Thought on OTL tube amps

Just do these sound/perform compared to tube amps with transformers? Why do you prefer one or the other? Any particular brands of OTL amps you would recommend listening to? You can see my system on my tag here...listen mostly to rock and roll with a smattering of jazz and a bit of classical once in a while. The next stop on my amplifier adventure is an OTL model, and I have no intention of trading it for either SET I currently have, as I'm very happy with both. One of the ways I enjoy experimenting with different "sound" is by switching up the amps. Just trying to solicit some opinions, of which I know there are many strong ones here at A-Gon. Thanks in advance!
don't mean to hijack the thread BUT was wondering why all the OTL amps I know of are tubes. Are there no solid state OTL amps? If not, why not? Just curious and hope someone will answer a question.
Blow a tube with your OTL and you could fry your speakers. Great sound, but some risk if you listen loud, or run tubes till they drop. Jallen
Most SS amps are OTL. Transistors are intrinsically low impedance devices and most circuit topologies don't need an output transformer. Tubes are high impedance devices and most circuit topologies do require a transformer to drive low impedance speaker loads.
I really enjoy OTL amps and have owned examples from Tenor (75w, 75Wi), Atma-Sphere (M60, MA1) and, Joule(VZN 100) and Berning (Siegfried, 270 and ZH 230...this one I use currently).

I have not had reliability issues with any of these or certainly any more than any other tube based amps.

They require careful speaker/system matching...your speakers would likely work well.

There is a transparency and immediacy to the sound that is very appealing to the OTLs they can also do bass and highs very well if set-up and used properly.

I would say that overall they are my favorite type of amp...but like you, I have several (types of amp) and enjoy the virtues of all.
I have yet to find anything that is perfect in all cases.
Macintosh is the only SS amp using output transformers that I am familar with.
"Blow a tube with your OTL and you could fry your speakers. Great sound, but some risk if you listen loud, or run tubes till they drop "

Properly designed OTL amp represent no more danger to speakers then properly designed regular tube or transistor amps.

For example, in many tube amp designs, output tube is connected to resistor which work as a "fuse of last resort". if output tube blows then it takes out this resistor.

I never heard that OTL amps by Joule-Electra and Athmasphere produce any troubles. I suspect that with proper maintaince these amps can work half century or longer - at least I see not a single reason why not?
The Atma-Sphere web site ( has several "white papers" explaining many of your questions. I own a pair of M60Mk2.2's. Absolutely stable, (you can even short the outputs, and switch output tubes while running, though I would not recommend doing so (I have seen Ralph Karsten do it) and I have had no problems. Happy Listening.
Jallen's comment above reflects a popular myth.

OTLs can be as reliable and safe as any other amplifier. Its all in the design. The only OTL design out there that really did have issues is the one that has generated a lot of the mythology around OTLs years ago (and in the case of that particular design, generally known as a Futterman, those stories are often true). The three myths thus generated were/are speaker safety, amplifier reliability and sensitivity to load. I'm only addressing the speaker safety issue here but will cover the others if anyone is interested.

Speaker safety is a design issue and nothing else with OTLs, the same as it is with a transistor amplifier (the concern is what happens if a tube fails/shorts).

A common myth is that OTLs run the same high voltages that regular tube amps do. This is false- if you think about it, the fact that the amp has no output transformer means that the tubes have to generate voltages and currents that are normal for loudspeakers, to do this requires a tube-type that runs at much lower voltages and higher currents. For example in our amps the B+ is only about 140Volts.

All it takes is a little testing and experimentation to find the limits of any OTL design, so its easy to set up the amplifier with a scheme to protect the speaker in the event of a tube failure. Otherwise the company goes out of business. I hope this is obvious for what it is: common sense! After over 33 years of business, its safe to say that we have built more OTLs than any other manufacturer so I think we can agree that if there was a problem it would have manifested by now :)
I have also owned and auditioned a variety of amps. Solid state; Classe, Krell, Gamut, Plinius, Threshold... Hy-bred; Lamm... Tube; Jadis, Lectron, Conrad Johnson, Convergent Audio... OTL; Atma-Sphere, Joule. I have found, as with nearly every piece of audio, gains and concessions in each approach. Presently, I find the Joule amp to be fantastically musical, yet capable of reaching the frequency extremes. Bass nearly as deep as Classe and Krell with greater body and density, shimmering highs, plus a near CAT midrange (not quite as lush as the Jadis amps). These amps, both the A-S Ma-2's and Joule Rite of Passage, have adequate power to drive the power hungry Avalon Eidelons to a very high listening level, which I require.

