest or cleanest sound does it for everyone, but am ready to be proven wrong.
" I'm not sure the purest or cleanest sound does it for everyone"

Nop doubt about that. Top notch technical performance and what people like best are not always the same thing.

The difference is the first is objective and performance claims can be substantiated by measurements.

The second is completely subjective.

The domain of tube amps and SS amps are different, with some overlap but significantly different paradigms at work. But performance is performance.

For me, I have found the best sounding tube amps I have heard sound mostly like the best performing SS ones. What they both share is very high performance well matched into the system as a whole yielding similarly good results, though each different in design operating within distinct paradigms.
09-23-14: Hazyj
Mulveling - thank you!
Great answer. Does every tube enthusiast agree with this?

LOL! Of course not! You are new to this. You will never find every enthusiast to agree on anything. Tubes, SS, digital, analog, cables, etc.

Mulveling loves high power tube amps, nothing wrong with that. My tube amp is 35 wpc and my speakers are 91 dB, no audible distortion. While I agree that no one wants to push their amp to hard, as distortion will rise quickly as you reach the clipping point. Most amps operate in a range where distortion is not a factor. I know of folks driving less efficient speakers than I have with even less power and loving it.

As Nelson Pass once said, if the first watt doesn't sound great, who wants 200 more watts?
Listen to the amp/speaker combo, and don't worry so much about numbers. I've heard amps with high power and low distortion specs sound horrible. Listen to the music and let your ears decide.
Why don't you just listen to some tube gear?
What jmcgrogan2 said. As long as the pairing of amp and speakers is appropriate the power rating is irrelevant.
"...So won't a high wattage tube amp have less of the special tube sound "tubies" like at their preferred listening volume?"

Probably... A lot of what people like is the compression and fatness that occurs when an underpowered tube amp is over driven. You'll have to try some combinations out, with your speakers, music and listening levels... The amount or lack of distortion that's pleasing to your ears, is up to you...
Jmcgrogan2 - you missed the point of the rhetorical question. were you really laughing out loud?

Swampwalker - define "appropriate"

Zkzpb8 - you got the point of the non-rhetorical question. thank you and nice response!

everyone else - i am listening to whatever I can as much as I can every other day or so. there's only so much time though, so your answers might sway me in the direction of more lower powered tube amp listening.
If you have relatively efficient speakers (89 to 90db or around there) and a relatively powerful amp that's not one of those tiny wattage esoteric gizmos, you can stop worrying and simply enjoy the damn thing. If the amp doesn't hum or explode and is made by a company that isn't insane, and it sounds good, you're only going to be using about 5 watts most of the time. Seriously...I use a classic old school (but new design with better than old amp innards) push pull amp with something like 65 watts per side and, fortunately, all of the watts sound great...who knew? One thing experienced guitar players have learned from tube amps is they all sound somewhat different from each other, and often WAY different based on output tubes, input tubes, yo mama...etc...but reasonably well made hifi tube stuff often defies the techno bullshit schools of design trendiness and simply works. Try one.
Nice rant!
Cool to hear that you're checking out different gear - have fun listening. Some lower powered amps can bring some magic- but one limiting factor could be with music that is more dynamic. Sometimes too much compression can set in and instead of enhancing the sound, the amp will act more as a limiter...
"Probably... A lot of what people like is the compression and fatness that occurs when an underpowered tube amp is over driven"

That's a very good point and a great example of how the best technical performance in terms of accuracy does not always win.
As I understand the question, you are seeking to determine if in general it makes sense to prefer a lower powered tube amp to a higher powered tube amp (assuming that both amps can provide sufficient watts for the application), due to an expectation that the lower powered amp may, under typical conditions, tend to distort more than the higher powered amp. And thereby provide sonic characteristics that are more "tubey" and less solid state-like than would otherwise be the case.

