Audio Quality
Single-ended triode (SET) amplifiers are considered a classic design among certain audiophiles and have achieved a cult status because of their alleged excellent midband performance (argued to be the most important part of the audio spectrum in music reproduction), "musicality", and "directness". This perceived high sonic quality is mainly attributed to the simplicity and minimalistic approach of the circuits involved and the triode amplifying tubes that are typically used. On the other hand, the legitimacy of branding single-ended triode amplifiers as adequate for Hi-Fi purposes is debated, as from a technical standpoint, SET amplifiers are considered to be generally far inferior to subsequent (and more common) push-pull tube designs or solid-state amplifiers: SET designs require output transformers which are able to cope with a strong DC component in the signal, which causes them to have worse performance in regard to frequency response, distortion and efficiency (although the latter is not generally a priority for most SET enthusiasts, or audiophiles in general). Furthermore, as SET amplifiers have a relatively high output impedance, it is hard to couple them effectively to a loudspeaker which hasn't been designed especially to be driven by a high output impedance amplifier, as this will cause the amplifier to be much more sensitive to the loudspeaker's impedance characteristics across the spectrum, resulting in coloration.
In general, the configuration will usually provide higher measured distortion performance compared to high feedback amplifiers. This distortion is predominantly second harmonic which is not unpleasant to the ear (the second harmonic for an A on the musical scale at 440 Hz is 880 Hz which is, obviously, also an A i.e., the same note just an octave higher), but by definition their high THD figures make SET amplifiers unaccurate. In a push pull amplifier this second harmonic distortion is cancelled in the output transformer. Several percent THD is not unusual at full power output, but will be much lower at normal music levels. Some builders and users have concluded that while global feedback reduces distortion across the harmonic range, it also reduces the dramatic dynamics associated with a SET amplifier and the highly efficient speaker needed to enjoy a low power amplifier, but this is also strongly debated.
Apart from the field of hi-fi amplification, SET amplifiers are well regarded by some in the guitar world precisely due to their distortion characteristics, which may considered desirable in the context of musical instrument amplification, as their aim isn't accuracy but expressiveness and harmonic complexity.
Read more about this topic: Single-ended Triode
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