OTL amplifiers

Can anyone explain to me, in layman's terms, the advantages or disadvantages of OTL amplifiers? I have heard a lot about them, but have not listened to any to date. I am really a tube fan and want to reach tube nirvana and don't know if OTL is the way to go. Are they suitable for all types of music? Any information would be greatly appreciated. Thanks.
OTL amps are a subject I really can get excited about in this hobby.

Advantages include, top to bottom clarity, resolution and tonal naturalness more akin to real music. The reason being there is no transformer that CAN add or detract from the signal. These amps when matched properly to the speaker they will be used with can take you to sonic nirvana like no other amplifying device I have heard including transformer coupled tubes, SET with transformers which typically don't offer the frequency extension of the best OTL designs and ss which with few exceptions doesn't offer the immediacy and presence that seems exclusive to tubes. So the best way to describe OTL's are the best world of both tubes and ss with few if any of the weaknesses with the possible exception of bass "slam" which seems more exclusive to ss designs. Bass resolution and natural impact is of much greater interest to me and is harder to get right than slam (dynamic impact, often exaggerated).

Disadvantages of OTL's are as long listed as their advantages:

Since the output tubes provide all the current to drive the speakers without use of an output transformer, it is important that the impedance characteristics not drop below 4 ohms in any case and ideally should be between 8 and 16 ohms at all frequencies without compromising the performance they are capable of. Unfortunately there are few speakers with these characteristics so use of an OTL is not ideal in most instances. Heat is another problem. They run very hot because many tubes are used in parallel to produce the current necessary. They can be designed for high watts but the price is many tubes and a lot of heat, up to 1,500-2,000 watts for a big pair of mono's.

Dependability has always been an issue with these designs but it seems this has become less so with current designs than in the past. The players include Transcendent, Atmas-phere, Joule, Tenor, Graff and the Berning Quasi OTL. They are all great performers with the right speakers.
...so basing on pricing of OTL amps you can easily get an output transformer with ferromagnetic core taken from the Moon(99.9% carbon-free iron only currently found on Moon) and brought to the Earth to make tube amp much more efficient and cooler running.

bottom line is large buck for the small watt.
Hi DFrigovt:


Output TransformerLess

They name it for what it does not have! Sounds stupid but the main advantage of an OTL is that it does not have an output transformer.

A transformer is used in most tube amps because there are major problems matching tubes to speakers. A tube is a high voltage, high impedance thing (thousands of ohms) and a speaker needs current and has a nominal impedance of 8 ohms or so. These are real problems. You hear folks talk about "power tubes" whcih usually means current. But even many "big tubes" do under .5 amp or so. The output circuit and the transformer creates the power. A transformer steps down voltage and steps up current and matches the amp's and speaker's impedances. Thus it solves some problems. You can see from Tubegroover's answer re speaker selection that the OTL, while gettin rid of hte tranny, has not solved all these issues either.

But the tranny creates problems too. John Broskie over at Tube Cad Journal says "The best transformer is not as good as the worse tube." Basically transformers inevitable cause distortion for a number of reasons. I'd be happy to outline the basics but I think I'd bore you to death.

OTL's tend to be expensive as Mark notes. Bruce Rozenblit's are not to bad though. They tend to be made in small shops so the economy of scale isn't to good and by people who are quite smart (Berning) and believe they deserve to make a decent living.

I remain,

That tubegroover fellow is grooving right! I agree with his post 100%
Clueless and Tubegroover, nice posts! Thanks!
P.S. Ever had a Futterman blow up on you ?
Those were the days...today, as TG has noted, the amps are much more reliable.
OTL bests with ear-speakers. There I see no compromise whatsoever.
Detlof. You had a Futterman blow on you?

I am reading through the Rosenberg book (from the Sherod thread) on Tube Electronics the last part of which is a long presentation (ad for?) Futterman amps. There was a recent thread here re warranties and everyone was impressed with Bryston's 20 years. That's small change.

The good news is that Rosenberg's book clearly states that the Futterman amps carry a "lifetime warranty!"

The bad news is I'm not sure if they meant your life of Harvey's.

You'll enjoy the book. It has a picture of the pressure cooker Futterman used to prepare the varnish to varnish his power transformers which he then "put on the window sill to cool."

Times have changed.