That is a logical question, but I think it focuses MUCH too narrowly on one single factor in tube amp sonics for any answers to be useful. There are simply way too many variables involved in the design and the performance characteristics of an amplifier, and in its interaction with the technical characteristics of whatever speaker is being used, for a useful answer to be defined. Certainly among different makes of amplifiers. And even when considering amps from a given manufacturer having different power ratings, that are otherwise generally similar in design, I see no reason to expect much consistency among the product lines of different manufacturers as to whether the higher powered or lower powered amp would be preferable. Or even much consistency across a variety of speakers when a comparison is performed between a given higher powered and lower powered amp, as a given tube amp can sound quite different depending on the magnitudes and phase angles of the impedances of the speakers it may be used with, and especially on how those parameters vary as a function of frequency.

Also, the question focuses on just one kind of distortion, harmonic distortion. There are other kinds, e.g., intermodulation distortion, transient intermodulation distortion, crossover distortion, etc. Some of these are neither generally specified nor even measurable in a standardized manner. And all involve a complex signal-dependent mix of different effects and different degrees of effects at different frequencies.

What I would suggest as the most useful line of inquiry, when you reach that point, is that you start a thread indicating the particular speakers that would be used, your budget range for amplification, the room size and listening distance, some idea of the volume levels you prefer, and (especially) whether or not your listening includes recordings having particularly wide dynamic range (i.e., particularly large DIFFERENCES between the volumes of the loudest and the softest notes, such as is often the case with well engineered, minimally compressed, classical symphonic music). The latter factor will dramatically affect how much power you need. Providing that information will get you many knowledgeable and useful suggestions from the members here, from which you can develop a “short list” of candidates.

Good luck. Regards,
-- Al
Zkzpb8 & Mapman - thanks!

Almarg - very helpful, thanks! Yes, my post is focused narrowly on this one question, but I'm not. When I listen I listen for what pleases my ear. If this is one of the things that does or does not then I'll note that fact and keep listening. I'm not really looking for specific amplifier suggestions, at least it didn't occur to me to do that. If I do it will be in another post, but I doubt that'll be necessary.
Hazyj, There is something to the idea that smaller amps sound better. In the case of push-pull tube amps, this has almost entirely to do with the output transformer itself. The reason for this is that the more power the output transformer can handle, the less bandwidth it has. So in the larger designs, usually the designer has made a tradeoff based entirely on what he thinks is important.

If the amp employs negative feedback, usually the high frequency bandwidth is the area that is sacrificed. This is because there is not a lot of energy at high frequencies so its possible to get away with this if you also run negative feedback to help output a little with the voltage response of the amp which will be degraded (another way of looking at this is the output impedance will be higher at the area of the rolloff and beyond).

In the old days, 60 watts was about the limit where the output transformer could make the bandwidth required to really be hifi. That bandwidth BTW is well past 20Hz-20KHz; its nice to get response up to 100KHz if you can so phase shift at 10KHz is not adversely affected. This really helps with the presentation of the soundstage, depth in particular.

These days designers seem to have pushed that power level up to about 100 watts before bandwidth really starts to suffer.

Now the problem with push-pull is often that there is a phase-splitter circuit that introduces some distortion. This is not true of all P-P amps, but it is true of most of them. This generally is not an issue until you get into the lower power regions of the amplifier, at which point the distortion of the phase splitter comes into play. For this reason a highly efficient speaker (+98 db or so) may not be the best choice with a plus-100 watt amplifier as you may never get the amp out of that region of higher distortion at lower power levels.

SET amplifiers get around this problem by not having a phase splitter (that's how we do it too although our amps are push-pull). This allows them to have a distortion characteristic that becomes unmeasurable as the power decreases to zero. To really take advantage of this, you really do need a high efficiency loudspeaker! The reason is two-fold with SETs- first, that bandwidth limit discussed with the OPT of P-P amps is more profound with SETs, the practical upper limit being only about 7 watts before bandwidth is degraded (for this reason many SET designers don't bother trying to get the bass right in the OPT as the speakers that have that kind of efficiency don't play the bottom octave either). The second reason is that you never want to push an SET past about 20% of full power or else the higher ordered distortion products come into play (when this happens, its usually on transients, and because the human ear uses the higher orders as loudness cues, the result is that the SET sounds a lot more dynamic than it has any right to because the loudness cues are occurring on the transients. You read about this 'dynamic' character with SETs all the time, but its an indication that the speaker used is not efficient enough).

