SET amps and speaker sensitivity.

There is no hard and fast "right answer" but there are common guidelines. As swampwalker suggested, space/volume will have an effect. Speaker impedance (and how it varies by frequency) could also play a very strong role. Second, not all amps of X watts are created equally (power supplies may affect amp response to speaker load).

That said, I find the article to which Kehut refers above to be at best oversimplified and misleading, because I have think most speakers with 2x that sensitivity (87db/6ohms) would present trouble with the amp being reviewed with music which had real "needs" (Holst Planets' Saturn, Reger organ music, and the list could obviously go on). 845s are supposed to have more oomph at the low end than other SETs but my personal experience with SETS and relatively efficient speakers (96db on the bass, far more on the mids/tweets) is that one needs to test even higher-efficiency woofers with more power than recommended to see if one is getting the best out of the speaker. My experience with 96db woofers and 13 watts (despite a power supply I rate quite highly) suggested that to get real clarity out of the bass (and bass that goes deep with clarity makes the the rest of the spectrum stand out that much more - positively shiver-inducing in my book) I needed probably double or quadruple that at times. 100% of the time while listening to Mozart chamber music I would be fine. And even most of the time on orchestral music I would be fine, but when push comes to shove comes to tympanies pounding, cymbals crashing, and the organ in the background grinding out the low notes, I needed more oomph.

Of course, that and a dollar will get you a dollar cup of coffee.

Wow! great to have such an overwhelming response. The more I know, the more confused I get. I guess I am realizing that I might not even have a clear understanding of what an SET amp is.

I am assuming (again an assumption) that not all tube amps are SET amps. Could some of you give me some examples of SET amps. On that note, could some of you give examples of good speaker SET amp combinations that you have run across in your expereinces. Truly I am a novice here.

On another note, my room size is 22L by 13W by 8H. That in feet. Is this big enough to put a floor standing speaker in? I've read on other forums that this is a common mistake, trying to fit too big a speaker into too small ar room Any feedback would be appreciated.

And lastly, just to give polks and idea of what I am looking into for the distant, I mean far distant future, would be something like matching the audio aero prestige monoblocks or the lamm ml2.1's with a floor stander from focal like the alto ot nova utopia be. Any comments?

Thanks all,
Peter

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.