I remain,
OTL's are usually more expensive, have many tubes - expensive tubes (high current triodes), consume a lot of power, may not perform the same with different speakers, and may not even work with some speakers. Those are, in very general terms without a specific make, the downsides (inconveniences, IMO). The huge advantage is the absence of the output tranformer which means the total harmonic distortion is almost non-existent. That's what gives 'em that incredible sound.

I disagree with tubegroover about a disadvantage being that they run hot. That' a characteristic of Class A operation, not the OTL topology.
>>I disagree with tubegroover about a disadvantage being that they run hot. That' a characteristic of Class A operation, not the OTL topology.

I like your posts GS (cuz I usually lern somethin) but tis a bit form over substance no?

OTLs do, as you say "have many tubes" and they do, as you say, "consume a lot of power" which they dissipate as a lot of heat. That is.... they run hot.

I remain,

I would like to mention for your information a few other details. I like you have eclectic tastes in music. Without doubt the most difficult music to reproduce accurately as you might hear it live is orchestral and choral music. The natural layering and individual voices you hear in a great live performance is something that I treasure when I experience it. To build a system that can capture that magic in a home environment is a worthwhile goal. It requires committment, but you can really get close. I wouldn't say that OTL's are for everyone as there are many downside considerations as noted above. All I will add at this point is that you try to listen.

So far as expense goes, Clueless is right about the Transcendent, it costs 2.1K for the T8-LN. It produces 25 watts and is great for a very efficient speaker with a stable impedance of say 6-8 ohms in a small room. I've owned this amp and would recommend it in a heartbeat to anyone that meets those specific requirements. I also believe this amp is an excellent value because it does all the things that seems intrinsic to OTL's if to a lesser degree at a very reasonable price. It is a very serious amplifier.

So far as the Joule goes, I had the opportunity to listen extensively to the VZN 80 stereo amp in my system . At the list price of 8K it appears on the surface to be expensive but when one understands that it is handmade (hardwired, no circuit boards) by the owner Jud Barber, has features including a variac to slowly bring up the voltage of the tubes to the correct operating level, individual biasing of tubes for optimal performance, no guesswork, variable feedback setting along with impeccable fit and finish I think its a bargain. I wouldn't have said this before I experienced this amp . Excuse my excitement but I'm not finished yet!

The amp I currently own is the Berning Zh270 ZOTL. I'm not going to even attempt to get technical because I don't completely understand how it works (Clueless's comment about Berning being bright may be a little understated :) It runs cool 100 watts at idle, can be used without a pre-amp, has 3 feedback settings which effectively can match it with lower impedance speakers, those requiring more current, without changing character or showing if you will a "personality" especially one that may be offensive. It is very tweakable and in my 1.5 years of ownership my excitement hasn't subsided, it just gets better. This amp costs 4.5K retail and I would just love to hear the amp at this price point that competes in the area that this amp excells, top to bottom clarity, no grain, glare, grit, grain or any other gosh darn G word you can think of. It is just amazing. And it also does bass very, very well.

The above 3 I have listened to extensively in my system. The rest I have heard on several occasions in different systems so I can't comment beyond the fact that every system except one (I think it was the speaker's I didn't like) had some serious magic going on. They are definitely worth looking into if you are willing to live with the speakers that would qualify their use in your system. Music is all about connecting to the emotion, these amps are better at it because they just get closer to it. In my experience I would say that their price to performance is excellent contrary to Mark. Cavaet, YMMV and IMO a comparably good transformer coupled tube amp will cost more.

Gs & Cluless

The disadvantage (inconvience) I speak of is not of any but consideration of ownership of such an amp that would be a space heater in their room. So far as OTL topology goes Gs, even Class AB OTL's (Transcendent) do run hot because they have so many output tubes although the heat output is less than a Class A topology (Joule, Atma-sphere, Tenor) Many would not consider an OTL simply because of this factor, too bad. Living in Florida or a warm climate is a consideration but it becomes less so after listening.
Marakanetz, what is your problem? Did you EVER had chance to listen either SET or OTL designs? I noticed you were equaly 'angry' over SET amplifiers some time ago.
I'm OK with SET
I'm not OK with pricing of SETs that basically require a knowlege of elementary school graduate to build them. Should be at least 10x- cheaper. I'm not OK with pricing 300B triodes as well.
As to OTL's distortion-free listening I'm OK as well BTW SET and OTL are completely different since OTL-s use mostly push-pull circuits except probably for headphone amps.
It's not OK with me the rest of inconviniences and again pricing. I agree that Bearnings are priced OK and could be the exception from the rest of OTLs.

