Tube Amp for Martin Logan Speakers

That's right George. My ARC Ref 150 might (???) be able to handle the current demands because it has a beefed up power supply -- 1040 joules. And, as I mentioned above, the output impedance off the 4 ohm taps is roughly .5 or so ohms. As a result, my amp performs somewhat like a solid state amp.

Hopefully, Al will come back with a response to my follow up questions: (1) whether ZEROs will change his calculations; and (2) how does the phase angle spec at a particular frequency affect his calcs.

That folks is why I persisted with my comments. I can't do the math, but after 2 or 3 years of this back and forth with Al, Ralph and some of our other tech members, I better appreciate the challenge of matching amps and speakers. Also, when I see that a speaker's impedance plot is literally off the chart, e.g., the ML ESLs and some of the B&Ws, I raise an eyebrow.

So ... this goes back to what Ralph (Atmasphere) said all along. Optimally, if a speaker manufacturer could design a speaker with a flat impedance plot, say between 8 and 16 ohms, with a zero phase angle across the frequency spectrum, the SPCA would go out of business.

And once again, the SPCA stands for the Society for the Prevention of Cruelty to Amplifiers. :)

Cheers,

Bruce

Bruce, thanks. You’re welcome!

If ZEROs are used to double the ML's apparent impedance plot, would that change your calculations?

It would narrow the gap somewhat. The 11 db figure for the solid state amp wouldn't change, but the 6 db figure for the tube amp having a 2 ohm output impedance would increase to 8 db. So the difference would be reduced from 5 db to 3 db.

Also, maximum power capability for the solid state amp would probably be cut in half, with distortion performance being improved as long as the amp is operated within that reduced power range. I'm not sure that it's possible to meaningfully generalize about what would happen in those respects in the case of a tube amp, with this kind of wide variation of speaker impedance as a function of frequency. I believe it would depend on the particular amp, and whether it is OTL, SET, push-pull, high or low feedback, etc.

Did the ML's phase angle plots factor into your calculation? If so, directionally, how so (if phase angle is negative or positve)? I intuit that harsh phase angles can constrict the SOA of an amp (SS or tube). I am not clear of the impact on sonic coloration, assuming the amp is operating within its respective SOA.

As I indicated, the phase angle was approximately zero at 46 Hz, and I ignored the -20 degree phase angle at 20 kHz in order to simplify the calculation. For purposes of a rough ballpark calculation that seems reasonable. It might not be, though, in the case of other electrostatics where phase angles may be more severely capacitive (i.e., negative).

Off the top of my head I’m not completely certain what the directionality of the coloration effects would be in those cases, assuming the amp is capable of handling the increased difficulty of the load, and supplying the necessary increase in current. For a given impedance magnitude, such as 2 ohms in this case, I believe that a severely negative phase angle would mean that the resistive component of the impedance (which is the component that can absorb power and convert some of it to sound) would be smaller. I believe that would tend to increase sensitivity to output impedance differences between amplifiers, and therefore exacerbate the difference between the solid state and tube amp calculations I provided. As I say, though, I’m not entirely certain of that. You ask some tough questions :-)

Best,
-- Al

Al, perhaps the EPDR article I referred to above might put a little gloss on what you said. The underlying thesis of the EPDR article is that a speaker's "nominal" impedance is further reduced if it is coupled with a negative phase angle at the particular frequency in question. So, if the "effective" impedance, or EPDR, is even lower, I can only assume that the "db delta" between a SS amp and a tube amp, even with a "low'ish" output impedance, would likely be greater. And I believe you surmise the same in your last post.

I think the take-a-way for us is that we may not be able to tell whether a particular amp/speaker combo will be a good match by looking at specs alone. But we should be able to identify (or at least be wary of) a combo that may be a bad match. Of course, if someone likes sonic coloration -- hey, all the power to sonic coloration.

Yeah ... I agree with the comments above, either the manufacturers and/or the dealers need to do a better job of helping customers understand this stuff before pushing product.

Thanks again Al.

It's only cruel to an amplifier if it's not up to the task.

Ralph is here to sell his products.