The main function of a Preamplifier is to amplify small and weak signals for further amplification. Generally, weak signals from microphones, audio sources and other sound detectors must be extracted with compromising the intrinsic signal to noise ratio (SNR).
Hence, the best position of a preamplifier is close to the sensor or detector. The output of the preamplifier is further amplified by Power Amplifiers.
Preamplifier amplifies the signal with very high gain but doesn’t have the drive current or current gain to drive the output. Hence, the boosted signal from preamplifier is given to a power amplifier where the current is amplified.