Single-Ended Triode Amplifiers from http://www.theabsolutesound.com/articles/a-survey-of-amplifier-types-tas-217-1/
One of the most interesting trends in high-end audio over the past 25 years has been the return of the single-ended triode power amplifier. The single-ended triode (SET) amplifier was the first audio amplifier ever developed, dating back to Lee De Forest?s patent of the triode vacuum tube in 1907 and his triode amplifier patent of 1912. SET amplifiers generally deliver very low power, sometimes just a few watts per channel.
You heard right: Large numbers of audiophiles are flocking to replace their modern power amplifiers with amplifiers based on 100-year-old technology. Have the past 100 years of amplifier development been a complete waste of time? A surprising number of music lovers and audio designers think so.
The movement back to SET amplifiers began in Japan in the 1970s, specifically with designer Nobu Shishido, who combined SET amplifiers with high-sensitivity horn-loaded loudspeakers. Many who heard SET amplifiers were startled by their goosebump-raising musical immediacy and ability to make the music ?jump? out of the loudspeakers. Thus began the rage for SET amplifiers in Japan, which was about 10 years ahead of the SET trend in the United States. You can?t open a high-end
audio magazine today without seeing ads for very-low-powered single-ended triode amplifiers. The SET enthusiast?s mantra, coined by reviewer Dick Olsher, is, ?If the first watt of amplifier power doesn?t sound good, why would you want 199 more of them??
On the test bench, SET amplifiers have laughably bad technical performance. They typically produce fewer than 25Wpc of output power and have extremely high distortion?as much as 10% total harmonic distortion (THD) at the amplifier?s rated output.
Despite these technical drawbacks, my listening experience with SET amplifiers suggests that this ancient technology has many musical merits. SET amps have a certain presence and immediacy of musical communication that?s hard to describe. It?s as though the musicians aren?t as far removed from here-and-now reality as they are with push-pull amplifiers. SET amps also have a wonderful liquidity and purity of timbre that is completely devoid of grain, hardness, and other artifacts of push-pull amplifiers. When I listen to SET amplifiers (with the right loudspeakers), it?s as though the musicians have come alive and are playing in the listening room for me. There?s a directness of musical expression that?s impossible to put into words, but is immediately understood by anyone who has listened for himself. You must hear an SET firsthand to know what the fuss is about; no description can convey how it sounds.
The popularity and unmistakable sound quality of SET amplifiers pose a serious dilemma: How can an amplifier that performs so poorly by every ?objective? measure produce such an involving musical experience? How can 100-year-old technology eclipse, in many ways, amplifiers designed in the 21st century? What aren?t we measuring in SET amplifiers that would reflect their musical magic? Why do conventional measurements fail so dismally at quantifying what?s right in SET amplifiers? Do SET amplifiers sound so good because of their high distortion or despite it? As of yet, no one has the answers to these questions.
