Thought on OTL tube amps

I have also owned and auditioned a variety of amps. Solid state; Classe, Krell, Gamut, Plinius, Threshold... Hy-bred; Lamm... Tube; Jadis, Lectron, Conrad Johnson, Convergent Audio... OTL; Atma-Sphere, Joule. I have found, as with nearly every piece of audio, gains and concessions in each approach. Presently, I find the Joule amp to be fantastically musical, yet capable of reaching the frequency extremes. Bass nearly as deep as Classe and Krell with greater body and density, shimmering highs, plus a near CAT midrange (not quite as lush as the Jadis amps). These amps, both the A-S Ma-2's and Joule Rite of Passage, have adequate power to drive the power hungry Avalon Eidelons to a very high listening level, which I require.

I will respectfully disagree with Jallen regarding damage to speakers from blown out tubes. With both the A-S and Joule amps the internal protections have arrested any harm from reaching the drivers on my speakers. I too witnessed removal of an output tube from an MA-2. Not to be done on the Joule, though. The Rites of Passage require all output tubes for proper function.

I have much enjoyed the few solid state amps, especially with tube preamp, but ultimately found the midrange less fullfilling. The Jadis along with the other tube amps left me wishing for more slam and top extension, although they likely would have benefitted from additional tweaking.

The OTLs have come closest to achieving what I demand from the particular system I now own. Do keep in my mind, they run very, very hot and devour electricity. Certainly more than any other amp I have owned. As is typical, the associated components, listening space, and your tastes will best dictate whether these amps prove satisfying.

Jallen's comment above reflects a popular myth.

OTLs can be as reliable and safe as any other amplifier. Its all in the design. The only OTL design out there that really did have issues is the one that has generated a lot of the mythology around OTLs years ago (and in the case of that particular design, generally known as a Futterman, those stories are often true). The three myths thus generated were/are speaker safety, amplifier reliability and sensitivity to load. I'm only addressing the speaker safety issue here but will cover the others if anyone is interested.

Speaker safety is a design issue and nothing else with OTLs, the same as it is with a transistor amplifier (the concern is what happens if a tube fails/shorts).

A common myth is that OTLs run the same high voltages that regular tube amps do. This is false- if you think about it, the fact that the amp has no output transformer means that the tubes have to generate voltages and currents that are normal for loudspeakers, to do this requires a tube-type that runs at much lower voltages and higher currents. For example in our amps the B+ is only about 140Volts.

All it takes is a little testing and experimentation to find the limits of any OTL design, so its easy to set up the amplifier with a scheme to protect the speaker in the event of a tube failure. Otherwise the company goes out of business. I hope this is obvious for what it is: common sense! After over 33 years of business, its safe to say that we have built more OTLs than any other manufacturer so I think we can agree that if there was a problem it would have manifested by now :)

The Atma-Sphere web site (Atma-Sphere.com) has several "white papers" explaining many of your questions. I own a pair of M60Mk2.2's. Absolutely stable, (you can even short the outputs, and switch output tubes while running, though I would not recommend doing so (I have seen Ralph Karsten do it) and I have had no problems. Happy Listening.

"Blow a tube with your OTL and you could fry your speakers. Great sound, but some risk if you listen loud, or run tubes till they drop "

Properly designed OTL amp represent no more danger to speakers then properly designed regular tube or transistor amps.

For example, in many tube amp designs, output tube is connected to resistor which work as a "fuse of last resort". if output tube blows then it takes out this resistor.

I never heard that OTL amps by Joule-Electra and Athmasphere produce any troubles. I suspect that with proper maintaince these amps can work half century or longer - at least I see not a single reason why not?

Macintosh is the only SS amp using output transformers that I am familar with.

I really enjoy OTL amps and have owned examples from Tenor (75w, 75Wi), Atma-Sphere (M60, MA1) and, Joule(VZN 100) and Berning (Siegfried, 270 and ZH 230...this one I use currently).

I have not had reliability issues with any of these or certainly any more than any other tube based amps.

They require careful speaker/system matching...your speakers would likely work well.

There is a transparency and immediacy to the sound that is very appealing to the OTLs they can also do bass and highs very well if set-up and used properly.

I would say that overall they are my favorite type of amp...but like you, I have several (types of amp) and enjoy the virtues of all.
I have yet to find anything that is perfect in all cases.

Most SS amps are OTL. Transistors are intrinsically low impedance devices and most circuit topologies don't need an output transformer. Tubes are high impedance devices and most circuit topologies do require a transformer to drive low impedance speaker loads.

Blow a tube with your OTL and you could fry your speakers. Great sound, but some risk if you listen loud, or run tubes till they drop. Jallen

don't mean to hijack the thread BUT was wondering why all the OTL amps I know of are tubes. Are there no solid state OTL amps? If not, why not? Just curious and hope someone will answer a question.