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THE MOST dedicated audiophiles insist on stereo systems made up of "one box, one job" components. This division of labor makes it possible to upgrade individual sections of the system. It also allows design engineers to craft each component in an optimum manner.

There's no need to design the amplifier, for example, with an eye toward keeping heat and magnetic fields from directly affecting FM radio reception circuits, since they won't be built on the same chassis.

As a result, the best separate components provide better sonic characteristics than the best all-in-one receivers.

The combined component approach has certain operating and economic advantages, however. When you use a receiver that combines amplifier, preamp and tuner, you buy one enclosure instead of three and one power supply instead of three. When shopping, you make one buying decision instead of three. When you get your receiver home, you'll have significantly fewer signal and power cables hanging around the back of your system. And when you turn it on, one switch powers up all your system's major electronics simultaneously.

Although preamplifiers have switch-controlled back panel power outlets, careful audiophiles prefer not to plug their amplifiers into such outlets. The preamplifier's power switch might not be robust enough to handle the heavy current flow that occurs when a high-powered separate amplifier charges up. And some components produce low-frequency thumps when you turn them on. When the amplifier is on, these thumps will go right to the speakers. The noise will at least irritate the ears and might even cause damage to speaker cones.

The recommended procedure is to plug only light-load components (CD player, tuner, tape deck) into the preamp auxiliary power outlets. The amplifier power cord goes directly into the wall outlet and the amplifier is turned on separately after everything else is on.

This small, but significant, inconvenience can be eliminated with a power controller of the sort sold by Radio Shack for computer applications (catalog number 26-1396). The controller has six outlets, and one of them has an auto-sense function. You plug your preamp into that one and your amplifier and other components into the other five.

When the power center senses that you have turned on your preamplifier, it provides power to the other outlets after a delay of about 15 milliseconds. As a bonus, the power center includes computer-grade spike suppressors and noise filtering on all outlets to clean up power line grunge.

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