Atma-Sphere M60 monoblocks + ML ESL 11 = Inferior combo

Hi guys,

Today I borrowed home a pair of pure OTL-Tube M60 mononblocks to combine with my ML ESL 11s (which base are partially driven by a Class-D amplifer of 275 watts) with very poor results. Unfortunately, although the M60s where only to cater for the diaphragm panels (at least in theory), when combined with the speakers, the soundstage collapsed, vocals felt muffled and absent deep into the sound picture, almost hidden away, and the base went out of control (got larger and louder in a negative way). I also lost details. The Monos are a pair of brand new demos and shined this morning together with a pair of smaller Vivid monitors.

My current SS - a Bryston 4B3 delivers so much better combined with the ML ESL 11s. Someone that can explain what I did just experience? I thought that OTL tubes and High sensitive Martin Logans of todays generation where to be a marriage in heaven.

Kind regards 




Looking at ML website it shows the ML11's have an impedance of 4 ohms, 0.6 at 20kHz. Way to low for any OTL amp. OTL's like to see a high impedance of 8-26 ohms without dipping much below 6 if possible. That's why they did not match. Maybe Ralph of Atmasphere can chime in for his thoughts.

Not a total surprise. That OTL amp starts dropping power big time as the impedance drops. Your ears told the story, it happens.

It’s not that the atma-spheres are bad amps, it’s just that they were not designed to power speakers like MLs. They are designed for easy to drive speakers and many people find they do very well in that situation.  This has been discussed many times on these forums.

I wonder why you didn't call Ralph (atmasphere) and discuss this with him? He often answers the phone.

He is very open and provides honest information with regard to his equipment.

Atmashphere amps can work with difficult loads, but you can overtax them- to the detriment of sound quality. Perhaps his new Class D amps would be a better fit.


The M60 OTL's are very load sensitive! They function best with a flat high impedance speaker - of which few exist. The solution is to use an autoformer between the OTL amp and the speakers. That's what I have on my Futterman H3 - a pair of Altec autoformers. The original owner had Julius Futterman add a pair of them to the H3 chassis so he could drive his low impedance (below 4 ohms) speakers. An autoformer presents a uniform higher impedance load to an OTL tube amp without compromising sound quality!

Yeah, I don’t think I would mix the two ….  Those speakers need solid state bruisers to extract all they can deliver 


Years ago at Goodwin’s High End in Waltham MA I listened to  ML Statement system driven by Boulder Amplification and it was pretty impressive.   I can not imagine panels working well with any 60 watt tube amp especially one with no OPT

Agree with the comments citing a classic case of inappropriate amplifier-speaker impedance mismatch. Impedance load is more critical here than speaker rated sensitivity.  These amplifiers are very fine but are not intended to drive lower impedance speaker loads. Pair them with an appropriate speaker and they will perform at a high sonic level.


Autoformers do affect the sound of your system! I am not saying that they are not an option and may be better than the wrong load on an OTL amplifier but they do affect the sound. How could they not as they are a complication/break in the chain?

I am a fan of sensitive speakers and low powered amps....   While I think ML sound very good it's not really my thing.

THE most important match is the speaker / amp interface.    It is irrelevant of price.  I proved this when using high quality amplification with modest Klipsch kg 1 bookshelf speakers .  They sounded so good with a good tube preamp and amp that you would have never guessed I paid $50 for them.  It was the electronics that made them sound way better than they had a right to 

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Hi Guys,

Thanks for your input! Indeed I know that the MLs are demanding when it comes to impedance, but I for sure has a learning curve ahead of me when it comes to OTL and Tubes in general. I guess I didn’t expect the same energy and presence in the room with only 60 OTL watts, but now I learned what is what. At least it was obvious that the MLs are designed in a way that they demand something else. Today I swapped the M60s in for a pair of MBL N15 monoblocks. These are designed to attack MBLs omnidirectional speakers and have plenty of power under their hoods to cater for Extremes etc. Going to be interesting to see how the ML fare under their command.

  • Analog toroidal transformer with protective shield
  • Weight. Ca 40 kg.
  • Continuous power (<1% THD, 8/4ohm) 352W / 615W
    Dynamic power (<1% THD, 8/4/2/1ohm) 402W / 786W / 1.50kW / 1.0kW

Some sort of Class D.


