Otl's with AvantGarde Duos?

I am driving my Duos with Wavelength's Venus. Not entirely satisfied ( to lean?) i am considering going for more power (15-30watts). Would like to have some feedback with OTLs & the AvantGarde.
Nope, look into Lamm ML 1.1. Avantgarde's sing with Lamm's.
My semi useless two cents. You should really audition OTLs before going on reputation. They have a sonic signature which is hard to describe but most definitely present. I have horns but not Avant Garde everyone said use low power SETs the horns are so efficient. I didn't like the SET sound for the music I intended to play on that system. That being said I have heard OTLs which I thought were outstanding. I would be careful with the OTL and horn you may encounter a overkill and thus fatuiging upper register and a lack of good bass.
FWIW, we have lots of customers running Avant Gardes- Unos, Duos and Trios. One of the first reviews of the S-30 was on Unos. The combination definately works. Bass is of course no problem.

Naturally, since I am a manufaturer, you have to take any comments from me with a grain of salt. Your best shot in cases like this is always to go through the effort of audition. No amount of advice from others is really going to do it. You simply have to hear it for your own self.
Graham Tricker (GT Audio) design his own high end tube amps specifically for the AG horns (he is the Uk distributor for AG and he heled design the AG Model 5 amplifier).
With the AG horns, you need a really quiet amp with little noise. I've heard the TRONs at some length on several occasions. The background noise is silence with the AGs even with the volume turned up full using Trios. they sound great too.
Graham does have a US distributor, so you should be able to source a demo. If in doubt look at his web site (www.gtaudio.com) and email him.
I am an OTL fan. I settled on the Berning Siegfried amp with my Duos. They sound wonderful together. The Siegfried 811 amp is a SET/OTL, 10 wt/ch. To my mind it combines the intimacy of SET with the speed and transparency of OTL's. Highly recommended.
I love the sound of good OTLs. They have a presence and vitality that makes the music jump. With some horn systems that are a bit forward and emphasize the midrange, this quality may be too much of a good thing. I don't think AGs have that kind of horn sound so I don't see a reason why an OTL wouldn't work. Some OTL's sound a bit light in bass weight, but that is irrelevant with AGs.

The only OTL's I've heard coupled with an AG Duo was the SAP integrated (sounded quite good) and the Tenor (Tenors don't sound like typical OTLs so that doesn't count).
Your combination seems very compelling! Have you tried with other 10-20w combinations as AirTight, Viva Veronas, Art Audio, good Jap's 300Bs and other quality SETs?
Hi Jayarr, FWIW, the Berning is not an OTL. There is a lot of misconception that it is, but it is just that.

An amp that uses an output transformer (air core or not) and semiconductors in the output section does not qualify as an OTL. Unquestionably an innovative design, but it is more accurately described as a Zero Hysteresis Loss amplifier, owing to the fact that there is no hysteresis loss in the output transformer. A true OTL has no output transformer at all and is all tube.
I am afraid that Atmasphere does not quite understand the Berning design, yes, it's not a 40 or 50 year old "OTL" design, but since the transformers and semiconductors are not in the audio frequency signal path, then it's irrelevant if they are there.

This amp is really beyond the normal OTL, which is why Berning named it ZOTL.
The Berning amp is ZOTL and is all tube, completely transformerless and semicondutorless in it's audio signal path.
I think, even with OTL designs it's the same like everything else.
Some are better than others, work better or whatever.
I own OTL's but please can someone describe the "sound " of OTL ?
I use OTL (Atmasphere M-60II ) with a horn loaded design, 99db, 8 ohms and it is excellent. I use Pass aleph's, too and my OTL have no "sound". They amplify the signal the way it should be. Maybe they are much faster than regular tube designs, what they definetly own, is a excellent high frequency area, but that's the way it should be.
I know some AG's and like them, I don't own them, but listened a few times to them, I would use them with OTL without hesitation.
Allanbhaganinfo, you might do as I did and read the following patent at this link:


you will see that the title is misleading, as in the first two statements is mentioned a DC to DC switching power converter (solid state) and and output transformer.

Use of both of these technologies in an amplifier excludes it from being in a class of amplifier known as OTLs. This is a simple fact: you cannot have it both ways (call it an OTL and then have an output transformer as an integral part of its operation).

I read this patent some years ago. Sorry to have to correct you on this, but as you might expect it is my area of expertise, that is if one were to construe that I have an area of expertise at all :)

There was a bit of debate in POSITIVE FEEDBACK magazine regarding this issue some years back. To this day, it still amazes me that David continues to call this amplifier an OTL when he clearly knows it isn't (if you refer to PF, you will find that he admitted such in that magazine, but found that people asked less questions when he described the amplifier as an OTL). If it were me, I would have described the amp for what it is: a unique means of coupling tubes to a loudspeaker that represents a new type of amplifier. ZOTL is misleading, as the moniker has no meaning. ZHL amplifier might be closer, as Zero Hysteresis Loss more properly describes the advantages of the output transformer in this design.
As I said, I am afraid that Atmosphere really does not understand this technology at all. There are no Audio output transformers or Solid state audio output devices in the ZH-270, NONE.

