
Most audio
amplifier systems must have preamplifiers with many
different characteristics. These include high-gain
linear response for magnetic microphones, low-gain
linear response for tuners, and high-gain RIAA
equalization for magnetic phone cartridges.
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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