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In Reply to: RE: Touche', sort of... posted by Lew on February 16, 2015 at 20:55:03
"I have never heard any negative consequences "
Until you compare them to equally good no feedback circuits I can imagine you don't hear the consequences as it has to be in relation to circuits without it. I have no doubt you find the OTLs of this design to sound good...I do too but not as good as the best SET designs in some important ways. What the OTLs have is this sensation of absolute transparency...something that you find only with the best of the best SETs and even then the quality is slightly different...where I heard my OTLs ultimately lacking was coherence and this only with full range electrostats...not with conventional speakers.
This is why the idea of a SE OTL is so appealing because IMO SET has greater coherence and harmonic integrity and the OTL greater transparency and punch...so marry the two and Violà perfect amp ;-).
I have close to this with my SE(Transistor) hybrid from NAT. It is single ended, it is OTL but it uses a MOSFET on the output (only 1 device per channel though...so kind of like a big output tube), it is hybrid, it is direct coupled and it has no negative feedback. Does it sound like a typical SET? No, it sounds less colored than most. Does it have the otherworldly transparency of an OTL? No, it doesn't quite startle you in the same way...althoug after 2 hours of playing it does get kind of psychadelic sounding, where transparency approaches OTL but with less leanness (feedback related, IMO) and superior tone and coherence.
I recently heard the new integrated amp from Ypsilon, the Phaeton, and it was extremely good sounding...also a hybrid but WOW! It REALLY makes me want to hear the SE(Transistor) hybrid mono blocks...although they actually put an output transformer on theirs (a nearly 30KG one at that). The price is unobtanium but really nice machines
Follow Ups:
You wrote, "Until you compare them to equally good no feedback circuits I can imagine you don't hear the consequences.."
Right you are. 15 or 20 years ago, I encountered the Atma-sphere amplifiers. Run with no NFB, these clearly were and still are superior to even the best Futterman type amplifiers in my experience. Now, whether my preference for a circlotron output stage with no NFB to a Futterman output stage with gobs of NFB has anything to do with NFB per se, we cannot know. However, I do prefer the circlotron types, in my system, to my ears. (Not wishing to criticize the Futterman topology which still also sounds pretty great.)
I think I asked this before, but maybe not: How can an SE OTL work with no NFB and still be able to drive a real world speaker with reasonably low distortion??? I guess the answer from the company you mentioned at the start of this thread is…. use a very large number of output tubes in parallel. That requires a very robust driver stage plus a gaggle of tubes that are at least fairly well to very well matched.
Well, my NAT did it by using a big MOSFET instead of a tube and that gets low distortion without feedback but to get any power and a reasonable impedance it seems you need a forrest of tubes if you want to go feedback free for a pure tube SE OTL design.
..has its own issues, but there are a few very nice sounding amps with tube input and driver stages and solid state output stages, I admit.
LewIMHO original older J.Futterman OTL designs which uses triodes but not pentodes in asymetrical SEPP(Single Ended Push Pull) OPS can probably have almost that SE amp sound signature , without of any of my doubt that Futterman amps are really PP amps and also regardless of use of significant amount of GNFB .
But must say that only some THD analyzer probe can reveal real output THD signature , so if THD spectrum consist from mainly second harmonic followed with much lower level of third harmonic and even almost non existand traces of higher order harmonics only than I can say that Futterman amps perfectly mimics SE amplifiers sound .Best Regards !
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/23/15
The only such amplifiers I can think of were those old models made by Fourier. One used 6AS7s and the other ("Panthere", I think) used 6C33C, I think. I owned the smaller version for about two weeks, or until I could re-sell it. It was nowhere near in the league with a Futterman H3aa or needless to say an Atma-sphere amplifier. Its sonic failings may have had as much to do with its poor quality of construction, as anything else about it. But so far as I know, Julius never built a triode output stage for any of his various models, of which I owned at least 3, at one time or another. As you probably know, he eventually settled on the 6LF6 as an output tube. He did build a headphone amplifier; maybe that one used triodes.
LewYou are basically right ,
but at that old time (1956) Harvard Electronics Co. NY under license was made J.Futterman OTL( model H1 , 15W ) amps based on 12B4A power triode tubes for AB class SEPP OPS , actually every early Futterman OTL amp was based on that 12B4A power triode , I have few original J.Futterman articles from 1954 & 1956 which clearly show that .
But of course that later he switch to use only TV horizontal sweep power pentodes probably with main intention to even more rise up the AB class OTL amplifier efficiency .
Any way my original point was that Futterman OTL PP amp under few condition can easy reveal typical SE amps sound signature ,or even to sound much better than any typical low power SE amp .
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/23/15
Thanks for that information. Even as I was typing my post, I was wondering also about the very early models, built in the early to mid-50s. I have seen only one or two in the flesh, but I did not realize he used the 12B4A in those days. Too bad Fourier made a hash of it in the later years.
I have an old HiFi Guide somewhere from the early 1960s that lists a Futterman OTL that used 6AS7Gs for power tubes. It had distortion specs that were not matched with transistors until a few years later.
LewIt is not surprise for me that Fourier do not have big success by replicating original Futterman amplifier concept .
Since even if that early triode based Futterman OTL amp looks very simple on the paper ( schematic ) basically they are not so simple , since surprisingly they are designed almost as some modern SS PP amps , with very high OLG ~60db ,with~ 35 dB of GNFB , with one extra bootstrap loop around of phase splitter and with final target to represent close as possible pure voltage source for at that time standard 16ohm load(speaker).
Even simple wrong layout of passive and active elements on amp chassis can cause unstable operation for that type of amp, sometime inserting of some extra compensation elements is needed to suppress amp oscilation ...
Any way to make long story short ,original Futterman OTL amp concept can not successfully replicate anyone , pretty big knowledge and experience is needed for that , and even bigger for original Futterman OTL amps based on TV power pentodes.
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 02/24/15
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