Tube Amp for Martin Logan Speakers

"important to know whether the ESL was voiced to be driven by a SS or tube amp."

I own myself a pair of ML Monoliths III's with quite new fresh panels.
If I drive them with a my pair of Rogue 120 tube amps, they have highs but they are very distant and too polite, if I drive them with my solid state amps (similar to big Krells) low output impedance and gobs of current the high are where they should be and you know all about cymbal crashes.

And this thread is about the right amp for ML's not Quad 57, which I agree were most probably voiced with low power tube amps.
For Martin Logan to mention an amp should double into 4 from 8 and then increase again into 2ohm, this means current. And they also want one that's stable into capacitive loads like ESL's.
And lets face it a stable amp can be a 5 watter and have no current ability, so long as it doesn't ring or oscillate it's stable, current ability has nothing to do with stability into capacitive loads as ESL present.

Cheers George

Unless the amp is being tortured by the load and sensitivity of the speakers:

See this article:

http://www.stereophile.com/reference/707heavy#ovW5G38gcYb8whWE.97

Now is the reason this type of amp works is (assuming the speaker has a sensitivity (SPL) of 85db/1W into 8 ohms)

.25W into 32ohms = 85db
.50W into 16ohms = 85db
1W inot 8 ohms = 85db
2W into 4 ohms = 85db
4W into 2 ohms = 85db

so the SPL remains constant into all impedances?

Can anyone please comment on the above as to its accuracy? If correct, I dont understand this electrical concept as it pertains to how the speaker plays various notes at the intended (recorded) levels?

Considering a very simplistic example, if a bass note and mid range piano note are recorded at the same level and are played at the same moment, and assuming the speakers impedance at the bass note frequency is different from the speakers impedance at the piano note frequency, how then are the level of these two notes played through the speaker at the intended playback level? At the moment these two notes are played, does the speaker draw a constant output, (voltage/cureent) from the amplifier? If so, lets say it is 1W. If the above speaker sensitivity question is true, how can both notes be played at the same level at 1W being drawn from the amplifier?

Thank you, Lee

Simple ... it's a function of the transducer system itself. That is what is meant by how the speaker is voiced.

Take a simple example. A speaker system may present an impedance load of 20ohms at the mid/tweeter x-over point. Yet, assuming the drivers are phase coherent at the x-over point, and if the speakers are well designed, the FR should be flat over the x-over point. Similarly, a speaker's impedance function even outside the x-over points may fluctuate. But still, the driver system emits a level FR SPL. It's about how the speaker was voiced.

There are some, but not many, speakers whose impedance function is near linear. Unless the impedance level is either ridiculously low or high, and the phase angle plots are not wacko, such a speaker could very well be both SS and tube (Power Paradigm) friendly. Just throwing this out there, but I think Maggies might represent a somewhat level 4 ohm load and pretty benign phase angles. I'm sure there others.

Hvowell, said earlier:

Now is the reason this type of amp works is (assuming the speaker has a sensitivity (SPL) of 85db/1W into 8 ohms)

.25W into 32ohms = 85db
.50W into 16ohms = 85db
1W inot 8 ohms = 85db
2W into 4 ohms = 85db
4W into 2 ohms = 85db

so the SPL remains constant into all impedances?

The reason this is not so is because in the case of an ESL, the impedance curve is **not** an efficiency curve. The fact is that the impedance curve is based on a capacitor more than anything else. This makes ESLs fundamentally different from dynamic drivers in a box. In an ESL, the efficiency is fairly constant despite the impedance. Generally speaking, ESLs have an impedance curve that varies by about 10:1 over the range of the speaker, which makes any one of them tricky for all amps. The reason they work at all with solid state amps is due to the negative feedback loop in the amplifier which allows the amp to adapt somewhat (reduce its output at higher frequencies) to the lower impedance at high frequencies. Despite that its a common complain that a transistor amp will be too bright on an ESL (not the least of which is that negative feedback in and of itself can cause brightness; likely a topic for another thread).

For example the Sound Labs, early Quads and Audiostatics are easily driven with tubes as the matching transformer in the speaker sets the bass impedance fairly high (usually over 16 ohms). OTOH, Martin Logan use the matching transformer to set the bass impedance at about 4-5 ohms, making the 20KHz about 0.5 ohms. This is why they want an amplifier that can double power reliably into low impedances, else they would simply not be getting any highs at all!

Note: George is mis-using the word 'stability' and 'stable' in his comments (it may not be his fault; this is a common mis-use of the word). Stability in an amplifier is how well the amplifier resists *oscillation*. For example our smaller amplifiers, like the M-60 (which is an OTL) are perfectly stable driving 4 ohms even though they don't make as much power doing so. OTOH some transistor amplifier are unstable driving capacitive loads (IOW they can go into oscillation) even though they can double power as the load impedance is cut in half.

We have customers driving Martin-Logan ESLs quite successfully using either our old Z-Music autoformers from years ago or the ZEROs. Both allow our amps to drive the ML ESLs with ease with no high frequency softness. I've heard one of these systems; if anything to me the combination might have been a little bright, but some of that may have been caused by the solid state preamp that was also in the signal chain.