Now I do realy have a question about DC at the outpu on OTL-s... Is there an output capacitor I assume?
Oh boy, where to begin? Firstly, I should say that I have been an Atmasphere S30 owner for the past 4 years. It is simply the best amp I have owned. They have a speed and transparency that, IMHO, no transformer-based amp can match.

The original futterman design and the NYAL clones of Harvey Rosenberg where notoriously unreliable and unfortunatly this reputation plagues current OTL manufacturers to this day. I only have experience with atmasphere(AS) which has run flawlessly. AS/tenor/joule amps are NOT futterman designs/clones and it's unfair to lump them together under the banner of "OTL".

Regarding the heat issue, any class A amp, like AS, will run hot. This includes tube or solid state. Yes, the big AS amps (225 and 140 watts/ch mono) can mimic "space heaters" but in actuality, it is my understanding, that they run only 10% hotter than their class A solid state equivilents.

The big AS amps will drive 4 ohm loads easily. With the S30 and M60, speakers have to be chosen more carefully.

Finally, about price. My S30, as optioned, went for $3150. It smoked a pair of well known SET mono's that went for $6000. I don't want to call AS amps "bargins" because they are expensive, but for what you get I do think they are one of audio's best values.
One of the reasons I purchased Atma-Sphere M-60 amps was low tube cost. A speaker that is a good impedance match is a good idea, but can be overcome with the use of ZERO autoformers ZERO Autoformer Website. I couldn't be happier with the performance of my amp/speaker duo without the autoformers however. The Atma-Sphere's do everything I could ask for. I think they're incredible and I love them. In fact, they gave me the same warm fuzzy feeling I had when I met my wife.

I will have to agree with your Mar. I would also include resto of the Hi Fi products into the same category: OVERPRICED!
I currently have a pair of Tenors. My last amps
were Bat 150se's. Both amps are very good, but
there is a thing that the OTL does that makes
you smile every time you listen to them. I'm using
Verity Parsifal Encore speakers(4 ohm) and the
Tenors do just fine in a medium sized room. They
do run out of steam at loud volume, but at that
level I'm sure there is some ear damage happening
anyway. You just have to hear OTL's matched to the right speakers.
Fuzzy feeling....I loved that. You are a lucky man, Wellfed. Hope you have it again and again and off the same sources!
Clueless, yes, those were wild and reckless days, there was hype of course, as always, but it was less brash, less cynical and Fulton was the only one with cables and a little later Mark Levinson, the man, before ML became Madrigal, his by the way were made in Switzerland. Futterman died much too early. Harvey did the repairing but mine never made it, because he was again in one of his bankruptcies. ( Did not have enough grannies, obviously )
Detlof, I'm going all nostalgic! I still have a pair of Mark Lev IC's.

Great stuff...
Futtermans driving Quad 63's (no protection circuit, of course), blowing up on me on a Saturday AFTERNOON...
There was lots of suspense then with the F's: are they biased? (never), will they play? (that's the exciting part).

Back to the question. Can't give tech & other details well covered above.
So, subjectively, OTL's offer an immediacy in the sound and, often, musicality -- I don't mean "tube warmth" (even order, benign distortion). Good microdynamics, giving these amps the "fast" label.

On the other hand, they're picky about the load they can drive, they're expensive and, until recently, not very reliable.
Interestingly enough Detlof, Harvey's "wretchedly excessive" Futterman's are still being used by someone I know to good effect (last time I spoke with him) with nary a problem on a pair of Magenapan Tympani 4"s. I suppose he is one of the lucky ones or maybe this particular *Child* of Harvey has an intrinsically sweeter personality or received more attention during the formative years?
Tubegroover, Harvey DID have his inspired moments, which this "child" must have sprung out of. I alas, was not that lucky. By the way, I would imagine that this combination, you've mentioned, must sound absolutely heavenly.....
Thank you Detlof, I have no plans or desire to replace ANY of my "sources".