I am sure your expertise is in OTL's but the ZH-270 is again way beyond that simple terminology.

It's simply a way better design than normal OTL designs of the past.
It seems to me the primary goal of an OTL is to eliminate hysteresis loss and bandwidth limiting imposed by output transformers. Since David Berning's patent is about another approach to achieve that goal, may I suggest that the results are all that count. In the conventional sense Ralph is right, but it may be irrelevant if someone is able to invent a better mousetrap - but that's not to say the other is. An amplifier with zero hysteresis loss between the output tubes and the binding posts by virtue of no iron in the signal path and no output transformation in the audio band can, in a practical sense, be considered to be output transformerless due to the same benefits it delivers. Just my opinion, sprinkle liberally with salt, film at eleven, etc. Bottom line, use your ears to see which you prefer. Both are outstanding amplifiers.

Allanbhaganinfo, you seriously don't get it, do you?

And Ralph was being so polite...
Rushton, sorry but you are right, I don't get it!

Please explain.

I was equally polite in showing Ralph that he's not right in his terminology of the Berning design.

There is much, much more to it than that.
Essentialaudio, yes, exactly but again it's even more complicated than that because there is no such thing as a Zero Hysteresis Audio bandwidth output transformer, if Dave invented such a thing, he would instantly be rich :)

It's not an Audio output transformer, there are lots of transformers in the Berning design but none are Audio bandwidth output transformers.

It's certainly nowhere close to a conventional OTL, which is what should be said and correct but not that it has output transformers and Semiconductor outputs, which is not true.
The comment that Ralph made in regards to me having admitted that the ZOTL is not an Output-Transformerless amplifier in PF magazine is incorrect. What I stated is something to the effect that the ZH270 is full of transformers, but none are output (or audio) transformers. In fact, except for a small DC-blocking capacitor at the input of the amplifier, the ZH270 is DC-coupled, an impossibility for any audio transformer-coupled amplifier.

David Berning
Hi David, could you then explain the presence of the air core transformer in the output section of your amplifier? Seems to me that a rose by any other name is still a rose. So I guess what I want to know is how the air core transformer shown is not in fact a transformer in the output section.
As I read the patent, it seems to play a crucial role in the design.

Also, you *did* mention in PF some years back when this issue first appeared (in a letter to the editor) that people did not seem to understand the idea of zero hysteresis loss, and found it easier to understand when you told them it was an otl instead. I can dig the quote up for you if you like. Anyone with back issues of PF can find it.

This was a bit after the time that Harvey Rosenberg was promoting your amplifier in his reviews, having negotiated a contract with you to sell your amps. So that would make it in the 1997-1999 period. Harvey told me that at the '98 CES and you told me that the contract had expired at the '98 Philadelphia Triode Show. So I know the timeframe is correct.
The transformers, of which there is a pair for push-pull in the ZH270, are power-conversion transformers that operate at fixed frequency. They have low-loss ferrite cores (not air core) optimized for 250 kHz. They are indeed critical components for the impedance matching in the ZH270. Audio output transformers in conventional tube amplifiers perform this impedance-matching function, and are major limiting factors in obtaining the wide bandwidth needed to prevent the amplifier from having the filtering properties that both you and I are trying to avoid by not using them.

Now to try to get to the bottom of this controversy as to you thinking that the ZOTL has output transformers and my claiming that it doesn't, we need to come to an understanding of what an audio output transformer is. To me, an audio output transformer is a transformer used to couple the audio-frequency signal from the output tube(s) to the speaker, and for high-fidelity applications it is designed to cover three to four decades of frequencies starting from at least 20 Hz. To do this they must be physically large. In the design of these devices, compromise must be made between high frequency and low-frequency performance because of parasitic capacitance of windings and number of turns needed to achieve sufficiently low magnetizing current to reduce core saturation and distortion at low frequencies.

All I have been saying all along is that the ZOTL does not have any such device. I see the term OTL as an acronym for output transformer less; I believe that that was the original intent when the early pioneers such as Futterman had the dream of finding a way around the main limiting factor in audio power amplifiers of the time. Any amplifier that by its design eliminates this problematic device could be called OTL for that matter, including most transistor amps. Amplifiers that are free of these devices avoid the filtering (frequency-dependent) type coloration associated with coupling output devices to the speaker, assuming no other audio-frequency dependent coupling network is used.

The ZOTL falls into a generic class of amplifier described by the acronym OTL both technically and in spirit. The power conversion transformers called out by the patent cannot and do not operate at audio frequencies. The semiconductor devices called out by the patent drive these transformers at a fixed 250 kHz and do not operate at audio frequencies or as amplifiers. The patented ZOTL circuit is a two-way coupling circuit that performs impedance matching like an audio output transformer, but does not have any of the limitations and distortions imposed by audio output transformers. This circuit is in no way a buffer, such as a transistor output stage in a hybrid tube-transistor amplifier. There is no power gain, and more importantly, the speaker “sees” the tube transfer characteristics and the tubes “see” the speaker’s dynamic characteristics. This two-way power flow separates the ZOTL from the hybrid amplifier in that sonically it is pure tube. Further technical details and graphical demonstration can be found on my web site davidberning.com in the form of white papers.

