Sound recording and reproduction 1920s when several radio-related developments in electronics converged to revolutionize the recording process. These included improved microphones and auxiliary devices such as electronic filters, all dependent on electronic amplification to be of practical use in recording. In 1906, Lee De Forest invented the Audion triode vacuum tube, an electronic valve that could amplify weak electrical signals. By 1915, it was in use in long-distance telephone circuits that made conversations between New York and San Francisco practical. Refined versions of this tube were the basis of all electronic sound systems until the commercial introduction of the first transistor-based audio devices in
Valve audio amplifier technical specification it is referred to as the "phase inverter") to drive the power section. As an alternate to the long-tail pair, the "concertina" uses a single triode as a variable resistance within a potential divider formed by Ra and Rk either side of the valve. The result is that the voltage at the anode swings exactly and opposite to the voltage at the cathode, giving a perfectly balanced phase split. the disadvantage of this stage (cf the differential long-tail pair) is that it does not give any gain. Using a double triode (typically octal or noval) to form a SET input