I will respectfully disagree with Jallen regarding damage to speakers from blown out tubes. With both the A-S and Joule amps the internal protections have arrested any harm from reaching the drivers on my speakers. I too witnessed removal of an output tube from an MA-2. Not to be done on the Joule, though. The Rites of Passage require all output tubes for proper function.

I have much enjoyed the few solid state amps, especially with tube preamp, but ultimately found the midrange less fullfilling. The Jadis along with the other tube amps left me wishing for more slam and top extension, although they likely would have benefitted from additional tweaking.

The OTLs have come closest to achieving what I demand from the particular system I now own. Do keep in my mind, they run very, very hot and devour electricity. Certainly more than any other amp I have owned. As is typical, the associated components, listening space, and your tastes will best dictate whether these amps prove satisfying.

Stan, in addition to Mac, the Sansui BA-5000 solid-state amp used output transformers:

Note the weight of the amp. Interesting because Sansui began as a transformer manufacturer.
FWIW,I've owned an Atmasphere S30 for 10 years almost since it's introduction currently updated to the latest 3.1 configuration.
Over that time there have been no significant mechanical/reliability issues. IMHO, these "OTL concerns" date back to the Futterman amps and their clones. Unfortunatly, modern day OTL manufactures are unfairly lumped in with them.
AFC, I have not heard the ZU Essense but on paper it's a match made in heaven with the S30. With the Zu's minimal crossover there would be very little between the output tubes and the Z4 driver except the speaker wire.
Have fun.
There are some very nice options to choose from on the OTL amp market these days. Atma-Sphere, Berning, Joule Electra, and Transcendent Sound to name a few. All of which are very reliable. It's rumored Roger Modjeski at Music Reference is working on a new OTL design. In addition, there are some great speakers that are OTL friendly, including Merlin (Bobby P. uses a Joule Electra set-up at the audio shows), Zu, Audiokinesis (Duke uses Atma-Sphere at the audio shows), Classic Audio, and Tonian Labs.

I myself own an Atma-Sphere S-30 that I use with my Audio Kinesis Jazz Module speakers. The S-30 was actually used to voice the speakers. I also own a Berning Micro ZOTL which although 1 watt, can drive my Tonian Lab TL-D1 speakers nicely in my small room.
Thanks all. What are the purported advantages of OTL amps over those with transformers, aside from eliminating possible transformer hum and obviously making the circuit more "pure" in a sense? Hum isn't an issue with the MastersounD or the Frankensteins I have. I'm just looking for ways to get different sound, see what I prefer with different types of music.

