REQUIREMENTS FOR AUDIO PREAMPLIFIERS
For the listener the most common source of program is undoubtedly the radio, followed by disc and tape in that order. The microphone is normally used only in connection with a tape recorder, apart from public address applications.
Radio tuners specially designed to feed a low-level audio signal to high quality amplifiers are now becoming popular. Tuner outputs vary, but it is normal to expect a signal level between 150 mV and 250 mV. Provision for an input to accept a tuner signal is, therefore, desirable and this input should have a high impedance normally from 100 kΩ to 500 kΩ.
Crystal pick-ups working directly into a preamplifier require a high input impedance, otherwise there will be considerable loss ...
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