(Some)OTL amps are amongh the best sounding amps on the planet. They mostly are very expensive but their performance justifies the investment. As Wellfed pointed ZERO Autoformers are the solution (except for the heat dissipance). And not only for OTL designs! You should check the Paul Speltz Website and read/learn about them. After a month (or so) of extensive use i became a believer. They are my next "investment"!
I really cannot add much to the advantage/disadvantage of OTL amps, except to say that they certainly do possess a magic and realness that is hard to beat.

But, I would like to say something about the Zero autoformers. If they work for you, and you like them, then please just use them and be happy. However, I feel they are not a good solution whatsoever. They steal most of the OTL magic out of the sound, imparting a very white, bleached sound which makes me feel as if I am listening to solid state.

In my experience, the way to do it is to find speakers which present the amp an OTL friendly(that means high impedance) load.

Not to take anything away from Paul Speltz; the man is obviously a genius, when it comes to AtmaSphere amps, and his mods boggle the mind. Using the Zero autoformers will allow people with not the best matching speakers to be able to use OTL amplification. But again, it simply does not sound nearly as good as doing it the right way.
I agree with Trelja that finding an OTL friendly speaker is the optimal way to go, but I did have exceptional results with the ZEROs and a previous set of speakers that, on paper anyway, were a good match for OTL amps (nominal 8 ohms that never dip below 6.5 ohms). I obtained these results by using an alternative 2X configuration that I understand involves using less of the windings in the autoformer. Using this configuration, I was able to retain the refinement of the Atma-Sphere M-60s, in the process though bass performance suffered. I then put a second pair of ZEROs, using standard 2X configuration, on the woofer binding posts and the overall performance of the system went through the roof. The whole spectrum was improved, bass performance was phenomenal, and the mids and highs benefited in major way as well. The resolution of the system improved enormously. I had strong expectations that by using the second pair of ZEROs I would get the bass performance back that was lost when switching to the alternative 2X setting. I ended up with performance that vastly exceeded the sum of the parts - making me a wildly enthusiastic believer in the ZEROs. I now use a pair of speakers that are rated at a 10 ohm nominal impedance and I achieve better results without the ZEROs.
I have heard reports that OTL's like Tenors have been known to blow out speakers due to the lack of there being a (transformer, buffering factor?) Any truth to this?
Wellfed your points are well taken. I however agree with Trelja in my experience with the autoformers. I found the 1.37 setting the best. 2X was ok but anything above that the sound became whiter or more solid statish losing much of the natural sweetness intrinsic to the Transcendent amp I used it with. Indeed the bass was improved. It seems this device designed with the Atma-sphere in mind decidedly didn't do what I needed in my system. I liked it at first but found myself over time going back to direct most of the time. What I needed was more OTL power.

I feel the zero autoformer is a very well priced well designed product that will benefit some systems more than others but bear in mind that the character of the sound changes the higher the multiplier.

The Zero Autoformers have another great advantage: they won't alow that amps blow out your speakers. I believe that was one of the idea's when constructing the Zero's.
I agree that results with them vary, but do try them... I'd rather invest in them (because of their universality-they work even with solid state amps!) then in speaker cables of that price. But that's just my five cents...
The alternative 2X setting I refer to use the white and black leads attached to the speaker binding posts and the blue and yellow leads attached to the amplifer posts. This configuration brought back the "magic" OTL sound, but resulted in a small loss on the bass side of things. If I only had one pair of ZEROs or did not have bi-wire capability I would have easily, and happily settled for this setup and accepted the loss of bass output for the improvements gained elsewhere.

Not entirely content to leave it at that however, I contacted Paul Speltz and he suggested I try a second pair in standard 2X mode on the woofers and like I said before, the results were phenomenal. The best of both worlds and then some.

I don't know why Paul doesn't promote this setup more at his website. I don't know if my results would be achieved on other speaker models, but on N.E.A.R. 50Me II loudspeakers the gains were absolutely awesome.

One method I never tried, but would be curious to hear the results of, would be to bi-wire using just one pair of ZEROs.

On the N.E.A.R.s I would use the configuration described above on the high frequencies and another alternative 2X such as brown/green or gray/green on the low frequencies. If anyone tries this, you might as well try the reverse as well, brown/green or gray/green on the highs and white/black on the lows, to see if there are any gains.

Refer to Group 2 Configurations at the ZeroImpedance.com website to see a diagram of the alternative configuration options I refer to.