Where is is all going is that if you want to really get the most out of your tube amplifier investment dollar, the selection of the loudspeaker is really important!

Something else to be aware of: Tube amps make more power and run more efficiently into higher impedances. You really don't want to make **any** amplifier (tube or transistor) work all that hard- the harder you make it work, the more distortion it will have. So you want to avoid 4 ohm speakers generally speaking- the amp will make less distortion into 8 or 16 ohms. This has nothing to do with whether the amp employs negative feedback or not- and it has everything to do with the fact that OPTs operate more effectively (wider bandwidth, lower distortion, less amplifier power converted to heat) into higher impedances. This is why you are seeing more 16 ohm speakers on the market these days- designers are slowly figuring this out.
Atmasphere, who out there today is making good quality 16 ohm speakers? With no impedance dips much below that I mean.

A google search on 16 ohm speakers turns up little at least on teh fist page.

WHat are some good ones at various price points? Which ones are full range?
As usual, Al's explanations are exceptional and I agree with everything he says in his post. I'm not sure if I'm getting this line in the OP's first post.

"If the rating is too high won't that mean lower distortion and hence less tube sound for a given volume for those speakers than a lower power tube amp (in general that is - I realize not all Watts are the same). So won't a high wattage tube amp have less of the special tube sound "tubies" like at their preferred listening volume?"

When I read that, it sounds to me like people with tube gear are choosing lower power amps so that they will distort. And the distortion is pleasing to the ear. That's not the case. People with tubes, just like people with SS, don't want their amps to distort. But if, and when, distortion does occur, there are those who feel that tubes sound "less bad" when they distort. I think that might be the missing piece you were looking for. But I think it would be a mistake to say people buy tube amps to make them distort. (Unless they play guitar. lol) Also, something else to consider is that tube amps and SS amps sound different from each other when they are not distorting. And both designs are liked and disliked, for the way they sound when operating under normal circumstances. That's why in my first post, I said why don't you listen to some tube gear. Its something you really need to hear for yourself to form an opinion. Its not easy to put this stuff into words.
Zd - thanks for this most helpful reply. I do understand the idea of "less bad" distortion at at or near clipping (I really do!), so it's not a missing piece if there was one. Rather you hit the nail on the head when you wrote "But I think it would be a mistake to say people buy tube amps to make them distort." I would have believed that a month ago before I started reading ... whatever I've been reading (I know - maybe I should stop reading, but then again what are the rest of you doing here? You should be listening to your systems! Sadly mine is missing an amp). And now there's a couple people commenting here that seem to be contradicting your statement. So, are there any closeted tubies here willing to come forward and confess to their tube clipping fetishes?
By appropriate, I mean a speaker that is efficient enough to provide the volume you need in the room you are using, that has the high-ish, relatively flat impedance curve that tube amps work best with. For the reasons that Almarg and Atmasphere elucidated, that efficiency had better be pretty high (>95 DB or more) if you want to make reasonably high volumes in a moderate to large rooms using an SET amp, but there are a good number of push pulls tube amps, w or wo OPTs that can drive speakers in the 85+dB range if the impedance is flat and >= 6 ohms.
Hazyi,
I can't speak for all owners of lower power tube amplifiers other than to say
I doubt they're intentionally seeking a "distorted" sound
experience. Al and Atmasphere's post are worth re reading. My speakers
are 94db at 14 ohms (minium 10 ohm load). My amplifier is an 8 watt SET
my average listening level is roughly 75-80db C weighted I sit 10 feet from
the speakers. Under these conditions I'm using only "fractions"
of 1 watt to achieve this very adequate listening volume. Most of my jazz
CDs have a dynamic range of 25-30db (some have an even wider range).
So there's ample head room in reserve when necessary to account for
musical peaks. The distortion levels are miniscule as I'm using mere
fractions of a single watt. The sound is pure and beautiful and there's no
need for "induced" distortion, In reality, quite the contrary. I
agree with Atmasphere's point regarding the advantages of high speaker
ohm impedance based on my daily listening experiences. Hazyi, I hope this
perspective helps make the point that gratuitous distortion isn't the appeal
for tube amplifiers.
Charles
From my experience I've seen a lot of tube amp owners who like distortion. Not SET guys with highly sensitive speakers, but the 10-20 Watt tube amp owners who mate their amp with a speaker rated much below 90dB.

