Amps Atma-Sphere M-60 Mono blocks OTL design

I just purchased a used OTL Atma-Sphere M-60 mono blocks that I have sent to Atma-Sphere to be upgraded to the current model 3.3 and I also added the option of a higher quality power supply and V caps.

I have sold my old trusted Eggleston Andra 2’s speakers and have built some monitors using Aurum Cantus ribbons (102db) and Aurum Cantus midwoofers (90db) that are both rated as 8 ohm nominal. I have a DEQX Premate and will be crossing over to (2) JL Audio F-113 subs at 80hz.

Currently I am using a solid state high power stereo amp (Pass Labs) that I used with the Andra 2’s.

The Atma-Sphere M-60 is rated at 60 watts class A and is said to work better with higher impedance loads.

It will probably be a few weeks until I get the M-60 and was hoping someone could provide opinions of what to expect.

I listen to late 60 early 70’s classic rock music mostly. Sometimes loud.


In a word......musical! I owned these then went to the Novacrons. What an OTL will bring to the table is clarity and a presentation that just says "that's right"!
Enjoy and please report back your thoughts.

(Dealer disclaimer)
Funny you should post this thread.
After 3 months of waiting. I finally got my Atma-sphere MA-1 2.2 (upgraded to 3.3) and MP-3 put into place and running.
My significant other kept hounding me-'This better be worth it'.

Well, after 30 seconds, even he noticed the improvement.
Everything was clear and defined. Drums, horns, piano, you name it, just sounded right- Alive!
I always thought my system sounded good. Now it sounds great.
I have to email Ralph and tell him, but your post got my attention and I had to reply.
Those Egglestons are really nice. A quick google shows they dip down to 4 ohm. The M 60 should be up to the task, but I would ask Ralph if you should get some Zero's.


Thank you so much for your comments. So with my set up these Amps should work fairly well?

My thinking is the Atma-Sphere M-60's will not have to power anything below 80hz and with my present 90db+ speakers they should work really well?

Did either of you have the V cap and power supply upgrade?


Unless your speakers are very efficient (e.g. say 94db+) you will most likely want to get a set of Autoformers from Anti-cables to boost the impedence of your speakers to reduce the load on the M60's.
ozzy that's the only way we have ever ordered any Atmasphere gear, with both the V-cap and Power Boost upgrade.

As for will it work well with the speakers you describe? Ralph would be the best to answer that.

(Dealer disclaimer)


Thanks for the info.

You really think I will need the Zero's if the M-60's will only need to power 80hz and above?


Can you comment on the V caps as to how many caps are involved, and what exactly is the Power boost upgrade?

You said the speakers you have built are 8 ohms. Assuming that there are no widely varying impedance dips below 5 or 6 ohms (particularly in the mid to upper bass region) I don’t see the necessity of using the Zero transformers.  Someone above mentioned a 94 dB sensitivity as a cutoff point and I disagree with that.  Zero transformers are used to increase speaker load impedance as seen by the power amplifier.  It has no meaningful bearing on the speaker sensitivity. 
@ozzy, the M60s are great amps and they should be a super match with your new speakers WITHOUT zero transformers. Congrats. @charles1dad is correct, it's all about the impedance match not about bass (sorry Meghan Trainor!) 

FWIW, IMHE those much praised zeros can cause a greyish, whitish haze over the music that really ruins it for me. I would never make them part of the solution. There are plenty of speakers better suited to OTLs, you shouldn't need them. Cheers,
Hi Ozzy,

I took a look at the photos in your system description thread, and also at the very comprehensive technical info provided at the Aurum Cantus site

My first comment is that it looks like you’ve done an extremely impressive job of assembling a very sophisticated system!

It appears that your G1 ribbon tweeter and whichever of the Aurum Cantus mid-woofers you are using have benign impedance characteristics, which don’t descend significantly below their 8 ohm nominal ratings at any frequency, and don’t have highly capacitive phase angles outside of the deep bass region (in the case of the mid-woofers) which they won’t be handling anyway. Also, it looks like the external crossover you’ve built consists of second order LC filters (low pass for the mid-woofer and high pass for the tweeter, of course), with a resistive pot in the high pass section to pad down the tweeter’s sensitivity to match the mid-woofer’s. I don’t see any of that as presenting any problem for the M-60, used without Zeros.

