Tube Amp for Martin Logan Speakers

Excellent post, Bruce, with which I agree. I also agree with the first two-thirds or so of the paper George referenced, although I would point out that the author has taken the reasonable step of simplifying the circuit analysis he presents by only taking into account the resistive component of the load impedance.

I would take exception, however, to much of the last part of his paper, beginning at the point where he states that:

Loudspeakers are not constant power devices. Loudspeakers must have a constant drive voltage to provide a constant acoustic output with changing frequency.

IMO (and I feel safe in saying in Ralph's opinion as well), that statement is incorrect and misleading. I would agree with it if it referred to the majority of loudspeakers, rather than being expressed as applicable to all loudspeakers. Many electrostatics being notable exceptions to his statement, IMO.

The fundamental error he makes, IMO, is that he assumes that with all speakers flat frequency response in (in terms of voltage) corresponds to flat frequency response out. That will be true, at least approximately, for the majority of speakers these days. However it will not be true for a substantial number of speakers, including many electrostatics. As Ralph has pointed out many times, it comes down to the intention of the designer.

Consider the classic Quad ESL-57 George referred to earlier, which is revered to this day (impedance curve shown here). It was designed before solid state amplifiers existed! And it is very successfully used with tube amplification by many audiophiles to this day. Although in fairness I'll say that I recognize that **at least among solid state amplifiers** the vintage Mark Levinson ML-2 which George referred to is recognized as being a particularly synergistic match, and that combination (with two stacked Quads per channel) was the heart of the highly regarded HQD system produced by Mark Levinson's original company (together with a large Hartley woofer and a Decca supertweeter, with a total of six ML-2 monoblocks driving the two channels in a triamped configuration).

Regards,
-- Al

Thanks Al. My apologies for the numerous typos in my last post. I was typing with my laptop literally on my lap, drinking coffee with one hand and typing with the other hand. Truly a multi-handed effort.

That aside, I wanted to say that this has been one of the best threads to get posted in a long time. So kudos to all of the contributors.

I believe that if and when I ever change out my speakers or my amp, the terrific comments posted in this thread will make selecting a good amp/speaker combo a less haphhazard (sp) exercise. At the very least, I'll be able to ask better questions ... and know what to expect in terms of getting better answers.

Thanks again to all, including of course Ralph, Al, George, Mapman, Unsound and Judy.

Cheers,

Bruce

Output impedance does go down with nfb - that's a basic tenet. Easy to check: take a non-nfb amp and change the load d
from 8 to 4 ohms. You will see that the output voltage drops a bit. Now apply nfb, do the same thing and you will see that the drop is less, showing that the output impedance has dropped. In fact, it drops exactly with the feedback ratio. Apply 20dB nfb and it drops by a factoe of 10.
Of course we all know that Zout is not physically a resistor or sumptin'....

Jan

Just a little background on my experience with the Manley Snappers.

I really think what can make a difference is how hard you're pushing the Snappers.

I was very concerned about the Snappers - given the specs - which is why I insisted on auditioning them before purchasing them. I had read that they were a tad more sweet in the midrange than the Rogue Audio M-150s (which is what I was planning on after reading some very positive reviews), but I suspected they wouldn't have the power to drive my Aeon i's - especially in the higher frequencies where the Aeon i's can drop down to just under 2 Ohms. And I should tell you that I am also using a pair of subs to augment the Aeon i's since they are totally passive and can only get down to 43Hz. But a practically brand new pair of Snappers came in at my local dealer's shop and he called me up and asked me if I wanted to try them out for a week. I decided to give them a try after speaking with Paul over at Manley Labs - being cognizant of the fact that my listening room is quite small and I would not likely be pushing the Snappers too hard.

All I can say is they sounded amazing through the Aeon i's. I brought over two audiophile friends - call them P and W. P is in his late 20's and can easily hear frequencies above 15kHz (which is actually pretty rare) and both were blown away by the sound. We we're comparing the Snappers with my Parasound HCA-3500 which has ZERO problem driving my ML's and the Snappers outperformed the 3500 in practically every way - especially on musicality and sound stage depth. What really shocked me was when W said he thought the Snappers had more slam than the 3500 - especially since slam is what the 3500 is really known for. P indicated that the highs were a bit more subdued with the Snappers, but they sounded much more pleasant in his mind. For W, there was no comparison - the Snappers sounded MUCH better in his mind - even to the point that he has now decided to move over to tubes. And we listened to all types of music - classical (full orchestra and smaller ensemble works), jazz (traditional and modern) and classic rock. Female vocals sounded especially life-like through the Snappers.

But keep in mind, the caveat to all this is the levels we were using for the audition given my room size. If you have a large room that needs some serious volume, then all the data does suggest that the Snappers will probably run out of steam and you might be better off with something like the M-150s or M-180's. But in my room at least, the Snappers had plenty of power to drive my Aeon i's with a level of quality that was leaps and bounds above the 3500.

Would a pair of M-150's or M-180's do better? Or one of the top-end Pass amps? Maybe. But all I can say is the Snappers were available, they were less than 8 months old and in like-new condition and they were priced at $3600 for the pair (they now retail new for $7200). So in my case at least, getting them was a no-brainer.

Specs are certainly important but they are not the end-all-be-all. At the end of the day, you really need to listen and decide for yourself how a component integrates with your existing system and room topology. For me, the Snappers were a major step-up and I'm totally happy with them.

Jan, please re-read my non-tech take 2 or 3 posts north of this post. What you describe is exactly the effect of using NF -- the amp compensates for the output voltage drop based on the amount of NF applied to increase the output voltage to compensate for the interaction of the amp's internal impedance load and the external load of the speakers.

The effect is more precisely described as the amp is behaving as though it has low output impedance. So ... the NF circuit does not really change the amp's internal impedance. Instead, it acts like a servo to cause the amp to compensate for the Ohm's Law effects.

Of course, as Al, Ralph and others have said, the amp can only do this if it operating within its power delivery limits -- i.e., SOA (safe operating area). Also, there are rarely any free rides. Hence, NF has its own problems, e.g., increased odd ordered harmonic distortion ... and its detractors, e.g., Ralph. IMO, amp designers make trade-offs in order to design and build amps that perform at a certain level at a certain price point.

Probably still not saying this correctly, but it goes to show how the terminology can get in the way even though we are all really saying the same thing.