I know that this bi-wire with one pair of ZEROs is an option, but for safety, please check with Paul Speltz before trying.
Dfrigovt, since much of what has been posted here is way outside the scope of what you originally requested (especially from me) and is probably very confusing, let me answer your question more directly as pertains to Atma-Sphere M-60 Mk. II.2 amps.

Advantages: high-resolution, effortless sound; extended in both directions, rich (but not syrupy) and dimensional

Disadvantages: speaker choices are more limited, unless you use a large OTL or a corrective device like the ZERO autoformer, and possibly heat (hasn't been a problem for me though)

Since there are plenty of excellent speakers out there that work well with OTL's, I don't see OTL's as being problematic in any appreciable way.

BTW, do you have a particular speaker that you would like to pair with an OTL?
Henry to answer your question, this might be a possibility if there is not a built in circuit or shut off in the event of catastrophic overload, a voltage surge from the outputs. Most OTL amps have some sort of protection circuit in the event this manisfests itself for any reason.

I see you have asked this question on another thread and no one has answered. Maybe you should talk to a dealer. I believe Jtinn is a dealer for Tenor and he should be able to answer your question. This was one of the problems with the Futterman's, it didn't have protection between amp and speaker not to mention its inherent instability when matched to the wrong load. Most of today's OTL's have addressed this problem and do have some type of protection.
Said simply in general

--OTL-s give out full power only at specific optimal load. If load drops they loose the power so the bass is weak weak weak. If impedance goes up the output peak power increases to the point that it might damage speaker. Problems with DC at the output are also added in some OTL designes.

--OTL-s are tube abusers where tubes work under its peak values and deadly deadly hot(low plate voltage and high POSITIVE gride voltage) so there is the reason of OTLs to be so hot and expect to change tubes(input tubes and power tubes together) more often than you change oil in your car. Wanna try NOS and see how it "shines"? Tubes might age differently and if OTL loosing balance it can also damage speaker so there we have a large reliability issues.

--There is a solution for OTL amps that relatively recently was in effect to implement the current transformation using solid state inverterting amps that allow tubes to work as a main amplification devices and so efficiently transofrm voltage to current to drive today's speakers. The input stages are also implemented with differential cascade using transistors since input tubes also require reverse
connection for OTL-s. Such OTL consists of differential input SS circuit, Amplification circuit(tubes PP or SE or SEPP in normal common-cathode connection) and output SS inverter. The only problem with SS inverters and drivers that someone would say that he/she's listening to SS instead of tubes. Well, these meanings are only stereotypes and in reality we're listening to the product that was designed for a specific characteristics objectively and music subjectively. The design I studied from the following page:

Hybrid OTL 150W/ch amp

Please note that japanese readers will benefit much more from that page than non-japanese since description of design is only written in japanese(figured from .jp domain he..he..).

I don't know how truthfull are the characteristics of such design as well as I don't know how this amp sounds when implemented, but my objective review just only based on schematics and characteristics makes me to conclude that this product realy should be reliable and great sounding after implementation. Moreover it runs "colder" than many of SS amplifiers and has a larger design room for larger power but certainly with sacrifice of reliability.

--Zero tube autoformer does nearly the same as SS inverter but with much greater loss, larger neg. feedback and distortions. Sorry but tubes arn't flexible enough to be as a dynamic load.

--Last thing pure tube OTLs are great with headphones.
Some thoughts about reliability and feeding tubes to an OTL faster than spark plugs to a Hemi...

I would be out of line to speak about amplifiers other than the AtmaSphere M60, but I can tell you I have nothing but the utmost respect for what Ralph Karsten has designed.

I have to say that when he says his amps are all but bulletproof, BELIEVE HIM! The literature states that pretty much no matter what goes on, it will not take the amp or your speakers out. That's a fact. It may cost you a couple of fuses while you are learning about biasing, "zero offset", or not shorting the amp, but that's about the worst of it.