@andy_wonderwall You've gotten good advice here. Martin Logans are traditionally hard to drive for any tube amp due to the 0.5 Ohm impedance at high frequencies- so an amp like the M-60 will sound muffled. You can fix this by using a set of ZEROs

-which will be helpful with most tube amps on that speaker. MLs are designed to try to take advantage of the solid state market; the problem is the ESLs in general don't behave the way the usual driver in a box does. ML tries to get around this 'problem' (which normally is solved by a tube amplifier) by making their speakers very low impedance. This prevents the usual doubling of amplifier power that a solid state amp might otherwise do. It also means that speaker cables are critical to the sound; how 'open' the speakers are.

Other than the ZEROs, your other solutions are obviously a different amplifier, or a different set of ESLs that are not so low impedance. For example M-60s work quite well on ESL57s or ESL63s made by Quad (and several of the newer Quads too).

We do have customers using MLs, some for decades on now. All of them use the ZEROs.


Thanks for reaching out. 

First all of. Let me tell you the M60s sounded just fantastic with a pair of Vivid Kaya S12 in my local hifi shop. So intimate, wonderfully vocals. Organic sound. A midrange to die for. Got me intrigued. Maybe the Zeroimpedance could be one way to solve the problem. I know MLs are hard on all Tube designs. People who have been successful on large MLs seem to be those who have invested in some heavy "hybrid-tube" stuff from ARC or VTL (like 250watts and above in strength). How do you think the ma-2 would handle the ESL 11? Zeroimpedance, still to be required?

Kind regards



Zero auto transformers may work BUT they also do change the sound. Definitely not as transparent. Yes I've heard them in a system.

Why do people keep trying to jam mismatched components together? If you want ML speakers then you need a big brute solid state amp. Why fight it?


As have I. Evidently certain members believe that if they cant hear something it doesnt exist. Problem is that this person gives advice which might be taken by others without due diligence. 

@andy_wonderwall no worries, and sharing some of the same, probably should have mentioned I run custom made hybrid electrostatic speakers on dual mono tube amps myself. 6ohm, 93db sensitivity would seem easy to drive. Not as much as I hoped with prior attempts using lower power parallel single ended amps in strapped triode mode. Back and forth I went with higher powered tube and revisiting some capable SS amps to find the best match. Your Atma 60 amps and your 4B3 amps are both on my bucket list. I can imagine your 4B3 amp lights those MLs up. When I put a good SS amp on my hybrid ES speakers, it does open a larger door and deeper room into what a capable electrostatic speaker can do.

If I had your Atma 60s I’d be fast on the hunt for a good matching speaker just to be able to truly appreciate and hear those fantastic tube amps. Your 4B3 solid state would be a nice match for my own speakers, and have kept that on my favs list to acquire as a future keeper amp. Lucky challenges to have, you have great taste, happy listening to both :)

ML's + Zero auto transformer = Very good sound. Used that set up for years.

BTW... same PP 40w tube amp + ET LFT8 = similar sound but no autoformer required.

My 11As sound far better with the Lux 509X than they did with ARC Ref 75/3 combo. Albert Sportis turned me on to this synergistic pairing after he told me of his extensive experience. Great guy.

Too bad more audiophiles don’t investigate the suggestion of @ticat, myself, and a few others, and look into the Eminent Technology LFT-8b. A planar-magnetic loudspeaker, in some ways very much like the Magnepan, but in other ways dissimilar.

The LFT-8b is more like an electrostatic than are Maggies, in both sound and power amp appropriateness. No offense to either Magnepan or owners of their outstanding loudspeakers (I have owned four pair, one currently). But if you want a planar loudspeaker that will mate with the Atma-Sphere M-60 (or generally any tube amp), the LFT-8b is a great option. It’s modulus-of-impedance is a very even 8 ohms, the p-m panel itself (which sits atop a sealed 8" dynamic woofer for the reproduction of 180Hz down, via a second set of binding posts) an almost ruler-flat 11 ohm load, ideal for all tube amps, none more so than an OTL such as the Atma-Spheres (with which I am very familiar, having used a pair of M-60’s with my QUAD ESL’s, a/k/a 57’s).

