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In Reply to: RE: Western Electric posted by morricab on May 22, 2014 at 13:35:02
Once you get rid of the transformer the argument for SET sort of goes away. Just like an SET, as the power level on our amps is decreased, the distortion decreases to unmeasurable. That with a good noise floor makes them an excellent choice for horn loudspeakers.
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Well that is true for an OTL as long as you are not using negative feedback. You can get the same drop in distortion also with SS devices if you don't use feedback.
As for not needing single ended...I disagree with you on this one because push/pull is inherently not perfect and as such creates distortion just on the mismatch between the two pushing and pulling devices will generate additional and unnatural distortions. Also, the cancellation of even order harmonics results in a less natural harmonic progression that is not seen in nature...nature is single ended.
With phase/time coherent speakers one can hear these trait differences regardless of how good the amps are. It was particularly obvious when I was using Acoustat full range electrostats and nearly as obvious with my current Reference 3a speakers.
MOST OTLs use a fair amount of negative feedback (even your small one the S30 does if I am not mistaken), which pushes them even further from the ideal distortion pattern, so while the benefits of greater transparency and usually tighter bass are there the tonal balance is shifted sadly more towards a typical "solid state" kind of leanness. If you read the reviews of your M60 OTLs, many of the reviewers note a slight leannes to the sound. For sure much of the warmth of the SET amps is derived from the output transformer distorting; however, I think that the best ones keep a natural sounding tone without undue warmth or leanness and they are almost always without negative feedback.
I recently had on hand the very nice sounding VAC Renaissance 30/30 MKIII, which has a nice tone and very high resolution. However, I could hear that it was push/pull because some things just didn't have the coherence one gets from my SET amps. It took quite some listening to nail it down but once I did I couldn't keep the VAC. I also tried an Einstein hybrid...it sounded relatively dead compared to a good SET.
You might be interested to read about my current amplifier, the NAT Symbiosis SE
http://www.nataudio.ch/cms/index.php?page=symbiosis-se-single-ended-integrated-amplifie
It is rather unique in that it is truly single ended (only one output device per channel) but that output device is a big industrial MOSFET. THe input and driver are 9 pin small signal tubes. It is all direct coupled and zero negative feedback. There are also 2 other industrial MOSFETS acting as regulators for the output ones (that is why there are 4 big heatsink towers).
What does it sound like? A big SET without any fluff around the notes...it is very precise and very dynamic but the tone is quite natural. When it fully warms up (over 1 hour of playing) it also has that otherworldly transparency of OTL. Until then it has the transparency of a good SET. Bass is very well controlled and powerful but natural as well. It is there that one can tell the output is a transistor.
That said, I think that a true SET OTL could be even better but it would be tough to do without adding a fairly significant amount of negative feedback...or would it?? I put it to you as a challenge. I know of only one on the market from Transcendent Sound but I would have to buy monos to get a reasonable 12 watts. Still I am considering...
morricab I see you are selling your NAT Symbiosis SE amp.......Why and what will you be getting in it's place? Maybe a Novacron? :-)
Looking at some very high sensitivity horns and don't need the power. I would look into SEOTL amps like the Transcendent sound kits.
I think you will find that even with high efficiency loudspeakers that if the amplifier is clean enough you will find yourself using more power than expected.
I've not seen an SEOTL that really makes an appreciable amount of power. Plus the ones I have seen have been really inefficient- using several high power tubes in parallel to achieve what can be done with just 2 6AS7Gs. I can pretty well guarantee that the 2 6AS7Gs will be audibly lower in distortion as well. Food for thought.
The primary distortion component we see in our amps is the 3rd harmonic. A lot depends on setup, but that is pretty much what we see. There was one test that showed something else, but the reviewer mistakenly grounded a speaker terminal with resulted in uneven drive of the power tubes and a lot more distortion as a result.
The contention that nature is single-ended seems peculiar to me- I don't know how such a statement can be justified. In that regard its not push-pull either- the whole things seems a Red Herring.
The leanness you comment on is not universal with all reviewers by any means. Certainly our amps are not as lush as an SET because they lack the 2nd harmonic, which the ear translates into tonality (in fact seems to weight the effects of distortion to a greater degree than actual frequency response errors). We have lots of access to live microphone feeds as we run a recording studio; in working with various amps in the control room its pretty obvious that the SETs are simply less neutral.
IMO if you really want to hear what an SET does, it should be on a speaker that is efficient enough that the amp never goes above about 20% of full power. In this way you get to experience greater detail and less coloration as the higher ordered harmonics tend to stay out of the picture, and of course at lower levels the distortion is absent entirely.
This makes them very similar to the sound of our amps, except that we have more bandwidth and power. BTW, the S-30 uses 4 db of feedback and is the most of any of our amps (the Novacron, MA-1, MA-2 and MA-3 have zero feedback). We have installed switches for customers to shut off the feedback entirely as the amp does not need it to be low in distortion. Typically we only get about 1% THD at full power if the tubes are set up right. So while there appears to be an argument against push-pull with respect to higher distortion, in practice it does not seem to manifest.
When doing AB comparisons, this difference of lower distortion in our amps is easily audible. As a result our S-30, provided it is installed on a loudspeaker that both amps can drive, has taken on some pretty heavy hitters in the SET world, ones that often cost many times more.
Yes, I know your amp gives predominantly 3rd harmonic as it is push/pull that cancels out even harmonics. Nature tends to have monotonic harmonic decay (2nd bigger than 3rd bigger than 4th etc.) unless it is completely anharmonic (like a cymbal crash). This is what Jean Hiraga was going on about 30 years ago and Cheever much more recently that the PATTERN has to be monotonic and in line with ear/brain distortion mechanisms (aural harmonics as Cheever puts it I think). A push/pull amp cannot do this correctly...by design.
Your OTLs do not sound overly lean, IMO, but I can see where that comment could come from. THe OTLs I had were incredibly transparent and powerful sounding but not as fluid and coherent as I get from SET. No OTL I have heard does that even as they are doing some things far better than all but the best SETs. For correct tonality I have to give the nod to really good SETs...those that don't have significant issues with their output transformers.
"IMO if you really want to hear what an SET does, it should be on a speaker that is efficient enough that the amp never goes above about 20% of full power."
This is probably true of any amplifier that is not using any negative feedback as the distortion is rising with increasing power. An amp with heavy negative feedback almost has to be played hard just to "wake up" a bit. I don't think it is a SET or PP thing, more of a feedback or not thing.
"This makes them very similar to the sound of our amps, except that we have more bandwidth and power"
Well, my NAT makes 100 watts of SE(T) power so it is right up there with your MA-1 in terms of speaker compatibility...probably better because of the damping factor. Bass is superb, deep tight and yet natural in a way that feedback bass is not.
I have no doubt that the S30 is a price/performance champ, but if you have the budget for better then it and the M60s do have competition from both the SET and hybrid worlds.
Again, make a 10-20 watt SEOTL and you will have a customer ;-).
If I get the really high sensitivity horns I am looking into I will probably be trying one the Transcendent Sound SEOTL models
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