Otl's with AvantGarde Duos?

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.

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.

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.

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.

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 :)

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.

Allanbhaganinfo, you might do as I did and read the following patent at this link:

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=5,612,646.WKU.&OS=PN/5,612,646&RS=PN/5,612,646

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.