I like both SET and OTL amps. Both kinds of amps can deliver very clear sound and very good, natural dynamics. OTLs, in particular, sound extremely lively and dynamic.
OTLs can be easily scaled up in power by adding more output tubes, though heat and power consumption can be very high. The more tubes in parallel, the lower the output impedance, an issue which some people are concerned with when it comes to both OTLs and SETs (high output impedance can be a concern with speakers with a low nominal impedance).
I like the Atmasphere and Joule OTLs. The big Joule I heard was richer sounding and had more midbass than the Atmasphere. Perhaps it is a matter of tradeoffs, but, the Atmasphere sounded more nimble. The OTL I did not particularly like was the Tenor. It was a nice sounding amp, but, it completely lacked the kind of explosive dynamics which is the raison d'etre of OTL designs.
A well designed SET can also be very clear, fast and dynamic, but, the designs that have that quality tend to be extremely low powered. The better SETs, while simple in design, put an extreme demand on a quality, air-gapped output transformer. These things can be really costly. Personally, I think that some of the clarity and low level dynamics of SETs are lost when higher powered tubes are employed in SET designs (845, 211), though some of these are still terrific sounding.
I own a SET designed around 2a3 tubes. It is a parallel SET (two output tubes per channel). The amp can deliver almost 6 watts per channel. I like the sound of 2a3 tubes, but, compared to 300B designs, for example, this is a leaner, and faster, sound. Like everything else in life, parallel output tubes in a SET is a compromise. Purist will say that multiple output tubes muddy the sound and cause a loss of low level detail (don't tell that to the OTL crowd).
Overall, I like the SET sound, but, most SET amps can effectively be used with an extremely narrow range of speakers. My speakers are 99 db efficient and I probably could use a bit more power. OTLs are WAY more practical when it comes to matching with common speakers. |
Sns,
I actually do like a number of 845 and 211 SET amps. For speakers needing more power than what a low-powered SET can provide, good SETs using either tube are contenders. But, when driving extremely efficient speakers, I don't think they are, at lower volume levels, as detailed sounding or deliver small scale dynamics (sound lively) as 2a3s or 45s. Of course, I've only heard a small number of any kind of amp/tube combination in my own system, so I am only roughly generalizing here. When it comes to SETs, the output transformers are really hard to design and build properly so a LOT depends on getting that part right. I bet that is particularly the case with either 211 or 845 amps because of the high standing current in the primaries.
For the vast majority of applications, that would be an academic argument because a low-powered SET would be unsuitable. I believe that first and foremost, one picks a speaker system that sounds great and then looks for a suitable amp. If I needed more power, I would certainly consider 845/211 amps (e.g., I really like the Wyetech 211 amp). I've heard some really strange sounding systems where someone "fell in love" with a particular low-powered amp and then went looking for suitable speakers (some single driver systems and horn systems are truly weird sounding to me).
It is a matter of picking the best compromise. The SET purist out there would certainly have a lot a bad things to say about the parallel single ended approach employed in my amp. I am not convinced any particular design is inherently superior. One of the very best amps I heard was a pushpull amp that is over 50 years old (a huge, rack-mounted Western Electric amp that currently sells for something like $70,000 per channel). |
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. |
Atmasphere,
I agree that multiple output tubes is really only a theoretical source of less than perfect sound (my SET has parallel output tubes). I am merely pointing out that "theoretical" concerns can be raised about any design choice.
A friend of mine has a direct-coupled OTL that has performed flawlessly for years. I would worry about some kind of catastrophic failure, such as an output tube shorting. Can that happen and is it a realistic concern? Wouldn't a servo circuit be the best approach to maintaining good sound while assuring safety?
Thanks, your posting are very informative. |
Thanks Atmasphere for clearing up the issue of safety/reliability of direct coupled. A friend has direct coupled OTLs hooked up to Sonus Faber Stradivari speakers. When asked about what might happen if a tube shorted, he just shrugged his shoulder and said: "I guess you just buy another speaker."
On the which-low-powered-triode-is-my-favorite debate, I like 45s, they are sort of in between: not as dry and lean as the 2a3, not as warm, and lumpish as the 300B. 45s have a big low end, but that low end is also tight. The other nice thing is that a lot of decent older 45s can be had without spending crazy dollars. The PX25 is a nice tube too. I am not a big fan of 300Bs, but some amps using them can be quite nice, particularly when equipped with Kron 300Bs. |
Although I am one of those that expressed a personal preference for 45s and 2a3s over 300bs (I own a pushpull 45 amp and a parallel SET 2a3 amp), I hardly think it is fair to say someone is wrong or inexperienced for have a contrary preference. A friend of mine who owns more than a dozen different amps, many he built or modified, prefers a 300b SET amp for his setup. His custom horn system is 105 db/w efficient and his room is not very large, and he listens at fairly low volume, so power is NOT an issue. In this case it is more of an issue of system matching. The horn loaded bass driver does not go low enough for the punchier, better controlled deep bass of 45s and 2a3s to matter as much, and the slight leanness of his system's upper bass/lower midrange is nicely compensated for by the upper bass bump of the 300b. This is a case of different horses for different courses so no there is no absolute superiority of one over the other.