As to tube longevity, the prototype of the AtmaSphere M60 is supposedly still running its original set of tubes - 20 years later! I kind of stupidly used one suspect tube(thanks Sovtek...). It was fine for a while, just had a permanent bolt of lightning out to the glass, and pulsated strongly with the music. But, when I ran another pair of amps for 2 months, and put them back into service, that tube just didn't feel like being social. It did all kinds of misbehaving, time after time. And despite some really awful pops and bangs, true to Ralph's word, the tube's pyrotechnics didn't harm my amp or loudspeakr in the slightest - try that with a Futterman. Other than seeing this kind of once in a while Soviet QA, the 6AS7G tubes last a long, long time.
Thanks everyone for your input. The discussion went well beyond what I was looking for, but that's not a bad thing. My feeling right now is that these things are far too fussy for my liking. I am willing to work for my audio heaven, but I do not want it to rule my life. Changing tubes frequently, searching the ends of the earth for the proper speaker, blowing speakers, etc. is not my idea of nirvana. Thanks again, all, for the insights.
Dfrigovt, I think you have received a lot of misinformation about OTL's that has confused the issues.

Like Trelja, I can only speak about my experience in relation to Atma-Sphere M-60 amps. I have used my M-60's for over 3 years without one problem arising. Yes, the M-60's do require the right speaker, but that can be said about just about every amp out there (too small, too large, etc.) You can pair the Atma-Sphere M-60 and the smaller S-30 successfully with just about any speaker with a nominal impedance of 8 ohms or higher, this is not a small number of speakers by any means. With the ZERO autoformers you can pair them with any speaker that is efficient enough to be driven by 30 watt and 60 watt amplifiers.

With the Atma-Sphere's you will not have to retube frequently, you will not have to worry about blowing your speakers or amps, you will not have any difficulty finding speakers that sound good with these OTL's. I have no knowledge by which to assess other OTL designs, but it is obvious that Tenor, Joule, Transcendant and Graaf are other OTL designs that people are using successfully as well.

OTL amps are obviously not the only amplifier type that will bring nirvana, but the special qualities they possess just might be nirvana for you personally. As you mentioned in your initial request, you are a tube fan, there is no practical consideration that should prohibit you from auditioning these aside from, perhaps, geography.

There is nothing about using Atma-Sphere amps that will rule anything in your life other than a simple 5 minute biasing procedure that should be done every 6 months. It would be a shame if you missed your nirvana by dismissing OTL's out of hand because of wrong information provided by others. There is nothing pertaining to modern era OTL's that will throw you any curves. I don't know why people keep bringing up issues that don't apply any longer, and haven't for years. This is like saying that it is problematic to fly more than a couple hundred feet because the Wright brothers could do no better.

If you haven't already done so, I encourage you to read the ZERO's Story at ZeroImpedance.com, it elaborates on the effect impedance has on the matching of an amplifier to a speaker. If you don't have the motivation to read all of the document, just read the Technical Sidebar near the bottom. This information is useful whether you end up with OTL's, ZERO's, or none of the above.
Yes, I agree with Wellfed. I think that basically the only challenge in running AtmaSphere amplifiers is finding a loudspeaker with highish impedance.

And, if you don't, there are still two options available. First, you could go with the Zero Autoformers. Although I am not the biggest fan of them, they multiply the impedance of your loudspeaker. So, if you had a 4 Ohm speaker, and ran off the 2X taps, you would have a OTL friendly 8 Ohm load. The 4X multiplier would provide the "ideal" 16 Ohm. Second, you could just buy a bigger AtmaSphere amplifier, with more output tubes. Simple as that.

Again, like Wellfed stated, the AtmaSphere amps are about as low maintenance as things get. Biasing them is a walk in the park compared to my Jadis. The most difficult part I find is waiting the 1 hour for the amp to warm up before I begin the biasing procedures. After that, you can literally do it in less than a minute. Even with an amp with its settings waaaayyyy off, it shouldn't take more than a couple minutes.

And, like Ralph Karsten says, they are all but bulletproof.

I could list the two things I consider as negatives, the awesome amount of heat they throw out and the high amount of power drawn, but nothing like tubes, speakers, reliability, or fuss is anything like a negative.

