Tube Amp for Martin Logan Speakers

The fact is they want an amp that are stable and also that can double it's wattage from 8ohms down to 4 ohms and yet still increase even to 2ohms, (which means an amp that can do good current)
This means that if an amp cannot do this, then at the low 1ohms or 2ohms that their (ML) speakers present to an amp, then that amp will then start to behave like a tone control, and not stay flat in frequency response over the entire impedance range of speaker it's driving.

As simulated speaker load graphs will show you of amps that cannot deliver current at those low loads, therefore their frequency response will not be flat.

All you have to do is look at Stereophile speaker simulated load graphs of tube amps especially to see that they cannot give a flat frequency response into those types of loads, especially ones that dip down to 1ohms.
If you need proof I will post links for you to see what happens, but I think you should go and look for yourself.

Cheers George

Great posts by Bruce (Bifwynne), with which I fully concur.

09-13-15: Georgelofi
... amps that cannot deliver current at those low loads ... cannot give a flat frequency response into those types of loads, especially ones that dip down to 1ohms.

This is true, but I would emphasize the word "into." The frequency response characteristics of the signal at the input terminals of a speaker that will result in flat frequency response in the acoustic output of the speaker will depend on the design of the particular speaker. As Bruce said, it is "important to know whether the ESL was voiced to be driven by a SS or tube amp."

And in that regard it is worth noting that the Quad ESL57 was designed before solid state amps existed. Although admittedly, as I believe you (George) mentioned earlier in this thread or in another similar thread, the vintage Mark Levinson ML-2 solid state amp in particular, rated at only 25 watts or thereabouts into 8 ohms but capable of supplying huge amounts of current into low impedances, is considered by many to have been a good match for the ESL57. While at the same time that speaker has been and still is used with tube amplification by many audiophiles.

Regards,
-- Al

George, you posted "[a]ll you have to do is look at Stereophile speaker simulated load graphs of tube amps especially to see that they cannot give a flat frequency response into those types of loads, especially ones that dip down to 1ohms."

I generally concur, but as I am sure you know, not all tube amps are made the same. For example, take a look at JA's bench measurements of my amp, the ARC Ref 150:

http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements#52FW4Aq5RbfXP6dv.97

As you can see, the amp's voltage output plot is a bit wavy when a simulated load is plugged into the amp's 8 ohm taps. That said, JA mentioned that "[t]he figures for the 8 ohm tap [ranged between] 1 and 1.4 ohms; for the 4 ohm tap, they [ranged between] 0.55 and 0.87 ohm." I surmise that the same simulated load would be less wavy if plugged into the amp's 4 ohm taps.

Now ... an actual "off the bench" report from me. If you get a chance, check some of my posts on the "DEQX Game Changer" thread. I bought a DEQX PreMATE, which effects both time domain alignment adjustments and room equalization correction.

To set up the DEQX, actual mic'd measurement are taken at the listener position. The FR of my speakers was frankly a mess. I surmise most of the FR aberrations were caused by room anomalies, not by my amp's output impedance interactions with the speaker. Btw, I drive my speakers off the 4 ohm taps.

One other point of interest. I sent Al my DEQX data files for review and comment. I think Al might concur with my observations. And right now, my speakers are pretty well adjusted via the DEQX. I am enjoying a very pleasant musical experience.

So, based on the foregoing, I suspect that if my amp/speaker combo was checked in an anechoic chamber, my tube amp/speaker FR plots would measure pretty close to the results obtained if my speakers were driven by a high quality SS amp under similar conditions.

Last point. The reason my amp's output impedance is low'ish and output voltage somewhat constant is because ARC uses a prudent amount of negative feedback. There is also some sort of local negative feedback effect achieved by reason of a circuit configuration between the power tubes and the output trannies. Ralph or Al can better explain how that works.

Kudos to all for the good comments.

Cheers,

Bruce

"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