Single-ended triode
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In a single-ended triode (SET) vacuum tube electronic amplifier the entire audio signal waveform is amplified by the power triode, in contrast to push-pull amplifiers where two output tubes work together in phase opposition. A typical circuit will have a driver that provides voltage gain, coupled to a triode (like the very popular 300B) connected through an audio transformer. Some audiophiles claim that the output transformer is the most important part of the SET.
SET circuits do not perform well at the frequency extremes (mainly because they don't use feedback), produce great amounts of THD (as much as 10%), have very high output impedance (i.e. 2 Ω) and very low output power (typically less than 12 watts per channel). It is therefore essential to match them carefully with efficient loudspeakers, like horn speakers.
Single-ended triode (SET) amplifiers are considered a classic design and have achieved a cult status among certain audiophiles, because of their excellent midband performance (argued to be the most important part of the audio spectrum in music reproduction), directness and ability to communicate the essence of music. This high sonic quality is mainly attributed to the simplicity and minimalistic approach of the circuits involved.