Also, while most of the energy that is contained in most music most of the time will be present at frequencies that will be handled by these drivers, as opposed to the subs (my understanding is that 350 Hz represents a rough rule of thumb for the frequency at which most music most of the time has equal amounts of energy above and below), the 60 watt capability of the M-60 into 8 ohms should get you to peak volume levels in the area of 101 to 104 db or so at a 10 foot listening distance, with a 90 db speaker that can handle that amount of power without significant thermal compression (as appears to be the case here).

Some further points:

While I’m not totally certain, I think it might be worthwhile to re-do the DEQX speaker calibrations once the new amp is in place. I’m envisioning that the interaction between the amp’s 4.1 ohm output impedance and the characteristics of the cross-over/speaker combination might change the results you obtained with the Pass amp, and its near zero output impedance. Although I’m not certain of that, as I said. Fortunately your DIY speakers are not multi-hundred pound monsters, and should be fairly easy to move, if that is necessary to minimize room reflections during that procedure.

Second, keep in mind that the interaction between the 4.1 ohm output impedance of the amp and the 8 ohm speaker impedance will reduce gain by a few db, relative to what you have with the Pass amp, and will require a corresponding rebalancing of the speakers with the subs.

Finally, I note that the frequency response plots for the Aurum Cantus drivers show some significant variations within the frequency ranges they will be handling, such as a rise of more than 5 db in the response of the tweeter in the top octave, and significant rises in the upper mid-range/lower treble responses of many of their mid-woofers. The DEQX should be very helpful in providing whatever corrections may be needed for those variations.

Best of luck. Regards,
-- Al

charles1dad, sbank, almarg,

Thank you so much for the comments and with the information that you have provided. You guys are the greatest and I really appreciate the time and effort provided in your posts.

Some more info.

My final crossover point between the midwoofer and the tweeter is now [email protected] 12db. This is based on trying to match the advantages of both of these drivers. I went to all Mundorf silver, gold, oil caps and Janzten foil inductors. I also ended up using Mundorf extreme resistors with fixed resistors points and not using the variable L-pad. (I will post new pictures in my systems page tonight.).

And, I will redo my DEQX settings once everything is done and burned in a little. I am getting fairly good with the DEQX and have compiled some easier to follow operating instructions than what is provided from DEQX.

Again, thank you for the help.


I just posted some more pictures of the DIY speakers and my final external crossover on my systems page.


Ozzy, I did not opt for the upgraded caps or power boost. My amps are vanilla MA-1's upgraded to 3.3
As things 'burn in', it just keeps getting better.


Did you find the tube heat to be a problem? My Pass Labs Amp gets very hot to touch.

I won't be touching the tubes but I wonder if I will have to listen in my skivvies with the M-60's 24 tubes.


I agree with Al,  what you've assembled looks impressive and I am sure sounds equally so. Curiosity leads me to ask how the music presentation differs in going from the esteemed Pass to the highly regarded Atma-Sphere mono blocks? Congratulations on a terrific audio system. 


Thank you.

In the end I am not rich and after a fair comparison period, I will sell off one of the amps.


Believe me I understand.  These two amplifiers are of such different design and topology and I'm always very interested as to what someone hears in comparison. 
I do not think the ZEROs will be useful on this speaker.

The ZEROs should always be regarded as a problem solver (and for any amplifier). The problem is that the amp sounds worse without them, in a nutshell. If the amp sounds better without them then it should be a no-brainer that you don't need them.

M-60s do quite well on an 8 ohm load- the only issue I question is whether 60 watts is enough power, but since this system also employs a subwoofer and crossover, I don't expect that to be an issue.


Thanks for the comments. My Pass Labs amp is rated at 350 watts A/B rms per channel but I think it only puts out 30 watts in class A. Supposedly, once the meter starts to move it is leaving class A for class B.

With my present DIY speakers, the needle does not budge, so I'm thinking I may be only using less than 30 watts.


The Atma-Sphere amps do get hot. In fact, they raise the temperature about a few degrees 2 feet away (I have to relocate my thermostat to the opposite side of the wall now). Though I have opted to mount them on the wall rather than a rack, so they get lots of air circulation.
Do they warm up the room, not really, but it doesn't get cold as fast as it used to.
The M-60 is spec’d as consuming 300 watts per monoblock, and since it operates in class A it will draw that amount of AC power all the time (when it is being operated, of course). And it will convert all of that power into heat, except for the amount of power it sends to the speakers, nearly all of which will be converted into heat by the speakers anyway.

