SET vs OTL

The guy perhaps most equipped to answer questions about OTL is Ralph of Atma-sphere (apologies to you other lurking OTL experts), who sometimes finds the time to post on Audiogon. Until he gets here, I will attempt to not embarrass myself unduly.

SET means Single-Ended Triode. A triode is a three-electrode amplification tube, and a single-ended triode amplifier is one where only a single triode tube acts as the gain stage. This is in contrast to a 'push-pull' amplifier where the signal waveform is 'split' into two parts, and one half of the signal is amplified by one device, and the other half by another device.

OTL stands for "Output-Transformer-Less". This means that while most tube amplifiers are coupled to the speaker by an output transformer (that big hunk of steel which makes the amp so heavy), the OTL amplifier is not burdened by the hunk of steel and the way that it might affect the signal. OTL amps are coupled to the speaker load either by (to my knowledge - and I dearly hope Ralph of Atmasphere can jump in if I miss something here) direct coupling (which prevent DC through circuit topology (and tube choice?)) or capacitive coupling (which does so with large-value capacitors).

Reading between the lines, this means that SET and OTL are not necessarily mutually exclusive by definition. It would be possible to have a SET OTL, however, the Cary 300SE is not one of them.

The Atma-sphere website has a section for white papers and there is some interesting reading to do there. From a quick look, it appears there was a 300B OTL project.

I also know that David Berning also has developed OTL and ZOTL (a variation on the OTL theme) technology and he probably has some interesting info on his website. At one point, I think he made a SET OTL amp for someone (if I remember correctly, it was an Audiogon member).

As to your question:

Is it true that despite its operational inconveniences, a good OTL (eg Tenor Audio) will always sound more "natural" than a good SET (eg a Cary 300SE)?

I don't have a clue. I expect that implementation counts for more than topology, and besides, I do not have enough experience comparing OTLs (which when I have heard them have been wonderful) with 'regular' SETs.

With regard to your follow-on post, several of the extant OTL mfrs offer higher-power OTL-topology amplifiers - many of them a LOT more than 3W. If OTL is what floats your boat, it can be found with enough power to drive somewhat insensitive speakers (as long as their impedance is high enough and the load is relatively flat across the frequency spectrum). In fact, if you wanted 50W, it would likely be a lot cheaper to do this with OTL than SET (because with SET you need to find a single tube which can put out 50W (there are a few, but not many, and the tubes themselves are expensive, and the OPT (output transformer) will generally be really big (and expensive)).

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.

Larryi, I don't agree that the low level dynamics and clarity of my 845 SET are lacking in the least. In fact these very things are improved vs. 2a3 monoblocks, push/ pull tubes and SS amps I own or previously owned.

I would agree that my 845 amp can sound rather dull and flat at lower volume levels, but that is not the same as saying it lacks in microdynamics and clarity. Perhaps this is what you meant?

I also love the Joule OTLs and Atmasphere certainly is well respected. OTLs are fast and immediate as they don't have the liabilities of tons of wire in output transformers. My 845 amp shares this trait despite it's output transformer, in fact, I would say that is it's most distinguishing characteric vs. my push/pulls and SS amps.

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).