SET amps and speaker sensitivity.

Basically, unless your speakers are 93db or more efficient, forget the normal SET. I grant that there are more powerful and expensive output tubes that could be used with less efficient speaker, but few rival the sounds of 45, 50, 2A3, or 300B SETs.

In my experience there's a huge range of preferred loudness levels among different listeners.

Whether you listen at 70 dB average SPL or 90 dB average SPL will make a hundredfold difference in your amplifier power requirements.

So in my opinion, "this first - know thyself".

Duke

SET- Single-Ended Triode amplifiers are a special class of tube amplifier. They are all class A. Usually the circuits are quite simple- the meat of it is the output transformer, which transforms the impedance of the output tube to that of the speaker.

Since there is usually no feedback, the amps are best off driving speakers that have a benign (flat) impedance curve, failing that at least a curve that has more high impedances than low as the amp will attempt to make constant power into higher impedances. Having no feedback means less in the way of high frequency sheen or hardness.

SETs are *particularly* good at low distortion at low power levels. Unlike regular push-pull tube amps and transistors, the lower the power the less distortion- so you get a lot more low level detail. This comes at a price- low total power output and often limited bandwidth. To really appreciate what they do, high efficiency speakers (97+ db) are *mandatory*! -so that the amp is able to work in the low distortion region of its envelope.

If you use lower efficiency speakers you simply will not realize the benefits of SET unless you have a very small room/nearfield situation or the like.

Due to the nature of the bandwidth issue associated with the output transformer, the smaller you make the amp the better they sound. 15 years ago the 300b was the insider story- 7-8 years ago the 2A3. Today its the 45- only good for 0.75 watts (in truth...). So to take advantage of such low powers a speaker of 103 db or more is recommended.

Great thanks folks. I have a lot to learn. So is this where bi-amping comes into play. Can you bi-amp two lower output SET amps and pair those with less efficient speakers?

Cheers,
Peter

Peter,

I never think of biamping as a means of delivering more juice. Rather, I find that the biggest advantage derived from biamping is a significant increase in liveliness and microdynamics at lower volume. But, biamping is quite a complicated subject and good results can be quite tricky to achieve. I personally like the results I've heard from active biamping (a dedicated crossover that splits the signal at the line level before the signal is amplified by separate amplifiers). But, even when this was being done by someone who is experienced, it took a lot of experimentation and changes to the design of the crossover and even the amps to get optimal results. When not done right, the drivers don't blend correctly.

I think the better first course is to get amps suited to the particular speakers that you like/own. I would be less concerned with the operating type -- SET, or pushpull, etc.. I own an expensive SET, but, I don't think they are necessarily the best amps, and this is the case even with the fairly high efficiency speakers I own. I have heard some fantastic pushpull amps (e.g., 60-year old Western Electric amps) as well as several fantastic OTL amps (incredibly lively and upbeat sounding).

I know that getting enough power to properly drive a system is an important consideration. But, sometimes too much emphasis is placed on power. I have seen people pursue more and more power because something is lacking in the sound and they do not quite connect completely with the music/performance; these people think more power will do it. I have found that whenever I have made the right choice in improving my system, I tend to listen at lower volume than before -- I don't need volume to compensate for something else that is lacking. Good systems resolve musical detail, harmonic structure, etc. at low volume.