SET vs OTL

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

Aarif, removing distortion never causes an amplifier to have fake neutrality.

OTLs have an economy of scale- the bigger you make them, the more efficient they can run. A big OTL that can make 200 watts will very likely be comfortable on 4 ohms, some smaller ones that make 150 watts might be able to do that also. A 30-watt OTL will not like 4 ohms much at all.

IOW the smaller you make the OTL, the higher you will want the load impedance to be. Many OTL headphone amps are designed for 32 ohms or more, plus they do not have to make more than 0.2 watts. Headphone amplifiers also are a special case because most headphones are designed to operate with amplifiers that have a much higher output impedance than their own impedance.

Without more information about your speakers, it would be hard to make a certain case for either amplifier technology, and we have to include your listening preferences somehow too. Although I have heard very good things about the Graaf, I've not actually heard it, so the following is my opinion only. I think it would have a chance at being a more neutral, musical presentation if the speaker impedance is linear and 7 or 8 ohms at least. I don't agree with some of the tube choices in that amplifier, as I feel that they tend to make the amp sound brighter, so I do have reservations at the same time.

In the end, like so many other things in audio, you will have to audition to know for sure. Remember- its all supposed to be fun :)