SET vs OTL

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).

A single-ended triode amp will always have an output transformer. The transformer is both the boon and doom of the amp- it allows the tube to drive the speaker, but the bigger you make the transformer, the more limited the bandwidth will be.

As a result, the best SETs are the smaller ones (2A3s, 45s, that sort of thing) assuming you have a speaker that is efficient enough to work with them.

OTLs have no output transformer and so don't have the power vs bandwidth issue. Instead you have the opposite problem- its harder to make a practical low-powered OTL because you have to be more careful about choosing a speaker for it.

Usually SETs are zero feedback. OTLs normally have feedback (although ours tend to use little or none). A triode zero-feedback OTL operating class A, like SETs, will generate primarily lower-ordered harmonics, with very little higher-ordered harmonics. This will give either one a relaxed quality.

The ear uses odd-ordered harmonics to tell how loud a sound is, so if those harmonics are not emphasized by the system, the system will have a sense of ease and a lack of hardness.

OTLs are normally push-pull, if built single-ended will produce very little power for the number of tubes involved! Since push-pull operation allows for even-ordered harmonic cancellation, OTLs generally lack some of the romantic lushness of SETs as they will also lack the 2nd harmonic. This is an advantage if you are looking for neutrality and transparency: anytime distortion is present, detail is obscured.

Normally push-pull transformer-coupled amps will have an increase in distortion at very low power, robbing them of that 'inner detail' detail that is part of the 'magic' that SETs are known for. OTLs, like SETs, produce less distortion as power is decreased, giving them a 'magical' quality that they share with SETs; good 'inner detail' and good low level resolution while being very smooth at the same time.

I apologize for the nutshell quality of this post- I'm glossing a lot of things over as this can be a detailed subject!