Anyone have any experience with McAlister amps? Plenty powerful, and a unique build, using old television horizontal sweep tubes rather than standard power tubes. Claims to greatly extend tube life, since the old TV tubes tolerate much higher plate voltages than do standard audio power tubes. They also have one that's tremendously powerful- 195 watts per channel. Pretty salty for a tube amp. They also have a very, very attractive price point.
With my speakers, Merlin VSMs, the Atma-sphere M60s have proven to be the best all around amps for my taste, up against a few amps I have had in rotation in my system, the best of them being the CAT JL2, Pass XA30.5, First Watt Aleph J, Music Reference RM Special Edition, and Quicksilver V4s - that is, for me the OTLs produce the best sound up against some of the very best tube and SS amps made - though the Music Reference is right up there, and have no doubt that with more difficult speakers might edge out the OTLs, but with proper matching (not hard to do) OTLs are pretty tought to beat. I would put the Joule OTLs right up there with the Atma-sphere, you cannot go wrong with either IMHO.
A good OTL has the immediacy and presence of a SET amp while generally providing even better bass response. You will also hear a lot of people using the word transparency. OTL's have a liquid yet very neutral and open sound. I love both OTL and SET amps. I own an Atma-Sphere S30 as well as 45, 2A3, 300B and GM-70 SET amps. I would not part with any of them. To my ears the SET amps colour the sound more than an OTL does but the sound is so gorgeous I don't care to be a purest. I find I use the S30 OTL and GM-70 the most. They have more power than the smaller SET's and I find they are also better balanced - not unduly emphasizing the midrange like the 300B does (which can be wonderful in its own right depending on my mood). I think all audiophiles should experience both a SET and OTL amp as a right of passege during their audio journey.
I'm sorry, but OTLs can't begin to match SET amps on the right speakers. That being OTLs on the right speaker as well.
If you would address speaker load, I for one would be interested. I can express my preconceptions but it would be better just to have the subject addressed in a broader way. I can say that I would love to try some OTLs on my speakers (four way, horns for top three and isobaric woofer configuration on bass). Efficiency for woofers vs horns is about 8dB different and I am pretty sure the high three are padded down the crossover. I think the mids and treble (104dB) have a relatively high impedance but I expect the isobaric bass (96dB) may dip as low as 4 ohms (I remember reading that somewhere). I need about 30W or what I think is significant damping on the bass. My 13W EAR 859s just don't cut it on the bass, and my 8W AudioTekne PP 2A3 amps produce almost no bass.

Would an autoformer allow the low-power tube amps and an OTL to work?
Almost all of the OTL amps I heard had an incredibly appealing immediacy and liveliness that solid state never has and most tube amps also also lack. That comes with a bit of a touch of roughness with certain recordings (particularly classical music). Several models I heard also seemed to have a bit more tube noise than other kinds of tube amps that might be an issue with extremely efficient speakers (a friend sold an M30 because of noise with 107 db efficient speakers. I thought that was a mistake).

A really good SET amp can also have similar kind of liveliness and immediacy. But, low powered SETs are much more limited in the models of speakers they can be matched. Although OTLs are expensive because of the amount of wiring needed for the multiple tubes, really good SETS can be even more expensive because they demand a top quality output transformer. For the vast majority of applications, I like OTLs.

Personally, I run 99 db/w speakers with a parallel 2a3 SET (two output tubes per channel), but, I don't play my system that loud.
Most OTL amps to me sound lean. If you like this transparency you can have it. Go listen to live music then you will know.

Jwm you mention that most OTL amps sound lean to you. Have you listened to any of the Joule Electra models? It seems that Jud Barber has somehow managed to capture the harmonic realism so apparent with most SETs without any loss of transparency, the best of both worlds.

In my experience the perceived limitations by some concerning OTLs rest elsewhere in the system providing the speaker impedance/amp are a good match. A poor recording will really highlight problems more so because of the wide bandwidth performance of OTLs.
Here is the issue with OTLs and speaker loads. There are two major concerns. The first is this:

In short you want a speaker that is designed for tubes. The second is that the smaller you make the OTL, the more it will want higher impedances to really perform right.

A lot has to do with the circuit of the amp and you should check with the manufacturer on this, but in general you will find that the triode-based OTLs can driver lower impedances as their output impedances are lower. Now this brings on a discussion of loop negative feedback, which is a common method of lowering the output impedance on any amp, but here is something everyone should know:

The addition of negative feedback will not allow the amplifier to make more power into lower impedances.

This is because loop feedback really doesn't change output impedance! If it really did, feedback would give you greater power at lower impedances, and it doesn't.

There is an additional factor that comes into play here. Put plainly: There is no argument for 4 ohms or less in high end audio, if **sound quality** is your goal. No matter what kind of amplifier you have, transistor, class D, transformer-coupled tube amp, SET or OTL, the simple fact is that all of them sound better at higher impedances simply due to the fact that they will make less distortion in one way or another. In the case of transistor/class D amps, you get less odd-ordered harmonics, which BTW the ear finds to be very objectionable. In the case of all tube amps, you get less lower-ordered harmonics (2nd, 3rd, 4th) which the ear interprets as 'bloom', 'warmth', 'thick', etc. these lower orders mask detail so if they are gone the system is instantly more transparent without additional brightness (brightness is associated with the 5th, 7th and 9th harmonics).

