Tube Amp for Martin Logan Speakers

Bifwynne and everyone else,

Thanks for your efforts to clear up the technical issues involved.

03-29-14: Tomcy6
I think that ML designed their current line to be used with either tube or ss amps, the choice being the user's.

Yet if you Google images for previous big HiFi shows, and look at specific Martin Logan rooms demo's, as far as I saw nearly evey demo by ML was done with big SS amps like Krell and such.

The only ones I saw driven by tubes were not in a Martin Logan rooms, but were in an tube amp manufacturers rooms, who were using ML powered bass hybrids for their speakers.

Cheers George

03-29-14: Bifwynne
Perhaps Ralph or one of the other tech members can speak to how much SPL variation will result if the "wrong" type of amp is used. Perhaps, for discussion purposes, we should assume the MLs were designed to be driven by a SS "constant voltage paradigm" amp and the "wrong" type of amp is a Power Paradigm amp.

Hi Bruce,

I did some quick calculations for the OP's Aerius, based on John Atkinson's measured impedance curves shown here. It should be kept in mind that the impedance characteristics of the Aerius are significantly less demanding than those of many other Martin-Logan speakers.

The extremes are a 25 ohm peak at 46 Hz, with approximately a 0 degree phase angle, and a 2 ohm minimum at 20 kHz, with approximately a -20 degree phase angle. To simplify the calculations I ignored the effects of the -20 degree phase angle.

For a given input level to the power amplifier, a solid state amplifier acting as a voltage source will put approximately 11 db more power into the speaker at 20 kHz as it would at 46 Hz, as long as it is operated within the limits of its power capabilities.

For a given input level to the power amplifier, a tube amp having an output impedance of 2 ohms will put approximately 6 db more power into the speaker at 20 kHz as it would at 46 Hz, as long as it is operated within the limits of its power capabilities.

If we assume per your question that the speaker is designed to provide flat frequency response when driven by a solid state amplifier, the speaker's response when driven by that tube amp would therefore be rolled off by 5 db at 20 kHz, relative to its response at 46 Hz.

That difference certainly figures to be audible, although not necessarily objectionable in many circumstances. The difference will of course be significantly greater with amplifiers having 3 or 4 ohm or higher output impedances, and with many other Martin Logan speakers.

What seems likely to often be more significant, however, is simply the ability of the amp (regardless of whether it is tube or solid state) to cleanly generate enough power at deep bass frequencies, into the high impedance. Obviously that factor is highly dependent on the dynamic range of the recordings that are listened to, as well as on the listening distance, room size, and volume preferences of the particular listener.

Best,
-- Al

Thanks Al .... that's exactly what I surmised the directionality of the impact might be. Btw, if ZEROs are used to double the ML's apparent impedance plot, would that change your calculations?? And your comment about whether any amp, SS or tube, can drive the MLs within its respective SOA (see EPDR article) is a whole "nuther" kettle of fish.

Btw, 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.

Incidentally, to my sorry old ears, I find a sonic coloration of even 1.6 db to be noticeable. I base that assertion on my amp/speaker combo.

Based on John Atkison's review of my amp, he reported that the output impedance off the 8 ohm taps is about 1'ish ohms, resulting in output voltage regulation variation of +/- .8 db (ergo a theoretical spread of 1.6 db). By contrast, those figures are roughly halved off the 4 ohm taps.

In practice, I find that my speakers sound less forward and have deeper, more extended bass when driven off the 4 ohm taps. That is consistent with the impedance plot of my S8s, which have roughly a 20 ohm peak at 2K Hz (x-over point) as compared to a 4 ohm saddle in the "power range" of 60 to 500 Hz).

Thanks again Al. You manage to lift the veil of confusion for non-techs folks like me.

Regards,

Bruce

Someone should tell Martin Logan about all this. Those idiots are telling people that their speakers can be used with a wide variety of tube and solid state amps.