So assuming the spec is reasonably accurate, which I’d expect since it is one of Ralph’s products, the amount of heat introduced into the room by the two amps will correspond to 600 watts. About the same as the heat that would be introduced into the room by six 100 watt incandescent light bulbs.

Best regards,
-- Al

gdnrbob, Al,

Thanks for the additional information.

Looking at my Pass Labs X350.5 specifications, it  lists power consumption at 600 watts temperature 53C.


Hi Ozzy,

I wouldn’t infer too much from that 600 watt number. The amp only operates in class A up to "40 watts peak," and beyond that output power level its AC draw will presumably vary with the dynamics of the music. Also, given the amp’s power ratings of 350 watts/channel into 8 ohms (700 watts total output) and 700 watts/channel into 4 ohms (1400 watts total output), obviously it would be drawing considerably more than 600 watts from the wall outlet when supplying anything approaching its rated output to the speakers.  (It can’t put out more power than it takes in, or anything even close to what it takes in given that it is not class D).

Given your earlier mention that your use of the amp has been such that it has rarely if ever left class A, FWIW my rough guess is that the amp has probably been introducing something like 300 to 400 watts into the room.

Best regards,
-- Al

BTW, Ozzie, I suspect that the X350.5 spec of "40 watts peak" for the point at which it leaves class A operation is equivalent to only 20 watts based on the usual conventions for defining amplifier power (corresponding to volts x amps with each defined on an "RMS" (root-mean-square) basis, rather than a peak basis).

So you’ve probably been using very little of that amp’s power capability.

-- Al


Thanks. I just got news that the used M-60's that I bought site unseen have arrived at Atmasphere for the 3.3, V Caps, power supply and extra binding posts (for Bi-wire) upgrades.

So maybe in a week or two I'll be able to try them out.


I am pretty sure you will love those amps.  One of the only equipment sales I regret is a pair of  the M-60s.  Luckily, I now have more efficient speakers and I got a good deal on used S-30 w the V-cap and power boost mods.  Love it.  An added bonus is having the best customer support in the business available to you.  


Thanks for the posting. My speakers are DIY so I only know the individual driver specs. I hope they are an easy load for the amps.


ozzy- FWIW, mine are too, although to be correct, they are else ;-).  The S-30 drives Feastrex quasi-full range field coils, reportedly about 94dB, crossed over at about 60 hZ to a Class D Crown for the low end.   So it is an easy load.  When I had the M-60s, I was using Merlin VSM-MMs. 
You're moving from very high power transistors to moderate power OTL with very high quality part upgrades. Keep us posted on this quite interesting comparison. I'm curious about what contrast you may hear.


I would have thought that the Merlin's would have worked well. I will be using (2) JL Audio F-113 subs crossed over at about 80hz.


I know what you mean. Just to keep everything fair, I bought another (same) power cord, another amp stand (Grand Prix) then I had to buy new isolation pads for the stands for the lighter load.


I have a pair of original Atma-Sphere Z-Music Transformers, if you or anyone else needs them. They will change the output impedance to whatever you need it to be for your speakers.


Ozzy- It was my bad; I was seduced by the warmer sound of a Joule-Electra VZN-80 but once I went to a better pre, I regretted it.  Eventually sold the Joule amp, too. 
Post removed 
To All,
I'm going to ask a question that I think is relevant.  A little less than a year ago I purchased the latest (3.3) version of the M-60's, without upgrades, directly from Ralph and have been a very pleased listener, using a little tube rolling. This my first venture into tube amplifiers, after starting with a couple different tube preamps, and, again, I'm very pleased with these lively amps.  My question is related to the three available upgrades: V-caps, power supply, or Caddock resistors.  Can anyone weigh in on the individual/relative benefits of the upgrades for these amps?

If the question is which of the three options is the most impactful, I’d say addressing the power supply. Power supply quality is foundational and as a result mandatory for high quality sound performance. The other two upgrades are important contributors to good sound, everything seems to matter (of varying degrees) when  it comes to audio products. But again power supply would be my initial step.

Obviously Ralph can provide expert guidance as he both designed and built your amplifier.


I too have searched for info on the upgrades. What Ralph told me was that the best bang for the buck is the cap and power upgrades. The Caddock resistor upgrade is an improvement but rather costly compared to the other upgrades.


Thanks to charles1dad and ozzy,

I, too, spoke to Ralph.  I hope I won't misquote, but he told me that the cap upgrade would improve transparency and that the power upgrade would improve smoothness at higher volume levels.  He said that the Caddock resistors would reduce "electronic haze".  I don't know if there is anyone else out there who could post about actual related experience.

