Considerations in the design of a high-fidelity radiogramophone

Abstract

Receiver performance has in the past been limited by inadequate audio-frequency range (50–5 000 cycles per sec.) and inadequate range of volume levels. The paper describes a radio-gramophone in which alternative upper frequency-limits of 3, 5, 7, and 8 kilocycles per sec. are provided, and the range of volume levels is increased by including a form of “negative automatic volume control” in the audio-frequency amplifier.The first section of the paper summarizes the requirements of the new radio-gramophone, which include radio-frequency, intermediate-frequency, and audio-frequency circuits each of adjustable and width. The radio-frequency section includes amplified automatic volume control and a selective quiet tuning and indicating circuit. For gramophone reproduction a new pick-up and needle, with a cut-off frequency of 8 000 cycles per sec., are described, together with a motor giving extreme constancy of speed. For the final acoustic reproduction, an improved electrodynamic loud-speaker is described having a paper cone with a metallic centre portion to give good reproduction up to 8 000 cycles per sec.Circuit diagrams and performance characteristics of the individual sections of the receiver are illustrated, and the paper concludes with a series of acoustic frequency-response curves taken under various conditions.