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When a number of 1_______________ are hooked up in series so that the output of one serves as the input to the next amplifier stage, the function of the early stages is usually to 2______________ up the voltage to a high 3_______________, while the last stage builds up the power to a level sufficient to operate an output device. Audio amplifiers amplify electrical ac signals that have a frequency range corresponding to the range of human hearing, or from about 20 to 15,000 cycles per 4________________.