Audio Amplifier systems with preamplifiers have various characteristics that includes low gain linear response for tuners, high gain RIAA equalization for magnetic phone cartridges and high gain liner response for magnetic microphones.
To meet this broad requirement, most amplifier designers include a single universal preamplifier circuit. Basically a high-gain linear amplifier, its characteristics can be altered by switching alternative resistor filter networks into its feedback system.
For example, when the selector switch is set to the Mag phono position, alternative input sources can be selected by S1-a, and appropriate linear-response gain control feedback resistors 220K, 470K, and 1M are now selected by S1-b. Those feedback resistor values are selected between 10 kilo ohms and 10 megohms to suit individual listener tastes. Circuit gain will be proportional, to the feedback resistor value.
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