A good friend of mine is a recording engineer and he always says "people love distortion"... In the studio throw a tube compressor on - and ''fatten" up the sound - warm things up. It's the same effect when someone uses a 10 Watt tube amp on a pair of speakers that are rated 25 Watts minimum. I see guys do it all the time - they're fully aware of the speaker's specs, and despite the manufacturer's advice to use more power they do it anyway. It's a seductive sound - but the product of distortion.
Charles - thanks for another very helpful post! I assume you mean wrt average power output, at your listening level, you're only using fractions of a watt. I suppose it doesn't matter since even if it is much greater at peak maybe we're talking only 2-8W which you can handle within optimal range. I do realize you have a very efficient setup so my math may be off, but the "fractions of a watt" does sound a bit surprising and if it's true then I'm impressed - jealous even. These are the Coincidents that you refer to?

Yes I understand how power relates to impedance. No need to go there.

Do realize that yours is another data point and opinion, albeit a very popular one. Others commenting here have stated that over-driven tube amps might be appealing to some listeners. Actually they said it more strongly than that, but I'm trying to keep this civil.
We all know there are plenty of 8 ohm speakers. 16 ohms is a little harder to find. Some examples:

Classic Audio Loudspeakers
Audiokinesis (several models)
High Emotion Audio (Bella Twin- one of the best monitors I've heard)
Quad ESL57 and ESL 63
Lowther, PHY and other full-range drivers
Coincident Technology (not always 16 ohms, but definitely higher impedance)
Rogers LS35A
many vintage horns- Altec, JBL, EV, etc.
ZU Audio has made 32 ohm versions of some of their speakers
Some Pipedreams are set up as higher impedance

Its not important in a 16 ohm load that the impedance curve be ruler flat. In fact that is true of 8-ohm speakers as well. What does seem to be important is that the impedance variation does not fall below about 1/2 the nominal impedance, and when so doing is at a crossover point. ESLs seem to be an exception.

An example of a speaker that is difficult for a tube amplifier is the B&W 802D. This speaker is rated nominally 8 ohms but employs dual woofers, which are both 8 ohms and in parallel, so really this speaker should be rated at 4 ohms since the most power made by the amp will be seeing this load. If you see dual woofers in a loudspeaker, it is really good idea to inquire with the manufacturer about the impedance at the woofer frequency! If it is 4 ohms I would ask if its possible to make a version where the woofers are in series rather than in parallel. If you get a lot of pushback on this topic, move on!

Something that is really important when matching a tube amplifier to a loudspeaker is to understand that it can't double power as impedance is cut in half. Where this gets you in trouble with speaker designers is that many of them use the sensitivity specification when rating their speakers. Sensitivity is 2.83 volts into the speaker and measured at 1 meter. If the speaker is 8 ohms, this is one watt. But if the speaker is 4 ohms, this is 2 watts- IOW the speaker is actually 3 db less **efficient** than it would seem to appear.

Where the trouble spot is: many designers don't really see a difference between sensitivity and efficiency- so they think that by paralleling a driver, the efficiency of the speaker is increased by 3 db! IME when you try to correct them on this issue, often you get an angry response.

