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A Transistor Preamplifier Design for Magnetic Pickups
This paper describes the design and performance of a transistor preamplifier equalizer for magnetic pickups for use in broadcast studios. It can be used with low impedance magnetic or high impedance variable reluctance type cartridges. The preamplifier uses three transistors and employs a feedback equalization which provides optimum response between 50 to 15,000 cps. The basic design equations for the transistor feedback equalizer are derived. The preamplifier can deliver an output level of -20 dbm to a 150-ohm load, with less than 1% harmonic distortion. The use of the 2N175 low noise transistor in the input stage results in a high signal-to-noise ratio. A three-position switch is provided for adjustment of the high-frequency response of the equalizer in accordance with the preference of the user. Alternating current line power rather than batteries is used to insure continuous operation. Etched wiring boards are used for automatic production and product uniformity. The preamplifier is designed to be mounted to the top of a studio turntable. The many advantages of transistors, which include ruggedness, lack of microphonics and the absence of heaters to burn out or to inject hum make the choice of transistors for this preamplifier design a very logical one.
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