I would suggest you give them an audition. That would be the determining factor for me, sound.
I have Transcendent 80 watt monoblock OTL's and Berning OTL's (according to some). I know someone who has Atma-sphere M-60's and someone else who has the MA-1's, as well as other Berning owners and people who have extensive experience with Joule OTL's. They are being used with a variety of electronics and speakers, ranging from Merlin VSM to Vandersteen5's. None of us has had any of the problems mentioned above. No blown tubes, no blown speakers, no constant fussing. Maybe we are all lucky, but I don't think so. The Bernings run so that tubes last a LONG time, and are auto-biasing; Transcendents need to be hand-biased, but not every time you turn them on. I forget if someone mentioned this above, but supposedly Ralph at Atma-sphere does a demo where he shorts out the amps with a coin, and....nothing happens. NO boom, nothing.
Dfrigovt, it would be a shame if you concluded that these OTL amps are troublesome or finicky or are only good for "optimal" setups with "perfectly matched" speakers. Proper amplifier/speaker synergy is something you need to concern yourself with regardless of what type of amplifier you choose. When I read comments from people warning of the need to "change tubes as often as they change the oil in their car" or other critical remarks, I have to wonder if this is coming from someone with personal experience with OTLs. As a Joule VZN80 owner I can tell you that my personal EXPERIENCE with this amp over the last few years shares nothing with the comments from Marakanetz. It has been nothing but a pleasure and, next to my speakers, is the smartest purchase I have made in audio. With respect to my amp, the idea that you need to frequently change tubes is absolute baloney. Quite frankly, it's a bit annoying to read comments that broadly label OTLs as having that problem. After three years I am still using the original tubes in my amp with absolutely no problems.

As for speaker matching, one thing to keep in mind (with respect to the Joule at least) is there is no problem using the Joule OTL with 4ohm speakers. In fact, Joule provides power ratings for both 4 and 8 ohm speakers. In fact, Jud Barber of Joule designed the amp to be capable of running his own speakers - Duntech Princess. Those speakers go below 4ohms. One nice thing about the Joule is that the feedback is variable - each channel, progressively. So, that means you can dial in the amount of feedback to suit your tastes - and the damping that your speakers seem to match well with.

As far as being "finicky" I will tell you this. I have had absolutely no problems with my Joule. Zero. From a practical use standpoint this is the way it works. You tell me if this is too finicky for what could be the best, and last amplifier you will ever need to buy... I take 30 seconds to turn a dial up (Variac) to operating voltage which you can read on a built in multimeter. It starts making music right away. 10 minutes later I get off my butt to tweak the voltage (once the tubes heat up). That takes about 10 seconds. That's it. Every half hour or so I may peak at the voltage while I change records. Sometimes I do. Sometimes I don't. No big deal.

In my opinion, saying an OTL amp is finicky is like complaining about engine noise from a V12 Ferrari.

I won't even get into the fact that the amp is built like a brick house, etc. I fully expect it to outlive me (I'm 31). Eventually I will have to replace tubes I guess. I don't view replacing tubes every 4 or 5 years as being a problem.

BTW, I have Merlin VSMs. You owe it to yourself to hear an OTL. I suspect many have no idea what they are missing.

Best wishes.

Like Jim, I've had my Joule-Electra VZN-80 for three years and have not had a single problem.
Dfrigovt, consider this fact, Atma-sphere has been manufacturing OTL amps since 1980, Joule since the early 90's, Transcendent since the mid 90's. The others I really don't know, but the Tenor is certainly more recent and the Berning which uses a completely different approach, was brought out I believe in 1996.

There were major considerations of owning such an amp in the past. The Counterpoint SA-4 and NYAL amps are really the only ones that contributed to the bad reputation of this design often because folks used them with the improper load. Harvey Rosenberg was very defensive of his design and was adamant that most of the problems were caused by audiophiles not paying close enough attention to this.

I am not trying to sell you on anything. If you like tubes I would say that a OTL is no more maintenance than a regular tube amp. The tubes should last a long time on the above mentioned amps. The outputs on the Joule the 6C33C are projected to last over 5 years. I believe the Tenor uses the same output triode. Audio Research 6550's are recommended for replacement every 2,000 hours, cost per tube is similar, about 30-40.00 each to replace. The Atma-sphere tubes are even less. The Transcendent's are about 20-30.00 each. Atma-sphere even less. I have not heard of ONE incident of catastrophic failure of either speaker or amp with the above designs. The advocates of these designs are speaking from experience. My guess is that Marakanetz is not but purely from an engineer's perspective based on what COULD happen with such a design from a purely theoretical viewpoint and how a transformer would act as a buffer in the event of failure. His concern seems to be about DC at the input of the speaker without a capacitor to act as a buffer. I can't speak for all the designs but I know the Transcendent uses fuse protection but is direct coupled. I'm sure the others have addressed this. Maybe someone else can offer input to Marakanetz's concern regarding this with the other amps. If not call the manufacturer and ask. Knowledge is king, misinformation is of no benefit to you or anyone else reading this thread. Get the facts.

