SET vs OTL

The harmonic structure of a good OTL will be the 3rd harmonic, which the only harmonic that the ear considers musical and thus does not object to. SETs have that and the 2nd harmonic.

Paul, what happens with these harmonics is not that they are not audible! It is that they are not **objectionable**. When GE did their experiments in this regard, they found that people will not *object* to nearly 40% of even-ordered content. That is not the same as 'you can't hear it', you certainly can, and audiophiles have terms for the presence of even orders (just like they do for odd orders) in small amounts. From small amounts to larger amounts: Warm, lush, rich, bloom, thick, murky, woolly, muddy. FWIW these terms refer to even ordered harmonics that are in greater supply than the terms that apply to odd ordered harmonics: hard, harsh, brittle, clinical, etc., where the odd-ordered enhancement may well be less than .001%.

With regards to the phase splitter- some OTLs that is an issue, others it is not. In our amps, there is no 'phase splitter' as a stand-alone circuit- that function is integrated into the single gain stage that exists in our amps.

Most SETs have more than one stage of gain- its my contention that if they were able to have only one, they would be a lot closer to the neutrality that our amps routinely express. I've got a 45 SET and a 2A3 SET- they sound great within their dynamic limitations, but they also sound veiled compared even to amps we were making 10 years ago, despite using the best parts available. As far as I have been able to experience, transformers always limit bandwidth, add distortion and rob the music of detail.

Given the example of the speaker above, I would recommend something with more power than the Graaf, or any SET for that matter. The speaker is simply too inefficient for an SET to strut its stuff. You would really need at least 10 more db for that. There are OTLs that can easily drive that load though. IMO 15 watts would not be enough power, unless you listen near-field and at moderate volumes.

Ralph,

Yes, there are studies that show even-ordered harmonics as musically consonant, but I do believe some of them or other also showed lower amounts - like 3% - to be inaudible.

It is very easy to keep THD down well, well below 3% - more like .1% or better - with copasetic speakers at moderate volumes. That's why I can't accept that the differences in HD spectra explain *everything* about why single-ended sounds different than push-pull.

As for the number of gain stages, most low-power SETs have one plus the output stage - just like your amps - right? I'm quite admittedly not an engineer but your OTLs do have two stages of gain counting the output tubes, right?

As for the lack of a phase splitter, that sounds like a plus, but please tell me, does crossover distortion still exist? I've always thought another of the big benefits of single-ended operation is that there is (obviously) no such thing as crossover distortion - because there's no 'crossover' - and distortion is *proportional to power output*.

Push-pull amps, all of them if I'm not mistaken, do suffer from the fact that crossover distortion is *inversely* proportionate to power, meaning the tiniest nuances are lost.

Not that your amps sound like they are missing nuance - they don't.

They also, incidentally, have some of the best low-bass control I've ever heard, even on 4 ohm speakers - an OTL myth shattered.

I do listen near-field and at moderate volumes and am looking to maximise neutrality.
It seems your advice Atmasphere would be to either get an SET with a single gain stage (who makes these pls??), or an OTL.
Thanks!

... and here is the famous SET that generates 150w per channel and a bandwidth of 20Hz-100,000 kHz. Too bad it costs $350,000...
The Wavac SH-833 :
http://stereophile.com/tubepoweramps/704wavac/

Aarif,

I don't think there really is any standard for "neutrality," or any other subjective quality. What I would consider neutral (neutral what? -- high frequencies, low frequency, etc.?). I am not familiar with the Audiovalve RKV that you like, so it is hard to say what it will be closest to in terms of tonal balance, or any other quality.

I don't think that the theoretical discussions are that helpful either. Sure, Atmasphere is a designer of OTL amps and honestly believes that they are inherently superior (transformers are FAR from perfect in their conversion of high voltage/low current to low voltage/high current output, and arguably SET transformers are WORSE than pushpull transformers). The other camp can point to theoretical disadvantages of OTL vs. SET (need to use multiple output tubes in parallel for high output and sufficiently low output impedance, need for a servo circuit or blocking capacitor to prevent passing high voltage DC).

Also, there is quite a bit of difference between specific models for each type, plus the different way each model may interact with the specific speaker it is used with.

So, as a very rough generalization, I would say that almost all of the OTLs I've listened to (Graaf, Fourier, SAP, Atmasphere, Joule, custom designs) were exremely fast, dynamic and exciting (the Tenor was the exception). Once you've had your ears pricked up by an OTL, it is hard to listen to more listless and dull amps. I liked the tonal balance, so I would call it "neutral," but, I could see how someone else might find certain of these amps a bit lean. The "excitement" also comes with a bit of rough and tumble quality that someone might describe as crude or "not refined" (someone who wants a smoother sound).

The SET amps I've heard vary FAR more than the OTL amps I've heard. The better amps deliver a pure, natural sound, dense and complete harmonic structure, and natural sounding attack and decay of notes (the notes seem to bloom into space like they would in a real hall). But, depending on the design and type of tube, one can get SET amps that sound lean and fast and dynamic (within their restricted power output), and others that sound warm, sluggish (smooth ?) and woolly. "Neutral" or not, there is something seductive about good SET amps that is harder to describe (you know it if you hear it), while the special qualities of good OTLs are pretty evident.

I would put the Cary 300 SE you mentioned well into the warmer, smooth side. If you think the Graaf is neutral in balance, the Cary may not be for you. I actually liked the Cary even though it is radically different sounding compared to my SET amp, but only after the tubes had been swapped (the stock amp can be substantially changed in sound by tube selection).

I only heard the Graaf briefly, but, I recall being favorably impressed. It sounded similar to the SAP OTL I heard and like quite a bit. You should also seriously consider the 30 watt Atmasphere OTL. A friend had that amp and I thought it sounded terrific (he traded it in for something else because of noise -- a bigger issue in his system than it would be in almost anyone else's system because his speakers are 105 db/w efficient).

By the way, I've never heard the WAVAC, but, I heard an amazingly good Audionote Sogon amp (211 tube) that cost something like $250,000. I know someone who has two pairs of those amps. Crazy world.