Parametric Input Stages in Electroacoustics, Part 1: Low Noise Playback Amplifier in Magnetic Recording
In this article the signal-to-noise conditions for the tape recorder input stage are discussed. Besides conventional input stages using tubes and transistors, possible parametric amplifier circuits are considered, and a varactor type is described in detail. Signal-to-noise conditions are compared on the basis of the equivalent noise resistance of the circuit. For all other circuits as well as for the noise contribution of the head and the tape, appropriate values have been determined and are tabulated as a function of frequency. It is shown that the transistor is inferior to the tube, which becomes noticeable at lower tape speeds. A parametric varactor circuit, however, proves to be equivalent to the best tube, making it possible to design a semiconductor input stage with good noise properties.
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