OTL amplifiers

Said simply in general

--OTL-s give out full power only at specific optimal load. If load drops they loose the power so the bass is weak weak weak. If impedance goes up the output peak power increases to the point that it might damage speaker. Problems with DC at the output are also added in some OTL designes.

--OTL-s are tube abusers where tubes work under its peak values and deadly deadly hot(low plate voltage and high POSITIVE gride voltage) so there is the reason of OTLs to be so hot and expect to change tubes(input tubes and power tubes together) more often than you change oil in your car. Wanna try NOS and see how it "shines"? Tubes might age differently and if OTL loosing balance it can also damage speaker so there we have a large reliability issues.

--There is a solution for OTL amps that relatively recently was in effect to implement the current transformation using solid state inverterting amps that allow tubes to work as a main amplification devices and so efficiently transofrm voltage to current to drive today's speakers. The input stages are also implemented with differential cascade using transistors since input tubes also require reverse
connection for OTL-s. Such OTL consists of differential input SS circuit, Amplification circuit(tubes PP or SE or SEPP in normal common-cathode connection) and output SS inverter. The only problem with SS inverters and drivers that someone would say that he/she's listening to SS instead of tubes. Well, these meanings are only stereotypes and in reality we're listening to the product that was designed for a specific characteristics objectively and music subjectively. The design I studied from the following page:

Hybrid OTL 150W/ch amp

Please note that japanese readers will benefit much more from that page than non-japanese since description of design is only written in japanese(figured from .jp domain he..he..).

I don't know how truthfull are the characteristics of such design as well as I don't know how this amp sounds when implemented, but my objective review just only based on schematics and characteristics makes me to conclude that this product realy should be reliable and great sounding after implementation. Moreover it runs "colder" than many of SS amplifiers and has a larger design room for larger power but certainly with sacrifice of reliability.

--Zero tube autoformer does nearly the same as SS inverter but with much greater loss, larger neg. feedback and distortions. Sorry but tubes arn't flexible enough to be as a dynamic load.

--Last thing pure tube OTLs are great with headphones.

Some thoughts about reliability and feeding tubes to an OTL faster than spark plugs to a Hemi...

I would be out of line to speak about amplifiers other than the AtmaSphere M60, but I can tell you I have nothing but the utmost respect for what Ralph Karsten has designed.

I have to say that when he says his amps are all but bulletproof, BELIEVE HIM! The literature states that pretty much no matter what goes on, it will not take the amp or your speakers out. That's a fact. It may cost you a couple of fuses while you are learning about biasing, "zero offset", or not shorting the amp, but that's about the worst of it.

As to tube longevity, the prototype of the AtmaSphere M60 is supposedly still running its original set of tubes - 20 years later! I kind of stupidly used one suspect tube(thanks Sovtek...). It was fine for a while, just had a permanent bolt of lightning out to the glass, and pulsated strongly with the music. But, when I ran another pair of amps for 2 months, and put them back into service, that tube just didn't feel like being social. It did all kinds of misbehaving, time after time. And despite some really awful pops and bangs, true to Ralph's word, the tube's pyrotechnics didn't harm my amp or loudspeakr in the slightest - try that with a Futterman. Other than seeing this kind of once in a while Soviet QA, the 6AS7G tubes last a long, long time.

Thanks everyone for your input. The discussion went well beyond what I was looking for, but that's not a bad thing. My feeling right now is that these things are far too fussy for my liking. I am willing to work for my audio heaven, but I do not want it to rule my life. Changing tubes frequently, searching the ends of the earth for the proper speaker, blowing speakers, etc. is not my idea of nirvana. Thanks again, all, for the insights.
Dave

Dfrigovt, I think you have received a lot of misinformation about OTL's that has confused the issues.

Like Trelja, I can only speak about my experience in relation to Atma-Sphere M-60 amps. I have used my M-60's for over 3 years without one problem arising. Yes, the M-60's do require the right speaker, but that can be said about just about every amp out there (too small, too large, etc.) You can pair the Atma-Sphere M-60 and the smaller S-30 successfully with just about any speaker with a nominal impedance of 8 ohms or higher, this is not a small number of speakers by any means. With the ZERO autoformers you can pair them with any speaker that is efficient enough to be driven by 30 watt and 60 watt amplifiers.

With the Atma-Sphere's you will not have to retube frequently, you will not have to worry about blowing your speakers or amps, you will not have any difficulty finding speakers that sound good with these OTL's. I have no knowledge by which to assess other OTL designs, but it is obvious that Tenor, Joule, Transcendant and Graaf are other OTL designs that people are using successfully as well.

OTL amps are obviously not the only amplifier type that will bring nirvana, but the special qualities they possess just might be nirvana for you personally. As you mentioned in your initial request, you are a tube fan, there is no practical consideration that should prohibit you from auditioning these aside from, perhaps, geography.

There is nothing about using Atma-Sphere amps that will rule anything in your life other than a simple 5 minute biasing procedure that should be done every 6 months. It would be a shame if you missed your nirvana by dismissing OTL's out of hand because of wrong information provided by others. There is nothing pertaining to modern era OTL's that will throw you any curves. I don't know why people keep bringing up issues that don't apply any longer, and haven't for years. This is like saying that it is problematic to fly more than a couple hundred feet because the Wright brothers could do no better.

If you haven't already done so, I encourage you to read the ZERO's Story at ZeroImpedance.com, it elaborates on the effect impedance has on the matching of an amplifier to a speaker. If you don't have the motivation to read all of the document, just read the Technical Sidebar near the bottom. This information is useful whether you end up with OTL's, ZERO's, or none of the above.

Yes, I agree with Wellfed. I think that basically the only challenge in running AtmaSphere amplifiers is finding a loudspeaker with highish impedance.

And, if you don't, there are still two options available. First, you could go with the Zero Autoformers. Although I am not the biggest fan of them, they multiply the impedance of your loudspeaker. So, if you had a 4 Ohm speaker, and ran off the 2X taps, you would have a OTL friendly 8 Ohm load. The 4X multiplier would provide the "ideal" 16 Ohm. Second, you could just buy a bigger AtmaSphere amplifier, with more output tubes. Simple as that.

Again, like Wellfed stated, the AtmaSphere amps are about as low maintenance as things get. Biasing them is a walk in the park compared to my Jadis. The most difficult part I find is waiting the 1 hour for the amp to warm up before I begin the biasing procedures. After that, you can literally do it in less than a minute. Even with an amp with its settings waaaayyyy off, it shouldn't take more than a couple minutes.

And, like Ralph Karsten says, they are all but bulletproof.

I could list the two things I consider as negatives, the awesome amount of heat they throw out and the high amount of power drawn, but nothing like tubes, speakers, reliability, or fuss is anything like a negative.

I would suggest you give them an audition. That would be the determining factor for me, sound.