Negative feedback involves taking some of the amplifier’s output signal and feeding it back to the input.
If you’d told Western Electric executives of the 1980s that in 1997 they would put the 300B back into production, they’d have thought you were crazy.(A single-ended triode power amplifier using the 300B output tube is shown in Fig.1 The 300B is the bulbous tube in the middle.)The triode is the simplest of all vacuum tubes; its glass envelope encloses just three electrical elements rather than the five elements in the more common (and modern) pentode tube.Triodes have much less output power than pentodes, but more benign distortion characteristics.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-endaudio magazine today without seeing ads for very-low-powered single-ended triode amplifiers.One SET designer told a reviewer, in all seriousness, “If you liked my 9W amplifier, wait until you hear my 3W model.”In addition to low output power and high distortion, SET amplifiers have a very high output impedance as amplifiers go: on the order of 1.5–3 ohms.This is contrasted with the 0.1 ohm output impedance (or less) of most solid-state amplifiers, and the 0.8 ohms of many push-pull tube designs.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.(Later in this excerpt we’ll look more closely at how this works.) SET proponents believe that because the triode amplifies the entire waveform, SET amplifiers offer the ultimate in sound quality and musicality.Moreover, SET amplifiers have no need for a circuit called a phase-splitter, making them even simpler.