So the bottom line is you want a higher impedance in the speaker if you can get it, all other things being equal.

Now OTLs, while having an economy of scale, do not suffer the way most other amps do when you make a bigger version. If the designer has done his homework, bigger OTLs will never loose PRat; they will be just as nimble as a smaller amplifier without loosing detail. The other advantage of OTLs is that if built right, they do not suffer crossover or notch distortion issues at the zero crossing point- the only other amps like that are SETs. The point here is that you can get a bigger amp to drive lower impedances if needed.

Now with respect to the impedance curve of the speaker: it is a common myth that OTLs need a *flat* impedance curve. That's not quite true- if the speaker impedance is too low for the amp, then yes, that's a good idea (and it usually is anyway). But otherwise, what an OTL is **really** looking for is that the entire impedance curve be high enough- often that is a lot more important than a flat impedance curve.

A good example is the Sound Lab ESL. This speaker has an impedance curve based on a capacitor and some inductors and has nothing to do with box resonance. This is why transistors have such a hard time with this speaker- its as high as 50 ohms in the bass and only 1.5 to maybe 3 ohms at 20KHz. That's quite a spread. Now a transistor amp can make lots of power in the highs, but hardly any in the bass. So they tend to be bright with no bass or just seem like they can't drive the speaker. You put an OTL on there and suddenly the speaker has bass authority and extension that the transistors could not do, and with an OTL that might be 1/3 the power of the SS amp will make the same volume.

There is an outboard device called the ZERO that is an excellent problem solver for tube amps (OTLs in particular) if you simply *have* to use a 4-ohm speaker. The most common question we get is 'doesn't that obviate the OTL quality?' and the answer is No. The ZERO loads the amp at 16 ohms and has only provisions for 4,3 and 2 ohms. The turns ratio is so low that it has bandwidth from 2Hz to 2MHz, exceeding the bandwidth of nearly any amp its hooked up to. It also does not block DC as its an autoformer. This sort of thing requires that you have an amplifier that already has a fairly low output impedance- you could never do this with a conventional output transformer!
Ralph, thanks very much for the highly informative post.

I have one further question. In this thread, and I believe in another current thread, you alluded to PRaT as something that can be designed for, within the constraints of certain tradeoffs that depend on the type of amplifier. Could you elaborate on what technical factors and design characteristics affect the PRaT of an amplifier? That's something which has never been clear to me. Thanks!

Best regards,
-- Al
I said yes to most, but you are correct the Joule is not lean I could live with this one.
Sure. The issue is this: there are human hearing perceptual rules and there are the specs that we see on paper, and surprisingly they have very little to do with each other. So if you design your amp to look good on paper, it at best will sound like a good hifi.

What we really want is for it to sound like music. For that, we have to get the circuitry/stereo to obey as many of these perceptual rules as we know about. This often results in amps and speakers that don't measure so well. For example, the human ear uses the 5th, 7th and 9th harmonics to determine the volume or sound pressure of a sound. Electronics have the ability to distort these harmonics (enhance them); this results in such electronics sounding artificially louder than they really are. We see this all the time- 90% of the time if an audiophile's wife is screaming them to 'turn that down!!' its likely because these loudness cues exist in un-natural abundance.

You can design to minimize the distortion of these harmonics. The first step is to not use loop feedback, as it is known to **enhance** such harmonics. Of course, then to get rid of distortions you have to use every design trick in the book to try and not make distortion.

IOW what we want to do is engage the human limbic system rather than the cerebral cortex. So in addition to distortion issues, the circuit or system has to be fast enough; if too slow the processing of the sound moves to the cerebral cortex. IOW the experience of the sound becomes intellectual instead of toe-tapping.

This is a sort of basic introduction but I think is also the easiest to understand. Obviously without feedback the THD of the amplifier tends to go up, but THD is a description of a settled-out sine wave and really says nothing about the behavior of the amplifier with a constantly-changing non-repetitive waveform, which is what nearly all music is. The evidence right now is that negative feedback actually *increases* distortion with non-repetitive waveforms- quite the opposite of its intentions.

