Tube Amp for Martin Logan Speakers

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.

Atmasphere,

Thank you very much for your response.

So, if I understand correctly, for the most part an ESL will be as efficent (output approximately the same sound pressure level or volume) at a given amplifier output, say 1W, regardless of the impedance? I assume at the impedance extreme of the ESL (less than say 2ohms) the SPL will be reduced?

I hear reference of "less musical energy" at the upper registers. Effect being these upper frequency sounds are by nature "softer/lower" in volume compared to lower frequencies and less amplifier output is required?

Again my apologies for my simplistic understanding.

Lee

Sorry guys amp that can't keep it's end up at those 1ohms loads, will not have a flat frequency response from 20hz to 20khz no matter what level it's being played at. It becomes a tone control, end of story.

Cheers George

George,

Would it also be true than that an amp that can't make power into higher impedances is also a "tone control" of sorts?

Respectfully,

Lee