Matching Power and Speakers -- Much Ado about Not-that-Much? (Tube amps and speakers)

Interesting conversation at the Part Time Audiophile's The Occasional Podcast.
There, tube amp maker Justin Weber of Amps And Sound makes these comments:

At about 24:03 in, they have an interesting conversation about power in tube amps and speakers' needs.

Justin makes an interesting comment about power and speaker efficiency. He's careful to caveat that room size might be an important differentiating factor, but for many (most?) audiophiles (with small-medium size rooms), there is just not that much to worry about regarding amplifier power and speaker matching. He is not touching on "synergy" in some larger sense -- he's just addressing the "power needed to drive speakers" question.

If I'm understanding the upshot of what he's saying, it's that lower-power tube amps made with quality transformers (and well made generally) have a very good shot at doing an excellent job for a much wider range of speakers than is typically assumed in conversations audiophiles have.

The whole interview is interesting, but here's the interesting bit -- I did my best to transcribe it, but go listen for yourself!

"Almost always, you need vastly less power than you realize....I've seen 1 watt power whole rooms and big rooms with moderately efficient speakers....What's a practical standard? 30 watts should do 99% of everything for everyone, and 15 watts should do 95% of everything for 99.9% of everyone....

Most audiophiles have small listening rooms.... [A more powerful tube amp may sound better, but] I think that’s a matter of it having a better output transformer and [that] output's transformer's core actually having a flatter frequency response and going lower.

So [in those cases where a more powerful amplifier is used] I don't know that it's a question of producing more power -- that you need more power -- as much as the transformer [in] the more powerful amplifier is [instead just] a better transformer."

Not sure what folks here think about these claims. Perhaps they seem so obvious as to almost not need repeating. But there are so many conversations about speaker sensitivity and watts that do not mention the quality of the transformers or which seem to overstate the importance of how powerful an amp is.

If Justin is right, then many, many pieces of advice related to "how many watts do you need" are basically wrong.


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Interesting and informed replies, as usual. Thank you. It would be cool if Justin Weber could weigh in, too, but I don't know if he's on this forum.

I have his Nautilus. It drives both my Devore O/93's and Spendor D7.2's very well. I do have a small listening room. And after about six months of using it to drive the loudspeakers it has been relegated to headphone use.

The Nautilus is a beast of an amp build-wise. It weighs about as much as my 10x higher wattage ARC Ref 80S. I think what Justin is saying is that speaker driving power is much more complicated than watts. 

Does the Nautilus sound the same or similar to my more powerful tube amps (also have an ARC Ref 150 SE)? No. And the differences go beyond mere sonic signature. I will concede the low-end grunt of even moderately low bass goes up as I go from the Nautilus to the Ref 80S to the Ref 150 SE. The Nautilus on the other hand has a sonic purity thing going on that is not there with the higher wattage amps. 

If I'm understanding the upshot of what he's saying, it's that lower-power tube amps made with quality transformers (and well made generally) have a very good shot at doing an excellent job for a much wider range of speakers than is typically assumed in conversations audiophiles have.

The issue is deciBels. The ear hears on a logarithmic curve; deciBels are a logarithmic unit of volume. 3dB is barely perceptible to the ear in terms of volume increase but requires double the amplifier power. So the difference between 30 Watts and 300 Watts is only a doubling in the perceived sound pressure.

This is why if you need more than 100 Watts to make your speakers work, you'll probably need 600-800 Watts or more to get the sound pressure you're looking for without clipping the amp. If you need more power than that the speaker is really impractical or your room is really large.

OTOH we have SETs which are a unique phenomena. Because they use no feedback (and so are 10% distortion before clipping), if you want to get the most out of them, your speaker has to be so efficient that the amp is never asked to do more than about 20-25% of full power. At power levels above that the amp will make more higher ordered harmonics on transients, causing the amp to sound 'dynamic', since the ear uses those harmonic to sense sound pressure.

That makes SETs impractical on most loudspeakers and is why horns made a comeback in the 1990s. SET users get by with so little power because the amp can sound 'loud' when pushed, so the user tends to think its plenty of power. When using a cleaner amplifier the volume tends to go up... but if such a user goes from a 7 Watt SET to a 30 Watt PP amplifier, they may find themselves running the 30 Watt amp out of gas because they are trying to get the same 'loudness' as they got before, if they don't understand that the perception of 'loudness' was caused by distortion.

Mr. Weber is ignoring a significant swath of high end speakers that have low sensitivity and a difficult impedance curve. Wilsons and Thiels are two prime examples. In the more extreme cases the speakers also have phase issues which makes them even more difficult to drive. Anyone who reads Stereophile and pays attention to the sensitivity, impedance, and phase measurements is aware that many speakers require a robust amplifier to operate at their best.

In my case I have a pair of Thiel CS6 speakers driven by a Krell KSA 300S (2400 watts into 1 ohm) and it takes everything that amp can produce to drive my speakers to concert levels. Because of the punishing impedance curve (around 2 ohms for part of the audio band) a tube amp will act as an unintended tone control on these speakers.

It's interesting that high end speaker companies are still designing speakers that have low sensitivity and a low, variable, impedance curve. This apparently is a tradeoff that some speaker designers are willing to make to get the sound they are after. These speakers need a solid state amplifier that can double it's output as the impedance drops by half.

Mr. Weber designs tube amps that cannot run speakers like mine. Sure, they will sort of work but they will sound anemic and the frequency response of the speakers will not resemble what the designer intended. His statements are uninformed and obviously intended for people with little experience in matching amplifiers and speakers. As this subject comes up over and over again I am disappointed that people like Weber, who should know better, espouse this drivel.

@8th- note I suggest you listen to a bit more of the interview before judging him so harshly.