When is it necessary to use separates ? What are the advantages, if any? And just how do you define separates? Most of us who started with stereo and moved on to surround sound multi-channel home theater began with a receiver, so-called because it is typically a fairly hefty black or silver box that contains a tuner or radio receiver for FM and AM radio, a control section or preamp (short for preamplifier) with input switching and surround processing, and several power amplifiers that boost the tiny electrical input signals into a more powerful form that has enough voltage and current to power loudspeakers. So the familiar receiver contains, on one convenient chassis, all the components of what older audiophiles called separates. But some surround enthusiasts still buy separate components and connect them together. Why would anyone bother, when a receiver is so much more convenient?
In this era of 5.1-channel to 7.1-channel surround sound, a receiver has to contain up to seven separate power amplifiers on one chassis, one for each of the five, six or seven discrete channels (most subwoofers have their own internal power amp). In the case of most A/V receivers, all of the internal amplifiers are powered by one transformer and power supply. To install five or seven amplifiers on one chassis, an engineer has to limit the receiver's maximum power output to 110 to 120 watts per channel in order to avoid problems of overheating or an unmanageably large and heavy chassis. And in many cases, the actual power output of an A/V receiver is measured with only one or two channels driven to maximum output while the other five channels coast along at one eighth of the rated output (so the advertised rating of most receivers doesn't give you the whole picture.)
Home theater enthusiasts in search of much higher real power output than A/V receivers offer, with big rooms and a taste for very high clean playback levels, and who have large speakers like the M80's that can handle many hundreds of watts per channel, will thus opt for a separate power amplifier of say, 250 watts per channel or more. It's the only way they can get enough real power to fill the big space with clean very loud sound. Since a separate power amp need only contain the amplification circuitry, it can be kept to a reasonable size, yet still have have generous heat sinks to cool the output transistors. Consequently, power amplifiers can typically drive lower impedances 4-ohm loads or lower with no danger of shutting down or overheating.
If you have a power amplifier, you still need a preamp or control section for source selection, surround decoding and processing, and level adjustment. That's where the A/V preamp-processor gets its name. Separate preamp-processors have lots of room inside for all sorts of extra inputs and outputs, so they are often more elaborate and versatile than A/V receivers. For instance, they might have phono inputs for both moving-magnet and moving-coil phono cartridges for vinyl playback or other features not commonly found on A/V receivers. And some preamp/processors will also have an onboard AM/FM tuner.
Specifications (distortion, noise, etc.) of preamp-processors and power amps are usually a bit superior to an A/V receiver, since there is better internal physical separation of wiring pathways (more room), hence less chance of interference (and resulting noise) from nearby hum-causing component parts. And having separates also means that you can upgrade to a larger or different power amplifier while keeping your preamp-processor.
Read the next tip: "Do I Need Two Subs?"