A pre-amp does the following: 1) it amplifies the low level signals from devices to the point where power amplifiers can then take the signal and further amplify it for rated output. 2) it is a control amplifier whereby it has volume control and switching capability so that you may switch between different input sources and 3) it is an impedance matching device. The theory is that if you separate devices and power supplies you will get cleaner, less distorted signal. Therefore, older devices contained the control amp, pre-amplification, tuner and power amp in the same chasis, sharing the same power supply circuitry. When each device was placed into their own chasis separately, using their own power supplies, the sound was much better. The power supplies didn't have to work so hard and cross talk and interference between circuitry was minimized or eliminated. Amps are not designed to pick up and amplify the really low level signals from turn table cartridges and other such devices. Their input sensitivities are listed for each amp and if you place a much lower level signal at the input of a power amp and the output impedance of a cartridge/phono stage may not (typically won't) match well with the input characteristics of a typical power amp. Also, an amplifier's job is to amplify the input signal such that the output signal is a exact match for the input signal only amplified. So if you have an all in one control/pre-amp/power amp the level of design is much harder. Hence the integrated amp. It is basically a control/pre-amp and power amp in the same chasis sharing the same power supply. If designed and constructed right, you could have an amazing integrated amp. But the cross talk/signal interaction possibilities are really hard to mitigate. Also, in the audiophile world where would we be if we went back to the all-in-one single chassis system?
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