The design of audio amplifiers is a hugely complex subject. This guide is intended to help you sort out some of the more obvious and important differences among audio power amplifiers, because every different design has it's own set of compromises along with it's own set of advantages. Because of the nature of an eBay Guide, I must be brief, and I will leave out many details.
There are many different ways to build a power amplifier, and then there are many different ways to package the electronics into a metalwork "box". The most important differences are: the number of channels inside the box, and the choices the designer makes to provide a balance among energy usage (from the wall socket), power output to the speakers, and overall cost, all while maximizing sound quality. And quite a balancing act it is!
Number of channels of amplification
At it's core, an audio "amplifier" is a single channel device. (It is monophonic.) A single channel of amplification in it's own box is a "monoblock" amplifier. There are stereophonic amplifiers (stereo, or two channel audio with a "left" and "right" channel) but that's just two individual amplifiers sharing a single housing. Now, with the popularity of multichannel home theater, and multichannel audio via SACD and DVD-Audio, what's sold as an "amplifier" may have many individual channels (generally, 3-, 4-, 5-, or 7-channels) of amplification built into a single box.
Monoblock Amplifer Advantages:
* Having only one channel of amplification in a chassis is regarded as the best--but most costly--solution; it is intended to provide the utmost sound quality, all other considerations are secondary. Since everything in the box is devoted to a single channel, a monoblock amp is generally (not always!) more powerful than a stereo or multichanel amp. There is no electrical connection between the channels except at the wall socket, and so signal seperation is maximized.
* The amplifier can be placed close to the speaker (as in, within inches) minimizing the amount of speaker cable needed.
Monoblock Amplifier Disadvantages:
* Cost: You buy a seperate amp for each channel in your system. It's not uncommon for a single monoblock to cost the same as a stereo amp.
* There are those who argue that it's better to have short interconnect cables than to have short speaker cables. One thing is guaranteed: short cables do less damage to the signal than long cables, whether we're talking speaker cables or interconnect cables.
* Some speakers (Polk SDA series, for example) are designed to work with amplifier channels that share a ground path for the two channels. (These speakers are not common.) Seperate amplifiers don't have that common ground connection.
* You need to be able to "plug in" each amplifier seperately. Your home wiring needs to have outlets near where each amp is located. You may need to hire an electrician to "beef up" the wiring in your house so that the monoblocks have a source of enough electrical power.
Stereo Amplifier Advanatages
* Two-channel amps are more common than monoblocks or multichanel amps, because high performance audio was all about "stereo" for thirty years.
* Cost: Compared to monoblock amps, you're buying half the amount of chassis metalwork. One "box" instead of two. Make no mistake: The metal box that houses the electronics is a huge part of the total cost of an amplifier.
* "Dual Mono" is a variation of stereo, in that while both are two-channels of amplification in one box, a Dual Mono amp is designed in such a way that it has nearly the electrical seperation as a pair of mono amps. The two channels of a dual-mono amp share a chassis, (and usually a chassis ground,) a power cord and power switch, and--perhaps--the primary winding in a shared transformer. Some dual-mono designs use totally seperate transformers, and some (generally the lower-powered or less-expensive) use a transformer with three windings--a shared primary winding, and a secondary winding for each channel.
Stereo Amplifier Disadvantages
* They are the compromise between monoblocks and multi-channel. So, you're buying less metalwork (chassis) than if you bought a pair of monoblocks, but if you buy several stereo amps, you're buying more metalwork than if you bought a single multichannel amp.
* If you actually need several channels of amplification for a multichannel music or home theater application, you'll generally spend less money if you buy all the amplifiers built into one box.
* Some multi-channel amps are designed so that pairs of channels can be combined into a single channel but with higher output power. As an example: You may buy a 100-watt per channel, 4-channel amplifier (100 X 4) that allows you to combine a pair of channels so that instead of 100 X 4, you could have 100 X 2, plus a third channel of 200+ watts; or even combine the second pair so that you end up with 200+ watts X 2 channels. Not all multichannel amplifiers have this capability--be sure to check before you buy!
* Since size and weight becomes a concern, and because all those channels of amplification are drawing electrical power from one power cord off of a single wall socket, maximum amplifier output power can be compromised. There are "clever" designs that can reduce the price, size and weight of a multichannel amp, and still provide tremendous "rated" output. The problem is, those "clever" designs often compromise the sound quality in the process of providing a more consumer-friendly package.