Tube Amp for Martin Logan Speakers

Its funny how there are always two ways to look at good sound.

1) technical
2) practical/case by case

Nothing guarantees each perspective will resulting the same choices/solution. Why should they each case is different even if for no other reason than room acoustics and how each person hears. Would we expect two people to choose the exact same pair of shoes? No. But we all kinda know what makes some shoes better than others. That's the technical perspective and the only one of global value. But then , the shoe gotta fit....

"In otherwords it will do the classic thing that can happen with low current or high output impedance tube amps "behave like a fixed tone control" instead of being flat into all loads."

That's a very interesting analogy!

I suppose, its a question of who is in charge, ie which component is going to dictate the parameters of the overall system that works best.

It makes sense that from the perspective of the amplifier or specifically the designer of a tube amplifier, they want their baby to be the one in charge.

The speaker designer trying to make a compact quality speaker with extended flat bass response that most might be willing to put in their rooms has a different perspective. The price paid in this case is more demand on the amplifier to help deliver the desired results.

Two different paradigms/ways of approaching things as Ralph likes to point out, each with unique strengths, weaknesses, and tradeoffs. Its good to be able to have the choice.

Someone should sign up to do a "paradigm" shootout, ie the best of voltage versus the best of power paradigms. Of course, finding an unbiased doer or doers for something like this could be the hardest part.

I would volunteer someday, perhaps after I retire eventually, although I doubt I will have the financial resources needed to put into two complete and alternate SOTA systems that could both truly push things to their limits. :-)

Or ... are you saying the relationship between an amp's output impedance and an ESL's varying impedance plot is simply not a relevant consideration to the ESL's sonic output because of some unique or different electrical characteristic peculiar to ESLs??

As I have pointed out before, with all ESLs, the impedance curve is not a function of a driver in a box. In addition, the impedance curve is also not the efficiency curve.

IOW, the speaker has the same efficiency at all frequencies. Note that I used the word 'efficiency' as opposed to 'sensitivity'. I am restating my second paragraph above slightly, but if I had used 'sensitivity' what would be happening is that the impedance plot would also be a sensitivity plot.

The reason is that the speaker's impedance plot is based on a capacitor.

The result is that an amplifier that operates in the Voltage Paradigm will make too much power at high frequencies due to the low impedance, and usually not enough power at low frequencies for exactly the same reason.

For this reason you will find that a lot of ML users that have solid state amps likely have them fairly close to the wall, which gets some bass reinforcement. Most ESLs, like any panel speaker, should be 5-6 feet from the wall.

I think now you see why the ZERO is so useful when dealing with this speaker. But I will explain: if you use a solid state amplifier one of the really likely results is that while detailed, it will also be noticeably brighter than reality. I've experienced this first hand many times.

Now if you use a Power Paradigm amplifier, as Al has pointed out, the 'output impedance' of the amp will interact with the low impedance of the speaker at high frequencies and you will experience a roll-off. This is why many tube amplifiers (in fact most tube amps) sound a little dark on that speaker if not outright rolled off. Those with negative feedback will of course sound less so (but of course negative feedback causes all amplifiers to sound bright due to trace amounts of odd ordered harmonics being introduced). Now if you just want things to sound natural with no roll off and no brightness, what to do??

That is where the ZERO is handy. It raises the impedance of the load enough that a tube amp can begin to operate within its constant power region. In this way you get high frequency speed and extension, while at the same time getting bass extension and impact.

A long time ago ML made a speaker called the CLS I. Unlike the newer iterations, it had a more traditional ESL impedance plot (meaning it was higher impedance like the older Quads) and it was a breeze to play it with a tube amplifier. As we all know (or should know) transistor amps don't make a lot of power into higher impedances (+16 and +32 ohms), IOW it may well be that such an amp might only express about 1/4 of its 8 ohm power. It is for this reason that a 100-watt tube amp can keep up with a 400-watt transistor amp on Sound Labs BTW.

Anyway, ML realized that if they were to sell speakers to the solid state market, which at the time was about 90% of the amplifier market, they were going to have to do something. The result was that succeeding speakers had much lower impedances, with the 0.5 ohm 20KHz impedance you now see. The ZERO corrects this issue so tubes can be practical on the speaker.

You'd be far better off saving the $1140us for the Zero's and putting towards the right amp or speakers, instead of masking the problem with the Zero's, which itself has it's own set of problems.

Cheers George