If you look at this from the field of Chaos Theory, what we find is that the formula that describes feedback in an amplifier is nearly the same as we see in Chaotic systems- and not what we want for reproduction. Chaos Theory suggests that amps with feedback exhibit a chaotic behavior which will include harmonics of the fundamental waveforms up to and exceeding the 81st harmonic! I find it interesting that as the science of math evolves, it is opening doorways to improvements in audio that many people have thought were far too cut and dried for there to be any serious evolution.

The Atma-sphere MA1 amp on my Soundlab A3 speakers is anything BUT lean sounding. I find it to be full of bloom and weight.
"...what we want to do is engage the human limbic system rather than the cerebral cortex "

That should be motto of every audio designer and it is ours, see -

Yet, I cannot agree with Athmasphere that negative feedback is bad.

Our entire method of amplifier design is based on the approach which considers an amplifier as a control system. Because of this, our amplifiers are designed with an emphasis on very, very, very high speed control loops. The ultra-short transit time allows the amplifier to correct for many small errors; and the control loop can follow the input much more accurately. It does not (or very minimally) create odd order harmonics, it does not create pahse discrepencies and other garbage conventional NF does. These electrical characteristics result in a more detailed, transparent sound with less noise and louder yet cleaner musical reproduction.

Its very difficult to compete with OTL in harmonic richness for any solid state amplifier. I admit it. We are doing our best but its difficult. However, if don;t like heat, weight, tuning up, warming time and may be have difficult to drive speakers then - you are welcome to try our (and others) ss amps.....otherwise, everybody would own OTL amp...

Oh, almost forgot - there were a few well deserved praises to Jud Barber (of Joule-Electra) OTL designs but Jud does use very judiciously negative control speakers even better.

I hope that after RMAF 2010, we will be able to add to this discussion of fidelity to true acoustic music we all strive to achieve and have in our systems.

Good Luck to Everybody.

At the risk of re-igniting the long-running debate about David Berning's designs, I would say in response to Spectron that the issues of heat and weight and difficult to drive speakers do not apply to David's amps, certainly not to the extent they do to other OTL's. I know someone with Joule 160's, and standing over them when they are fully warmed up is a good way to sweat off a few pounds in a hurry. In contrast, I can put my hand on the grill right above the power tubes in my Quad Z's and leave it there as long as I want, even when they are fully warmed up. And I have had preamps that weighed about as much as the Quad Z monoblocks.
Yes, let's not get into the issue of whether or not Bernings are or not OTL technically, they do seem to be in that camp to most users, and very fine sounding amps to boot.
The atmasphere amp is a great match for Soundlabs. Soundlabs generally are very full in the midrange and atmasphere goes very well with them.
negative control speakers even better.

Negative feedback cannot be used to control speakers. It can only be used to control amplifiers. The idea that it helps with speaker control is part of the myth of damping factor, a subject that has its own thread on this forum right now.

This myth has heavy subscription by the Voltage Paradigm camp- see the link I provided in my second post to this thread. It is true that higher speeds in the amplifier will reduce odd-ordered harmonic generation when feedback is applied, and it is also true that the amount of this is variable from amplifier to amplifier, and does account for some of the differences that we hear.

Most OTL manufacturers use feedback. I think that we are one of the very few that do not. It is true that this limits the speakers that work with our amps, but it is not a matter of speaker control- usually its a matter of tonal issues related to incorrect voltage response for that speaker. Our philosophy is that if the speaker requires feedback for the amp to work with the speaker, there will be no way that speaker will ever sound like real music as the amplification will be inherently incapable of the task. It might sound like a good hifi, but that is not our goal.

There is no question that this philosophy has been a major marketing problem for the company, and it has contributed to the idea that OTLs are load sensitive (again, see the link I provided earlier). But if you want it to sound real that's what you have to do as its all about human perceptual rules. In the end, this means that there are dozens of speakers that work with our amps rather than hundreds.
Hi Athmaspere.

"Negative feedback cannot be used to control speakers. It can only be used to control amplifiers. The idea that it helps with speaker control is part of the myth of damping factor, a subject that has its own thread on this forum right now. "

Dumping factor is a "myth", I agree with you 100%.