Despite very positive reviews in The Absolute Sound, Hi-Fi Choice, Hi-Fi World, Hi-Fi Pig, Positive feedback, and from Harry Weisfeld of VPI (who declared the LFT-8b to provide the best midrange reproduction of any loudspeaker he has ever heard. The ET LFT driver reproduces 180Hz to 10kHz, with NO CROSSOVER!), U.S.A. audiophiles continue to ignore the best value loudspeaker (imo) of any on the market.

Part of that blame can be attributed to the low number of Eminent Technology dealers (which pales in comparison to those of Magnepan), but E-T will ship you a pair to audition in your system. Some E-T owners prefer the LFT-8b to not only the Magnepan MG-1.7i (I have heard both---it’s not even close), but also the MG3.7i, which retails for more than twice the price of the LFT-8b.

I expect this post will be ignored as have been all like it in the past. It’s your loss, not mine ;-) .


Thanks for good advice. And yes. Both types of amplification, stellar in their own way. If I ever would have the budget to have a smaller separate system the M60s would be on the list together with speakers suited for the technology. But for the moment I am focusing on a large scale system with the ML´s 11s as foundation, (alternative ever the larger 13s one day). So I need to adopt suiting amplification. The 4B3 is an excellent partner! That SS has a lush sweet bold powerful sound together with the MLs for sure. Could definitely be my end game. But what would this hobby be without setting new personal frontiers? Another mountain to climb :-)? I am lucky to have a dealer that are willing to borrow me MBL N15s monoblocks so that I can compare different technologies on the MLs.


Thanks for advice. 

These speakers sounds interesting. Will not break the bank either. 

Its a shame no dealer in Sweden carries them. Would be interesting to listen to them to see how they compete (in my ears) to a pair of large MLs.


@andy_wonderwall have you tried pairing up a really good tube preamp in front of your Bryston 4B3 amplifier?  If not, this can yield another very nice result too.  What  preamp are you using with your Atma M60 mono tube amps now?

@andy_wonderwall -

' I am lucky to have a dealer that are willing to borrow me MBL N15s monoblocks so that I can compare different technologies on the MLs'.


The MBL's should be able to drive just about anything, but if I were to get them, I would just get a pair of Radialstrahler's.



Andy, Roger Modjeski designed his Music Reference RM-200 tube amp specifically to deal with low-impedance loudspeakers. When they occasionally become available on the used market, they fetch around $2,000 for the original version, a grand more for the Mk.2. A pair of KT-88 tubes are used to create 100 watts (per channel), into all impedances. The RM-200 is a VERY unusual tube amp.

In his original review in Stereophile, Michael Fremer placed the amp in the mag's Class A Tube category, and in a follow-up review found the Mk.2 iteration to be an even better amp. It has remained his "moderately"-priced reference tube amp for over a decade, and he recently declared the RM-200 MK.2 to be one of his most-cherished hi-fi possessions.


A Theoretica Applied Physics Bacch SP-adio. It is a crosstalk eleminating sound processor with a DAC doubling as a pre-amp. Works gr8 for me. But combining with a nice tube preamp is also something to consider! 


The pair I am borrowing are a pair of demos. Lets see at what price point they are for the moment.

…and you are right. They probably can handle any loads. Sounds terrific and have outgunned my beloved Bryston in most areas, Ive learned during the course of this day. 

100W per channel from a pair of KT-88s? Doesnt seem possible even for a quad of KT-88s.

Good observation, @audition_audio. Just one of the things that makes the RM-200 as unique as it is. And he gets 100 watts out of a pair of KT-88’s not by driving them hard (which of course shortens their life, something Modjeski was very much opposed to), but by creative engineering. How he did it can be discovered with a little research.

Modjeski often said he only introduced a new amplifier when he had something new to offer, not just to fill a market need. He gave talks at two or three of The Burning Amp Festivals in San Francisco, sharing the stage with Nelson Pass, one of the few of his contemporaries he held in high regard. Those talks were videotaped, and may be found on YouTube. Essential viewing!

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