Tube Amp for Martin Logan Speakers

Tom, if you read my posts, I framed my question by referring to those tube amps having a "high'ish" output impedance (say 3 or 4, or more ohms).

By contrast, some amps like my ARC Ref 150, have a "low'ish" output impedance, possibly because they use negative feedback. Specifically, the output impedances off my amp's 4 and 8 ohms taps is about .5'ish and .6'ish ohms, respectively. As a consequence, the Ref 150's output voltage regulation is pretty tight: about +/- .4 db and +/- .8 db off the 4 and 8 ohm taps, respectively. So in effect, tube amps that have a low'ish output impedance function somewhat like a low impedance SS amp -- constant voltage source.

I mention all of this because you say in your last post that "ML approves of using tube amps with their speakers." Well ... that may be. But then there are tube amps, ... and then there are tube amps.

Regards,

Bruce

Quick postscript: my posts are not just academic. The OP is asking about matching a tube amp with ML ESLs. My comments are trying to focus attention on whether the ML ESLs in question should be driven by a low impedance amp that acts as a constant voltage source, i.e., a typical SS amp or a low impedance tube amp.

As I said above, "there are tube amps, ... and then there are tube amps." My somewhat tongue-in-cheek point is that if the ML ESLs were designed to be driven by a low impedance amp, then the OP should be cognizant about the output impedance of the tube amp he has in mind. The consequence of using a "high'ish" output impedance tube amp like an Atmasphere OTL is that the sonic colorations may result in augmented bass and shelved treble. Of course ... even a low impedance tube amp may need some serious current capabilities, even if it performs SS-like. Consider the EPDR article and its reference to SOA.

By the way, as I also mentioned, while ZEROs may raise the apparent across the board impedance load presented to the amp (SS or tube), I am dubious that the device will smooth out the sonic presenation of the MLs if the wrong type of amp is used. 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.

I surmise that if the tech members respond, we will all gain a better understanding of whether one should drive MLs with a Power Paradigm tube amp if they were voiced to be driven by a SS/Constant Voltage Paradigm amp.

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