I also agree with you that more accurate describtion of Jud Barber use of negative feedback in his OTL amps leads to control amplifier for....for......for...better sound coming from ... from...speakers. So lets not play with words ( it is not presentation of peer-review technical paper here)

If you do not use negative feedback then its great for you, however, in my eyes its not the virtue either. I will propose that for every technology used in amplifier design there is its own advantages and its own disadvantages. Otherwise we all would use one "perfect" design. Solid state amps have class A, AB,C, D etc. Tube amplifiers have traditional push-pull, OTL, SET etc.

I firmly believe that its not "what" technology you use is important but "how" you use it and sonic results are that count at the end. Obviously, each technology has some advantages specific to it e.g. low weight per watt of power in class D - but by itself its totally useless

Finally, regarding gorgeous OTL amplifiers (and I am lover of them!!!!) I can only repeat you conclusion:

" In the end, this means that there are dozens of speakers that work with our (OTL) amps rather than hundreds." - Sure and when it works its spectacular !!!

In our approach (i.e. control theory, which I believe unique in this industry), we are trying to match not only hundreds but every speakers available to produce musically involving sound comparable to the sound of acustic music.
Sounds like the Spectron approach would lead to broadbase good and the Atma-sphere to a narrower range of as good as it gets - certainly a market for both approaches. With my speakers its the OTLs I would go with, with Thiels or B&W, perhaps the Spectron approach would make more sense.
Hello Jwm,

I just wanted to let you know you're not alone in your thoughts. At least with regards to the brand I heard driving my speaker. Although it does have some wonderful audiophile traits overall the faults I hear ruin the illusion for me.

I must admit I am a little confused here. Two posters have said OTL amps are thin sounding?

Goodness my experiece could not be more opposite. On my Soundlab speakers they have more body then two other great SS amps I have heard on them.Amps that had good body, but the Atma's had more!

I also used an OTL amp on a set of Silverline Bolero speakers and again the sound was rich and full of texture and bloom. My experience with OTL amps seems to be very different indeed.
Granntring, that is how I hear it. We must own one of the several dozens.
"I must admit I am a little confused here. Two posters have said OTL amps are thin sounding? "

You should not be confused. When (OTL) amp and speaker is matched poorly then sound is bad (thin, fat what have you).

When properly build (OTL) amp is match well i.g. Athmasphere/Soundlab then sound is glorious !!!
Hello Grannyring,

As Pubul57 pointed out my opinion is in the extreme minority and yours is in the greater majority. The Soundlab/Atma combo has virtually all their owners "dancing in the streets" so please enjoy as I'm sure it must sound fantastic.

In my personal experience with an OTL amp and my speaker of choice I concluded that 'for me and me alone' the combo didn't click and I was not willing to live with the trade off in sound quality that I heard.

In the end it is the individual user alone who must decide what works best for them and their systems.

I thought this would be so. Anyway, I run from thin sounding gear as my ears find that to be the #1 sonic dislike.
Soundlabs and Silverlines are on the warm side to begin with so the match with your OTL is an obvious one. I heard dozens of other speakers sound thin with your amps and they were not hard to drive in other words above 6 ohm loads.
Your missing the fact that I have owned many top end amps along with the Atma-sphere on these same speakers. So I could compare these other amps to the OTL and note any changes to my music. Trust me when I say I have heard many a great amp and the OTL was always the leader in body, weight, texture and realism. Again on direct comparison to other amps on the same speakers.

Surely if the OTL was thin compared to other great tube and SS amps I would have noticed it relative to the other amps.

Just my experience.
Every amp I've owned, including Joule and David Berning, sounded best with less, not more, negative feedback dialed in no matter what speaker I paired them with.

"In our approach (i.e. control theory, which I believe unique in this industry), we are trying to match not only hundreds but every speakers available to produce musically involving sound comparable to the sound of acustic music." - Spectron

Spectron, so what you are saying is that you want to sell your amps to the broadest market you can even though matching a less or no negative feedback amp with the right speaker produces better sonics for end users.
To Jj2468,

" Every amp I've owned, including Joule and David Berning, sounded best with less, not more, negative feedback dialed in no matter what speaker I paired them with"


".... matching a less or no negative feedback amp with the right speaker produces better sonics for end users "

suggest that not only Spectron but also Joule-Electra and David Burning (and many, many others) don;t know what they are doing by including negative feedback in their amplifier...