Martykl, I've heard AN/Es being powered by pushpull 45 amps in a very large room and was actually surprised by the volume that speaker can deliver, given its somewhat modest efficiency. I don't know how it would have sounded with a 45 SET amp however (SET would be roughly half the power). I prefer that speaker a little bit away from the corner because its bass response can be a bit too much in the corner. Because Audionote speakers have a fairly warm warm upper bass and lower midrange response, I would have some concerns with 300bs overemphasizing these qualities. Still, one never knows without a trial.
One more thing to add. There are certain 300b tubes that are better at controlling bass and reducing their loose and overripe upper bass response. I am talking about the Kron 300b. It is no longer made, though new stock is still available. Because of the close relationship between Kron and Emission Labs (EML), it would be worthwhile looking into EML's current production 300b (I use their fantastic 2a3 tubes myself). |
Paul,
The pushpull 45 amps were built by a Russian guy named Pavel and were sold through a Washington, D.C. area dealership (Deja Vu Audio). These amps used Acrosound TO-330 output transformers (the feedback taps not employed, obviously). The wiring was all point-to-point. In fact, Pavel laid out the amps so exquisitely well that there was almost no wire used in the amp (lead-to-lead connections).
I have one of these amps, though mine uses solid state rectification (most of Pavel's amps employed tube rectification). Some versions used big Chicago brand transformers.
I like the sound of these amps very much. But, my current amps (Audionote Kagekis) are, better to me (at a HUGE price premium).
Deja Vu Audio is an interesting store. They will run almost any speaker with low-powered tube amps which "conventional wisdom" says are way too underpowered. The store sells Quad speakers, and even the 2805 is connected to a fairly small tube amp. The one solid state piece in the store is a Halo JC-1 amp. I asked about it and was told that it is used purely for comparison -- Vu, the proprietor, wanted a decent, high-powered solid state amp for a reasonably fair showdown. |
Every OTL I have heard had the quality of being very immediate and exciting sounding; the music comes alive to a degree I have never heard with solid state amps. But, that is not to say that some pushpull and SET amps don't do the same, it is just not a common characteristic of other types of amps. OTL amps, like amps of any other topology, sound different from each other based on tube choice, voicing by the designer, choice of other components, specific circuitry, etc. The Atmasphere amps, for example, are leaner sounding than the Joule amps I heard. Which is better depends on taste and system matching.
I have no idea which form of amp is superior to the other. I run fairly efficient speakers (@ 99 db/w), so I can use a pretty wide array of amplifier types, including low wattage SET amps. I have two pushpull amps (45 tubes, and 249 output tubes) and a SET amp (parallel 2a3s). I don't know if I really have a "favorite" between the SET and the 249 pushpull amp (I currently like the 249 and have it in my system, but, I like certain things about the 2a3 SET amp). One of my all-time favorite amps, however, are OTLs that a hobbyist built that two people I know own. Another great amp I heard is a Western Electric 59A (252 meshplate output tubes) which is a pushpull amp that is incredibly dynamic and exciting sounding (also insanely expensive). So, I have no idea which approach is superior in delivering the sound I like, much less what anyone else would like. |
Jet,
I find that the upper bass on the AN-E, as well as other Audio Note models, can be a bit bloated, but, it is a very musically enjoyable speaker so, if that is a flaw, so what? I am not as big a fan of the 300b as I am of other small triodes (2a3 and 45) because the upper bass and lower midrange is a bit overblown. But, as you demonstrated, things can be tuned to ameliorate these sort of issues.
I have heard an OTL on the AN-E SEC and the sound was fantastic. I think it would be worth your effort to find an OTL to try in your system. The Atmasphere M-60 should be a good candidate. It is on the slightly leaner side, so it would not exacerbate the tendency of the AN-E to sound a bit bloated. All three M-60s I've heard had a slight bit of buzz or hum, but, given that the AN-E is not extremely efficient, I doubt that this noise will be much of an issue. |
The autoformer used as an impedance matching device is, in effect, an output transformer. It is doing exactly what an output transformer does. To me, if something sounds good, I don't care how it got there. I've heard many great tube amps, and I would not dare to say if any particular topology beats out another. My personal favorite happens to be an OTL amp, but, it is an utterly unique, one-of-a-kind design. A very close second (maybe not even second) is a pushpull amp that is extremely rare and costs six figures for a pair (Western Electric 59A). I own a pretty good SET amp (Audio Note Kageki) and I've heard many other good SET amps, but the amp I currently run in my system is a low-powered pushpull amp. |
No, it would constantly be in protection mode, meaning no output. |
I don’t think it is possible. The tubes in a SET amp are constantly outputting dc that is the modulated up and down in output by the audio signal. That changing output fed to the primary windings of the output transformer then induces an alternating current in the secondary. Absent a transformer, a SET amp would be sending a very large dc signal to your speakers, particularly when there is no music playing, and dc is very bad for speakers. |
