Despite the market swing towards active / powered speaker systems, the separate amplifier for many remains the core component of their live sound system.
There are several benefits of using a separate amplifier, though for many Dj’s starting out the most important of these is price. It is still considerably cheaper to put together a useable system of amp and passive speakers then it is to buy active units of the size quality that's good enough to gig with, and having the separate components means that either can be upgraded, replaced or repaired without the loss of your entire rig.
Power and signal management
One of the biggest features of a dedicated power amplifier is its capability to function in bridged mode, which gives the user the ability to add additional amplifiers to the system at any time and connect them together to work as a single power unit. This gives a lot of flexibility for future system expansion, be it upgrading to more powerful speakers or adding in subwoofers. Running amplifiers in that way also makes them run at their most efficient, giving you longer life and higher power output than if you were running one in stereo. You can then use a separate speaker management unit and crossover system to give you control over the frequency cut offs and channel distribution. This is the setup you will find in most live venues, night clubs or theatres.
You will find that as power ratings increase on speakers, the input connections will move away from the binding posts or spring tags used on hifi equipment and become more robust and heavier duty. This is both to better handle the increased power loading and also as large PA is often going to be connected and disconnected repeatedly if used in a mobile rig.
There are 3 industry standard connection types for pro audio, the ¼ inch (6.35mm) phono jack, the large locking Speakon plug, and the xlr plug which is used for balanced audio feeds to active PA.
Using any of these plug and socket designs is much more favorable to bare wires into posts as they give a far more secure physical connection, and the conductive core of the cable isn't exposed. They also allow the use of much thicker diameter cable which helps the system to work unstressed and stops signal loss from heat build up.
Inputs and controls
Power amplifiers generally forfeit the usual array of inputs you would find on a hifi amp or similar as they are designed to be fed from a mixing desk or other type of signal preamp system which will have all the required equipment inputs.
With the heat that a large power amp produces, it just isn't a good idea to have any circuitry or controls onboard that aren't necessary as they just get roasted, which eventually causes allsorts of issues. It's also done this way as for their intended usage, it's often the case that the amplifier will be away from your audio sources completely, especially in a Dj booth, so it would be very impractical to have to run your various devices direct to it as you would with hifi or home cinema.
Most power amps will just have a standard analog RCA / Phono line input, mirrored with ¼ inch Jack inputs and XLR balanced inputs for accepting either a line level or balanced feed from the mixer system.
Power ratings and cost
When it comes to amplifiers, more power equals more money. For a standard mosfet type power amp, its power reserves come from the size and quality of its transformer and of its amplifier transistor chips, and its ability to dissipate heat. Transformers are expensive, and they bigger and better they get the more money they cost. In a power amplifier the transformer can often make up 70% of the total cost of the whole unit.
Amplifier power output is also greatly affected by the buildup of heat in use, and its ability, or lack of ability to rid itself of that heat can have drastic consequences on its performance and longevity. Chassis materials, heatsinking, electric fan cooling all add to the cost, so whilst being great additions they also make for more expense in both material and construction.
Cheap amps are great. They have come a long way in the last few years, and will be perfectly suitable for general party use, home Dj’s, small rooms and pub sound systems etc. For bigger venues, professional mobile rigs, installations or rentals, you need to be looking at spending slightly more on one of the larger offerings that will give you the power and the stability in continuous usage.