I actually suspect that they DO know what they are doing...
but you are free to share with them your experience with "negative feedback" and "no matter what speaker" so they will know, finally the truth.

Also, I beleive Spectron approach is not to use MORE negative feedback (he never said so) but to use ULTRA FAST negative feedback which are entirely different things...............
Its very difficult to compete with OTL in harmonic richness for any solid state amplifier. I admit it. We are doing our best but its difficult.

I went down a similar path 25 years ago. It got a lot easier when we got rid of the feedback. All I can suggest to anyone who has not tried the same is, make sure all your ducks are in a row- run the most linear circuit you can and put it on a speaker that allows you to hear the results without tonal issues. Only then can you see if you really want to go back.

OTLs don't run hot unless, like any other amplifier, they are biased to do so. Heat and good sound happen to go hand in hand; sure, you can get an amplifier that does not make a lot of heat to sound OK but again its the sort of thing that once you try using a little more bias and thus making more heat, you may not want to go back.

Afc, getting rid of the output transformer has nothing to do with hum.

There are several advantages to getting rid of the transformer: wider bandwidth and lower distortion. One advantage that most people don't think about is the fact that because the output transformer steps voltage down, in an OTL that translates to less gain needed. This is why its possible to build an OTL that has only one stage of gain.

The claims of longer tube life with sweep tubes may be true, but not particularly because of the voltages used. The voltages used in an OTL are so low that even conventional audio tubes would have a long life if that tube could work in an OTL.
As a user of OTL's for over 20 years, and now an Atma dealer, I might add that anyone who has heard Atma-sphere amps over the years, should try and here Ralph's latest designs.

As fantastic, detailed, quick and lifelike as they always were, they have simply reached a new plateau with the latest versions.

I just recently added dampers to the output tubes ( I have had dampers on the input tubes for years) I was surprised at the improvement yet again.

I am currently using the Zero autoformer with 4 ohm speakers.

Great tube OTL amps I think are they way off the audio merry go round.

Yes they need to be paired with appropriate speakers (the Zero increases the lot of compatible speakers) but once the amps are chosen, one may now look elsewhere in the system to make improvements, confident that the amps are making much of the potential in the signal.
using an autoformer I think I will use my tube amp with built in transformers
" using an autoformer I think I will use my tube amp with built in transformers"

Totally agree, provided its high quality amp
I just had my Atma-sphere MA1's updated to the latest 3.1 version (from 2.2) plus all options available have been added.

I have got to share that the current upgrades are a revelation for this amp. Wow does this amp now sing. So rich and glorious with state of the art transparency. The stage depth is jaw dropping and the images so 3D and real.

This is my first OTL amp or amp from Atma-sphere and I am taken back now realizing what I have been missing. This latest generation simply outclasses the 2.2 by a wide margin.

Yes, I am pleased to say the least. I have owned many a tube amp and this one has all of that wonderful warmth, bloom and weight combined with SET-like intimacy.
My feeling too in upgrading to M60 3.1. The M60s replaced my beloved CAT JL2s which is a great amp, but with my speakers, which are very easy to drive, I prefered the Atma-sphere - my thought being that OTLs by Atma-sphere and Joule are pretty tough to beat with the right speakers.
I love the sound of the Atma-sphere MA1's on my Soundlabs, but I wish I had a bit more power.Up to 85 db or so they are so sweet and intimate. The bass has great foundation and weight.

My amps are still breaking in and I must remember that. They have the V-caps and I read they take time to break-in. They only have 30 hours or so thus far.

Funny thing is they seem to get more powerful with break-in. I know that seems nuts, but I swear it is happening.
They used to clip at sound levels they now play through? Strange, but true in my current experience.
10-02-10: Grannyring
I love the sound of the Atma-sphere MA1's on my Soundlabs, but I wish I had a bit more power.
Clearly, the answer is MA-2 monos!

What's the sacrifice of more money and more heat when it translates to better sound?!