Balanced Low Power Tube Monoblocks?

Tvad, FWIW, a 'dip' to 4 ohms is not a problem for the M-60! OTOH, having the bass frequencies be 4 ohms (while the mids and highs are 8) is, and is a problem for **any** zero feedback tube amplifier.

When you see speakers that are 4 ohms in the bass and 8 ohms in the mids and highs, quite often the speaker designer is using woofers that are 3 db less efficient, and expecting that the amplifier will double power with the 4 ohm load. This brings the woofer output up to the level that the mids and highs operate at, but you need transistors to do that properly.

No tube amp will sound right with a load like that since no tube amp can double power as you cut the impedance in half! You can make a speaker like that work (sort of) if the tube amp has lots of feedback in its design. The problem is that the negative feedback will foreshorten the soundstage and impart a sheen to the mids and highs- IOW it will not sound natural.

That is why our amps have little or no feedback- why bother try to drive a speaker that will always sound electronic? I agree it has limited our market, but limited to the speakers that can sound like music (work properly with tubes), and that is not a bad thing :)

Almarg, No, I don't seem to agree. The issue is that tube amplifiers are not always going to act like a true voltage source (that is the domain of transistors). You *can* get tubes to *start* to behave like a voltage source if you add enough feedback, but then IMO/IME, by doing so you obviate the main reason to go with tubes, which is linearity without feedback.

The problem with feedback is that it will *increase* the odd-ordered harmonics that the human ear uses to determine the volume of a sound. Since these harmonics are considerably higher in frequency (5th, 7th and 9th harmonics specifically) they also add an electronic brightness that everyone on this forum is very familiar with. Nelson Pass wrote a great article about this:

http://www.passlabs.com/pdf/articles/distortion_and_feedback.pdf

pay particular attention to the graph showing the odd orders.

So IOW we as audiophiles are always having to decide between sound that might not be perfectly flat but otherwise sounds natural *or* sound that measures flat but sounds hard and bright! Either way there is a coloration and either way neither one sounds like the frequency response is flat.

I for one prefer highs that are natural as opposed to bright. I don't like boomy bass or anemic bass either, so I choose speakers wherein the designer knew that the amplifier driving that speaker was not going to be a perfect 'voltage source'. see

http://www.atma-sphere.com/papers/paradigm_paper2.html

for what that is all about.

Almarg- OK, I see your point, but in practice such an amplifier will in fact 'produce an excessively bright response' not due to frequency response variation but due to generation of odd-ordered harmonics.

I apologize for repeating myself often, but the simple fact is that that audio industry is ignoring 45 years of research into how humans perceive sound! Were that research taken into account, I would agree that your statement is correct as today's amplifiers would be different. What amazes me about all this is how the audio industry has stuck steadfastly to design rules that, even while they were being implemented during the 1960s, were already being proven wrong by said research.

What I am talking about is the typical amplifier that you are describing violates the most fundamental rule of how we perceive volume, by exaggerating odd-ordered harmonics. Another way of putting this is that you can be convinced that a sound is a lot louder than it really is if the 5th, 7th and 9th harmonics are tampered with. These harmonics are not only cues to the human ear brain system, they also contribute to brightness- quite literally our ears are more sensitive to these harmonics than they are even to vocal frequencies! So there is almost no way an amplifier like that will not appear to have excessive highs.

This is the fundamental reason why tubes are still around after being declared 'obsolete'...

Almarg, I agree completely. As you have probably figured out by now, I regard the quality of the amplifier by its ability to not enhance these harmonics more than anything else.