Now it happens that if you parallel a pair of woofers or put them in series, they will act exactly the same in the cabinet designed for them, the only difference is the values used in the crossover. I am going to use Kirchoff's Law here to make the point- if you have a single woofer and put one watt into it, it might make 92 db at one meter. If you have two of the same woofers in either parallel or in series and put 1 watt into the array, the result will be the each driver will have 1/2 watt on them- and the output will be exactly the same 92 db. If the efficiency were to actually go up, it would violate Kirchoff's Law, and because that is impossible (just like its impossible to violate Ohm's Law) it doesn't happen.

I can't tell you how hard it can be to get this fact across to speaker designers- its enough of a problem that I consider it an embarrassment (due to lack of engineering knowledge) when I encounter it. When you run into this, its guaranteed that the designer is using a transistor amplifier for design and testing and really does not understand the relationship between the amp and the speaker.

Now if you know all this above, and you know that higher impedances are good for tube amps as they will make less distortion and more power, then you are well ahead of many speaker designers and you now have a better chance of extracting all the performance out of the tube amp that otherwise is left on the table. Its your dollars in that amp- why not use it to best effect??
Atmasphere - you have been most helpful. I won't sum up the list of your contributions quite yet though. Am hoping you're not done here ...
Atmosphere,
A few other "easy load" speaker brands to add to your list.
Rethm, Vaughn Audio, Tonian Labs, Ocellia, Trenner Friedl, Audio Note.
Ralph, there's something that doesn't make sense concerning your comments about speaker designers. These designers have to have at the very least a rudimentary understanding of science and engineering. Something as basic as Kirchoff's law has to be easily comprehended and applied. They have the equipment available to confirm that concept and its effect. I just don't understand why you would be met with such resistance to something so plain and fundamental. How do these designers counter your point/position?
Charles,
Great points by Ralph, as usual, with which I agree.
09-25-14: Charles1dad
Ralph, there's something that doesn't make sense concerning your comments about speaker designers. These designers have to have at the very least a rudimentary understanding of science and engineering. Something as basic as Kirchoff's law has to be easily comprehended and applied. They have the equipment available to confirm that concept and its effect. I just don't understand why you would be met with such resistance to something so plain and fundamental. How do these designers counter your point/position?
Charles, pending Ralph's answer, as I see it the key to reconciling that is the fact that the majority of speakers are designed with solid state amplification in mind. And with a few unusual exceptions, nearly all solid state amps (in contrast to tube amps) are designed to provide an output voltage which does not vary significantly as a function of load impedance, as long as the amp is operated within the limits of its voltage, current, power, and thermal capabilities.

Therefore, as a consequence of Ohm's Law and the definition of power, while of course not all solid state amps will double their rated MAXIMUM power capabilities into halved load impedances, nearly all of them WILL double the amount of power that is delivered into a halved load impedance (albeit perhaps with some increase in distortion), as long as they are operated within those maximum limits.

So when Ralph says:
If you have a single woofer and put one watt into it, it might make 92 db at one meter. If you have two of the same woofers in either parallel or in series and put 1 watt into the array, the result will be the each driver will have 1/2 watt on them- and the output will be exactly the same 92 db.
... Note that he refers to an input of 1 watt.

If he had referred to an input of a given number of volts, say 2.83 volts (which corresponds to 1 watt into 8 ohms, as he indicated), applying that voltage to two identical woofers in parallel (which together have half the impedance of a single woofer) would cause twice as much power to be delivered as if the same voltage were applied to only one woofer. Resulting (everything else being equal) in a 3 db increase in SPL. And conversely, applying that voltage across a combination of two identical woofers in series (which together have twice the impedance of a single woofer), would result in half as much power being delivered as if the same voltage were applied to only one woofer, resulting in a 3 db reduction in SPL.

Basically, the speaker designers Ralph referred to are thinking in voltage terms, rather than power terms. Which is arguably reasonable in the context of most solid state amplifiers, but can be very misleading when it comes to tube amps.

ZD, thanks for the nice words.

