@rusty_jefferson Just to be clear, I'm not Trolling you, or your company, just answering
the question in the thread title, "What's your OTL tube amp experience
and suggestion".
No worries- I didn't think you were. But most people don't realize the ZOTL amplifiers have not only an output transformer, but also semiconductors in the output section. It is quite unique!
By stating you made the first reliable OTLs
does indicate that some or all of your predecessors were in fact, not
reliable. And by safe, I didn't just mean not catching on fire, but
rather safe and reliable in our systems. The Berning ZOTL amps have no
chance of passing DC, no popping noises, and extremely long tube life,
again, on the order of many years to decades of daily use. If your
amplifiers are capable of similar safety and reliability, I apologize
for not knowing so.
Sooner or later, if a tube fails the Bering will make a popping sound. But it unlikely to threaten a speaker.
You might think about this for a moment: If our amps were damaging speakers, we would not be able to stay in business for very long, instead of the 41 years we've been around. I'd be quite nervous if I thought for a second that one of our amps could threaten my speakers- the drivers in them are quite expensive. That sort of thing has a way of coming home to roost!
We protect the speaker with a simple mechanism which is highly effective. But that is not the only reason our amps are safe- we are the first OTL manufacturer to make an amplifier wherein the power tubes are in fact properly under control (which is really important; think about what that means for distortion too) and also, the first OTL that is unconditionally stable (which is to say: stable regardless of input condition or output load).
We too get long tube life (we warrant all the tubes in our products for a year) often on the order of many years to decades... so apology accepted- no hard feelings :)
And if some think ZOTLs aren't really OTL
amplifiers because they don't meet some engineering criteria that fits
their needs, well, I don't know what to say about that. I don't think
anybody has ever sued Berning for false advertising. Here's a nice
discussion from the Berning website, including pros and cons of
different OTL implementations, for those interested.
When the amp first appeared I read the patent, as it is patented. This is from the abstract of 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." (emphasis added)
Although connected in a quite unconventional manner (hence the patent), it is the transformers that effect the transformation from the high impedance of the power tube to the low impedance of the speaker, which otherwise is a rather conventional way of describing the operation of an output transformer.
In a letter to the editor of Positive Feedback back in 1997, David mentioned that he was originally going to call the amplifier a Zero Hysteresis amplifier but he found that to be difficult to explain. When he told people it was similar to an OTL then the glazed expression went away. It didn't help that Harvey Rosenberg suggested the ZOTL acronym, which Harvey told me specifically didn't mean anything as he knew full well the amp was not an OTL.