The circuit topology of the ZH270 is completely different as to how the transformers are connected. I simply fail to see how anyone familiar with audio-amplifier circuits can confuse these transformers with audio output transformers. At least one winding of an audio output transformer is connected to the tube(s) in the non-OTL amplifier, and a winding is connected to the speaker. Neither of these connections is made with the conversion transformers in the ZOTL.

While I realized that the earlier OTL amplifier designs, that is pre ZOTL, have wonderful sonic characteristics when used with compatible speakers, I did not want to make OTL amplifiers before I invented the ZOTL because of the high heat and reduced reliability that comes from having many output tubes running in parallel. I also realized that speaker selection was more critical because these tube OTLs are starving the speaker for current. Since the ZH270 was introduced in 1996, it has slowly gained respect; so much so that I can no longer keep up with demand and there is a waiting period that now extends into next year. Clearly more people are finding that the ZH270 can provide sonically what previously only older-type OTL designs could provide, and can do so for a wider selection of speakers and without the excessive heat and fuss of replacing tubes and chasing drifting bias settings.
Hi David, we may disagree on whether on not conventional OTLs have weakness that other arts do not; that is a matter for elsewhere.

You misinterpreted my request for an explanation of the air core transformer's function. I was asking if it is an OTL, how can there be a transformer essential to its function in the output section. The acronym is for "TransformerLess". In plain English, this means that there is no transformer, aircore or otherwise.

So we also disagree on semantics. Even though the air core device you use does indeed allow for DC response, it is nonetheless a defined as a transformer by the electronic texts (any physics prof will disagree with you also). Even though it is supplying the power for the output tube... well, with this simplified description, that happens with a lot of output transformers (although not in the same way). And, if you strip the 250KHz out of the signal that the air core transfers, there is an audio signal that is the resultant. Plus, if you trace from the speaker terminals back through the circuit, you do not find it connected to the elements of a tube (not even through a coupling cap)- it is connected to the tube via a DC to DC converter and an aircore transformer.

This is clearly not an OTL. Technical explanations to the contrary have, for the last 7 years, amounted to obfuscation.

Now the odd thing to me is this: Before you came along, there were two techniques for coupling output tubes to loudspeakers: the transformer coupled way and the OTL way. All tube amps made prior to this invention operated on some varient of the the above two techniques.

Now there is a third way and you invented it! Why, for heaven's sake and for your own, would you not take advantage of the tremendous marketing opportunity that represents??

Of course, I know that you are not referring to me in some of your comments directed at OTLs :) Controlling bias so that is does *not* drift, and providing as much current as a transistor amplifier are two of the things that we do. But I agree that those are problems with (other) current and past OTLs.
Good discourse going on here by two of the audio paragons. For what it's worth, the opinion of the village idiot...

I am the owner of a pair of AtmaSphere M60 OTLs, and have also had the pleasure of being around a Berning ZOTL just over one month ago.

While I will not get into the circuit designs of either, as I have not immersed myself in them. I don't know what is and what is not this or that. I will say that I was more than a bit surprised to find out exactly just how similar both amplifiers sound to each other. And, in the end, is it not really the sound? However close or not the designs are, I have NEVER encountered an amp that sounded so much like my Atmas as the Berning.
Ralph, there are two things that Dave is really not very good at, in fact, he's very bad at, is visual esthetics and marketing :)

If you ask Dave if he's done any marketing, he'll say, oh yeah, just came back from there, got some apples to make apple sauce!

An I'm not kidding.,
I don't doubt what you are saying: One of the more brilliant concepts seen in amplifiers in the last 20 years or so and he has it mislabeled :)

Could you share your experience of the Atmasphere M-60's with your Duo's at a high level: great, good, not so good.  Thanks.
Well, Wyshak, it's 15 years down the road since my last post on this thread and I'm still listening to the Duos through the M60s. That should tell you something. The combo is excellent IMHO and I've had zero issues in that time (other than a few replaced tubes).

Thanks for your response; glad to hear you're still enjoying your system.  I was wondering if you think you need the 60 Watts from the M-60, or if 30 Watts out of an Atmasphere S-30 give the same performance?  Any thoughts on how hard you're pushing the amp?
Alan, you could answer that easily enough by removing half of the power tubes in the amps. That would simulate the S-30 to a great degree. Its been too many years since I heard the combination myself, but IIRC the S-30 does fine on the Duo.
@ Ralph     It was Wyshak asking the question about the S30. To answer your question from my perspective, Wyshak, the S30 could easily power the Duos no problem. I would still opt for the M60s regardless, tho, because I prefer their lower output impedance. Everything's just a wee bit tighter and more controlled this way. FWIW, I did experiment with pulling half the power tubes from the M60s and feeding the Duos that way but ended up ditching that configuration and going back to the full complement. The improvement was marginal but worthwhile in my estimation.