Tube Amp for Martin Logan Speakers

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

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