SET vs OTL

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!

Thanks guys this is absolutely fascinating!
3 specific follow on questions:

1. Other things being equal, the choice then is deciding what hurts naturalness more... (a) the lushness or "smearing" that occurs as the tube interacts with the transformer in an SET, or (b) the fake neutrality coming from the even ordered harmonic cancellation of push-pull in an OTL. Correct?

2. I am not looking for a lot of power as my listening room is small. As such, the choice could likely be between, say, a CARY 300SE (15W per channel) as the SET contender, and the GRAAF GM20 (20 Watts per channel) as the OTL. Not that much difference in price...Theoretically which amp should sound more natural?

3. "its harder to make a practical low-powered OTL", why is this? I have a low powered OTL which sounds great: http://www.audiovalve.de/rkv/engrkv.html

Thanks again.

Just to add to the above, my speakers have a very flat impedance curve (ASA Monitor Pro). Looking forward to clarifications on my 3 questions. Thanks

Aarif,

I've owned both types of amplifier and, before you can describe either, you have to bear in mind that the sound of these amps are entirely speaker match dependent. So, assuming that we've got the right speaker match, I'll paraphrase Ralph;

SETs amps tend toward candlelight (warm & romantic)

OTLs amps tend toward sunlight (neutral & revealing)

IME

Marty