I have just purchased the latest Sound Lab M1

Hi Ralph,
It looks like it’s time for me to get involved in doing the best I can to explain the ZOTL technology so more people can understand the real issues. It is good that you pasted my patent abstract into your post, and from the complexity of the description, it is easy to see why the ZOTL technology is hard to grasp. In the near future, I hope to write as non-technical an explanation as I can and post it on the Berning web site to help people understand a little better the implications of the patent, and why it works the way it does.

I maintain the term OTL in the description in the patent and in other discussions of the ZOTL because, simply, there is no output transformer in these amplifiers. No such devices are called out in the patent. What are called out in the patent are converter transformers. This is not simply calling one thing by a different name; it really is very different in form and function.

Since it will take me some time to pen an explanation of patent, I will just say for now that the interested person can visit my web site and look for the review of the ZH270 that Chuck Hansen did in Glass Audio some years ago. Chuck is a switching power supply engineer and was able to fully understand the technology and tried to explain it in terms that non-engineers could understand.

But for now, I think that it may be instructive to make some comparisons between your amplifier and my amplifier. When I look at your amplifier, I see that not only is the speaker connected to the tubes, but it is also connected to a pair of floating power supplies. Is it not true that these power supplies have ss rectifiers and transformers in them? Is it fair play now for me to call your transformers output transformers? I would not do that because it is silly. Your transformers operate at a fixed line frequency that has nothing to do with the audio frequency. Likewise, the converter transformers in my ZOTL amplifiers operate at a fixed frequency that has nothing to do with the audio frequency. Remember, also, that the ZOTL amplifiers are dc coupled except for the input stages, where ac coupling may be used to prevent dc from being passed to the speaker. Clearly, this would not be possible if there were output transformers. Likewise, your output and driver sections are dc coupled.

The real difference between your amplifier, Ralph, and my ZOTL is the power supply. You have simple fixed power supplies that are designed to strike the best balance between allowing the tubes to pass enough current to the speaker and not overcooking the tubes too much. On the other hand, the power converters in the ZOTL perform optimum impedance matching between the speakers and the tubes because they can independently furnish the operating power to the output tubes while providing the current to the speaker via an arbitrary tube voltage step-down ratio, which is the same as the speaker current step-up ratio! In other words, the ZOTL frees the amplifier designer from the requirement that the same current that goes through the speaker must go through the tubes. The ZOTL further adds the wonderful feature of isolation between the high voltage of the tubes and the speaker, and this isolation simplifies dc coupling in that there is no requirement that the output tubes be operated at the same dc level as the speaker.

With the ZOTL, the tubes can be operated at their best efficiency. In the ZH270 and the new Quadrature Z, I choose to operate with a dc plate voltage of around 1400 volts. This would be too high for an audio output transformer, as the insulation would fail if required high fidelity tight-coupling winding procedures were followed. Operating the tubes at this voltage brings the efficiency of the tubes up to what would be expected of transistors in a conventional amplifier. This is why the Quadrature Z can boast 200 watts out of only four modest-sized output tubes (33JV6) at 8 ohms. This goes up to 270 watts at 4 ohms continuous without the tubes showing color. Ralph- that’s 270 watts of OTL tube power into 4 ohms!

David Berning

This has turned into quite a great discussion with two luminary amp designers.

Hi David, your numbers are impressive and as I have always made sure to mention in the past, its an impressive design. I was privy to Harvey Rosenburg's plans when he coined the acronym 'ZOTL' back when he was making a deal with you to manufacture and distribute them.

I've not argued the term 'ZOTL', but when people say the amp is an OTL, well, it isn't. If you removed the aircore transformers from your amp it wouldn't work. I get that you choose to call them 'converter transformers' but the term transformer was sufficient as that term already meant what you were trying to say. A person could as easily say that a every transformer-coupled tube amp made has a transformer that converts from high impedance to low impedance with "an arbitrary tube voltage step-down ratio", to use your expression. Either way it sure sounds like a transformer is somehow involved in the matching between the tubes and the loudspeaker. Your text supports this as does the patent abstract. OTL it isn't.

My point is also that in saying you have an OTL, people come away with a very different idea of what your amp is. Why not call it a 'zero hysteresis amplifier' (isn't that the basis of the letters 'ZH' in the ZH270)?

In an email that you once sent to me when I was having a rough time, you said that high end audio needs companies like mine for the diversity they represent. I've always appreciated that, but in fairness the simple fact is that you are right, and your amplifier represents some pretty serious diversity!! -and one that I for one have always appreciated. Let it shine dude!

"If you removed the aircore transformers from your amp it wouldn't work."

This is not true, it will work, but it will have to work like a regular OTL, unable to handle low impedances, as well as power requirements without a zillion smoking hot tubes, as well as the other short comings that Dave mentioned above.

As I have said before, it is a quantum leap in OTL deign or any kind of tube amp design and as much as you want to harp about a transformer, you know it's an impossibility of it being a AUDIO output transformer, which is really what OTL means!

Hey man, you know all we are trying to do here is educate the audiophile a little and old school does not apply here anymore which is everybody doing the same thing but "only different" slightly.

Take it easy there man!

Hi Winston, There are several good amp recommendations here but if you can get a demo with the new Berning Quad Zs, I have no doubt you would be left with a big smile. When the MBL 9011 amps left me cold, I talked/begged David Berning into working his magic with amps that would power the most difficult speakers made, read M1s, Revel Salons and MBLs. The units offer large current delivery, COOL running tubes, slam and finesse. No other high output amp offers this combination that I know of.