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In Reply to: RE: REVIEW: Linear Tube Audio ZOTL40 MK II Amplifier (Tube) posted by jsm71 on January 28, 2017 at 10:31:36
For those unfamiliar with what Berning has figured out almost 20 years ago and patented, his amplifiers use tubes but no output transformers. Unlike other OTL amplifiers, Berning's design is truly unique. Read the notes on the LTA web site for an explanation
The statement above is false. The Berning amplifier, while brilliant in design, does use an output transformer and therefore is not an OTL.
From the patent:
"A linear audio amplifier includes a push-pull pair of vacuum tubes operating in a linear amplification mode coupled through a pair of dc-dc switching power converters to an external load impedance. Each power converter includes a transformer with one or more secondary windings that drive rectifier circuits, and the resultant dc voltage sources are loaded by their respective tubes."
See text in bold.
Its a different application of a transformer for sure, but it is the turns ratio of the transformer that effects the loading on the DC-DC switching power converters. This turns ratio is what allows the high impedance power tube to drive a low impedance loudspeaker. That's an output transformer.
Years ago Berning used to use the phrase 'zero hysteresis' to explain the operation of the amp (and is where the 'ZH' of the ZH270 came from) but found it (understandably) to be a bit of an uphill battle trying to explain the operation. Harvey Rosenburg suggested the 'ZOTL' acronym (which Harvey freely admitted to me that it had no meaning) and it stuck.
Follow Ups:
My intent with this review wasn't to spark a pissing contest between design approaches. If I stated a design feature that was untrue or perpetuated an already open wound I'm sorry.
I am however thrilled that this amp doesn't use the "more traditional" iron core paper weight output transformers that accounts for the majority of an amp's weight and negatively impacts the sound. How's that?
The sound of this amp does in fact sound as marvelous as other OTL amps I've heard, but does so with minimal tubes that are not run hard and it generates very little heat. The cost is also quite approachable for the level of quality sound it produces. Those cost benefits also extend when/if the tubes last as long as projected. Lastly, I didn't have to question if my speakers had the right impedance to mate well.
I've seen Dave Berning carry one of his amps in a paper shopping bag. I wouldn't try that with mine ... or yours.
Again, a brilliant design IMO.
Our amps aren't that heavy either, lacking an output transformer altogether.
My dream is MA-3's.........
ET
"If at first you don't succeed, keep on sucking till you do suck seed" - Curly Howard 1936
To follow up Ralph's comments, and as I've said before, we at LTA do try to emphasize the aspects of the circuit that make it unique, and explain that it is an entirely different thing than what "OTL" usually refers to. We have continued to use the ZOTL term for the same reasons David does: it's the easiest way to let people know that it's not a traditional tube amp, and possibly open the discussion to explain the circuit in more detail. I'm sure Ralph has spent more time than he'd like explaining that his amps are not Futterman designs.Thank you for the review! Always glad to see people talk about our power amps, as we've mostly had feedback regarding the MicroZOTL2 and its variants.
-Will
LTA
Edits: 01/31/17
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