Best regards,
-- Al
Hello Al,
Yes I understand the premise that most speakers are designed with the intention of using transistor amplifiers. It seems what Ralph was explaining is their(designers) reluctance (or even rejection) to "recognized" his point as it pertains to tube amplifier -speaker interactions.
Charles,
^^ Charles, I think human nature is the culprit: the most powerful human motivator being to look good, the second most powerful not to look bad. You fail both of those if you don't understand the difference between power and voltage if you design speakers.

BTW, the Tonian and Ocellia have both used the PHY drivers that I mentioned. I think Tonian has moved away from the PHYs but still makes very easy to drive loudspeakers.
I get the impression that 4 ohm impedance speakers became the default
load once transistor amplifiers became plentiful and affordable. It could
also be that they are possibly easier to design and build. These designers
realized that if there's a difficult load to drive, just purchase a higher power/
current SS amplifier and the problem is solved. Perhaps higher impedance
speakers are more challenging to implement and designers simply followed
the path of least resistance. Sound quality may not have been the only
consideration for this direction.
Charles,
Anyone other than Mapman want to respond to Zkzpb8 posts?
"09-25-14: Zkzpb8
From my experience I've seen a lot of tube amp owners who like distortion. Not SET guys with highly sensitive speakers, but the 10-20 Watt tube amp owners who mate their amp with a speaker rated much below 90dB."

I've never heard of anyone doing that. Are you sure they actually wanted the amp to distort all the time and not just prefer tube distortion if the amp distorts?

"A good friend of mine is a recording engineer and he always says "people love distortion"... In the studio throw a tube compressor on - and ''fatten" up the sound - warm things up. It's the same effect when someone uses a 10 Watt tube amp on a pair of speakers that are rated 25 Watts minimum. I see guys do it all the time - they're fully aware of the speaker's specs, and despite the manufacturer's advice to use more power they do it anyway. It's a seductive sound - but the product of distortion."

That's not the same thing. In the studio, what you're talking about above is signal processing. Its part of the creative process. Playback gear's job is completely different. The idea behind that is to reproduce the original event as accurately as possible. Making your amp distort on playback goes in the opposite direction of that goal. You don't want you home audio system to distort because you want to reproduce whatever is on the recording as best you can. Even if its distortion.
If anything, the move to tube amps in the last 30-40 years has been for the things that tubes do that are difficult or impossible for transistor amps. I don't see audiophiles that are into tubes doing it for any other reason then they want to get as close to the musical experience as possible.

I really don't think you have to know anything about tubes, how tube amps work or why people use them. All you really have to know to understand that they work really well for music, is that over half a century after they were declared obsolete, they are still around- and we still play them and even the tubes/transistor debate is older than many audiophiles! IOW the economics tells the story!
Ralph,
That sums up my preference for tubes. The only criteria that matters, the more natural sound quality.
Want to thank the OP and contributors, really enjoyed this thread.
"That's not the same thing. In the studio, what you're talking about above is signal processing. Its part of the creative process. Playback gear's job is completely different. The idea behind that is to reproduce the original event as accurately as possible. Making your amp distort on playback goes in the opposite direction of that goal. You don't want you home audio system to distort because you want to reproduce whatever is on the recording as best you can. Even if its distortion. "

That's definitely true that in the studio it's processing the signal - but at home, tube amps are often doing the same thing. It is a form of distortion. Not all tube based systems distort this way, but I do see lots of listeners using underpowered amps for the speakers they have.

Tubes are a great way to emotionally get close to the music - I think of tubes as a way to create an illusion that performers are in the room with you - but I never think of this as accurate or neutral - but it can be very involving...
Most SS based systems I have heard over the years were underpowered in terms of achieving optimal performance not on purpose but for not knowing any better and/or not having many practical and cost effective options available, or people just being satisfied and it just not mattering much.

Same true I suspect with tube amps. It's not that tube amp lovers seek distortion, but rather that it similarly may be even less of a real issue for some due to the more benign sound of soft clipping.