Jud Barber has been working with tubes since the 50's, and also is an engineer. Bruce of Transcendent is an electrical engineer. Ralph I don't know. I think their experience and success with the reliability of their products would qualify them to a greater extent than someone providing theories as to what COULD go wrong and making absolutely erroneous remarks about replacing tubes regularly. The same for Atma-sphere. This design is mature and refined. See if you can find any horror stories about Atma-sphere amps blowing up.

The point is this as it is with anything audio, it is up to YOU to find out to your satisfaction the reliability of these amps. The users of current OTL amps disagree adamently with the naysayers who probably don't own or have NEVER owned an OTL amp. First thing you need to do is listen. The newer designs DO work to 4 ohm loads but as stated above, the OPTIMAL load is 8 ohms or higher. There are many speakers that qualify. Talk to the manufacturers'. Jud of Joule, Ralph of Atma-sphere and Bruce of Transcendent will give you the low down of the do's and don't for their respective designs. Each of these designs are considerate of the past failures that has led to the current reputation (by some) that OTL's are somehow still unreliable.
I would like to say that I am sure the recount of Ralph Karsten demoing his amps' ruggedness by shorting the outputs with a coin and having NOTHING happen is 100% accurate.

The reason I say this, and I am not proud in doing so, is one a couple of occasions, I have shorted the amp by moving my loudspeakers around, frantically trying to wrest that one extra bit of performance via speaker positioning. The stiff loudspeaker cables were not as compliant, and the amp was shorted.

What happened? Nothing!

Except for maybe in one bad situation, no sound coming out of the speakers for a second or so, until I realized what I just did. As soon as I unshorted the amp, the music immediately began playing again. Same as it ever did.

I didn't even blow a fuse!

The only times I have blown fuses was when I really wasn't yet adept at biasing the amps, and I had one setting way out of wack. Other than the fuse, nothing else was wrong.

How many amps, tube or solid state, can match this kind of performance???
I have owned 2 OTL amps, and still own one of them. The ones I have had have been just as reliable and easy to use as any solid state amplifier I ever had. Absolutely no fuss whatsoever. But the sound is better than any other amp I've had by far. I see no reason to avoid OTL for reliability reasons anymore. That is simply a part of their early history, and has no impact on the current crop of OTL amps. I have the Berning ZOTL-type, and I think it may be the best design of all amps ever. And if you want a SET OTL, then it is the ONLY choice possible, because nobody else can make a SET OTL, except Berning.
I was realy speaking in general terms of OTL designes. In particular the protection and auto-bias circuits are implemented either using tubes or transistors depending on model. Amps that are pure tube-OTL with all implemented protection basically overwent tough reliability issues but "naked" tubes still drop the power twice when impedance drops twice and that's in the nature of all "naked" tubes.
OTLs can realy be used successfully in bi-amp applied for active systems where there is no need for higher power i.e. for freequencies from .8k to 20k and that's where most of our ears hear the best clarity resolution.
why doesn't someone ask Dfrigot where he is and see if he can hear one, feel one, seeing is always believing :)
You Berning guys are famous for that :)
Show offs :)
I agree with TWL since I have owned the Berning ZH-270A and I could not think that I will replace with another amp. I am listening every day to the Berning and it makes me addicted to my music more than ever before. But if you own the Berning and will add to it some more tweaks you will love it.
SET OTL amps

BTW, the Joule VZN80 is a Single Ended Triode OTL amplifier...

re: SET OTL amps

Same goes for Atma-Sphere.

Perhaps Twl is referring to single tube SET's.
Is anyone knowing about the new version of Berning ZH270 with new chassis?
Yes, I'm referring to single tube SET, with my statements. All others are simply parallel configuration to lower output impedance. As Albert Porter once described it to me, parallel is like a choir singing, and single tube is like a soloist singing. No matter how syncronized a choir sings, it will never be as clear and articulated as a soloist. I thought that was a very lucid description. I think the simpler the better, as long as it can get the job done. Of course, there are certain speaker requirements for efficiency in order to use low power single output tube SET amps.
I think it's very WRONG to use the ZERO,You get out of not using an output transformer and there you are using them again.What is the use of using an OTL with transformer at the OUTPUT??