Ozzy, I'm looking forward to you posting about your own listening experience.

As an owner of 'vanilla' MA-1's, I can say they exceed all my expectations. I can hardly believe that he can 'improve transparancy' or 'smoothness'. But, now you got me thinking maybe I should....

Finally got the Treo's installed and with the Atma-Sphere amp and pre, they sound wonderful.

Well it seems that my used M-60's that are at Atmasphere getting updated to the current model did have the power supply upgrade and the V-Caps previously installed. However, the V-Caps were the tin foil version not the latest version with copper. So, I still decided to go with the latest copper version, but I asked for the tin foil caps to be returned with the unit.

Looks like it won't be shipped out till next week. Darn...

Your patience will be well rewarded.

With V-caps and the 3.3 upgrade, I also opted for having dual speaker binding posts installed on each unit. My DIY external crossover was made so that each driver is independent of each driver. I thought that would be a nice add on. Ralph thought so also.

Ralph told me that this Friday they will be playing my M-60’s. I’m hoping that means its at the last stage and will be shipped soon.

I feel like a kid waiting for Christmas!



So you're the one keepin Ralph and his crew busy!  I'm waiting on an MP-3 that is being updated to 3.3.  The wait is torture!  I finally found a unit on the used market, but it was a very early model. I did get lucky, as the seller was in Minnesota, and kindly offered to drop it off. Its been a few weeks now, but I'm sure it will be worth the wait.  It will be replacing an AtmaSphere UV-1, and feeding a pair of M-60 3.1 driving Vandersteen Quatros.  I cant wait.  Though now I'm probably gonna want to send in the amps for updates... 
Bigboltz, do the M-60's have enough power for the Quatro's? I ended up going with the MA-1's because I thought it would be a bit too much for them.
BTW- My MP-3 took a while to complete, mostly due to the remote option, but as I said to ozzy, your patience will be rewarded.


Sorry, I'm waiting also. It probably will be a few more weeks before I get them. Hopefully sooner, but they are waiting for some electrolytic caps to arrive.


Thanks, I don't know about you but its hard to be patient when there are several if's that need to be explored. Such as; will it have enough power for my DIY speakers? I think so, but until I can place it in my system I can't be sure. Will I like the sound quality more than my Pass Labs equipment etc.? We'll see.


ozzy took a look at your pics nice job on the new speakers! Congrats on your new amps also, I've only heard atma-sphere amps once, at the Capitol Audiofest this summer, and the sound was fantastic. I am sure you will be very happy and finding out your amps are more up to date than you thought is a very happy outcome in a used transaction! Hang in there and savor the anticipation!

The M60s drive my Quatros with no problem.  I rarely get past 12 oclock on the volume and its more than enough.  Vandies have a rep of liking alot of power, but are a fairly easy load. 8ohms pretty much strait across. Plus, with the powered subs and hi-pass, no issues.  I thought about the volume control, but I'm already stretching my budget. What do you think of it?  

And I'm really curious about the phono stage. I'm hoping its great, I sold my Allnic 1201 to make this possible.  Tell me I wont be too disappointed!

I heard from Ralph today ( ok, he emailed back cuz I bug him once a week) and he said he's done the initial testing and expects to have it in the system next week.  Maybe my pre and your amps will end up testing together!    Anyway, I can't say enough about the level of customer service AtmaSphere has given me.


Thanks! Believe me I'm savoring...


Though I don't have the Quatros, I will be using subs and just the signal from 80hz and up will be powered by the M-60's. Do you know the crossover frequency of your subs / Quatros?

And, thanks for your update, mine is still on the work bench waiting for parts.



The Quatros require -3db at 100hz.  Ralph is actually building that into one of the outputs for me, so i wont need to use the external crossover(m5hp) that goes between the pre and power amps. Vandersteen has a rather unique way of hi-passing their speakers, whether it be the Quatro and up or model 3 and down used with seperate subs
You might have to upgrade or replace your current preamp that you are using at the moment since you will be missing the finest tubey sound.  If you have 8 ohms speaker that will work fine with M-60 but if you ask Ralph he recommend to use 8 and16 ohms speakers.  High end 16 ohms speaker such Classic with built-in horn mid range and tweeters are pricey.  The only alternative way to gain 16 ohms is through Zero transformers.