With a great deal of speaker systems becoming active, the need for separate power amplifiers has definitely dropped in recent years, but for many applications it is still the more preferable source of power for the system.
For the mobile Dj market, choosing a passive speaker set and power amplifier gives you the freedom to expand and upgrade at any time, without ending up with redundant equipment.
Additional amplifiers can be added into a system, in both stereo and mono use, and bridged together to form one large power unit. The heat generated from the units can be managed properly, and is not encased into the back of a speaker like in an active rig, so is not having a degrading effect on other components. The other benefit, should anything go wrong with an amplifier, is that it can be repaired or replaced without you needing to lose or change your speakers at the same time, so inconvenience or loss of earnings is far more manageable.
Amplifier technology has moved forward on all fronts, and quality is very high on even the most budget units now. Features and overall output power ratings will in the end be the determining factor for most people over the unit cost, as whatever type you buy, it has to meet the demands required of it from the rest of your system and its intended usage.
Power amplifiers generally now fall into two types which is the long standing Mosfet class A/B type, or the newer Class D switching type. Now explaining the difference between those two on an actual design level would take several pages, and be full of a lot of fairly meaningless tech talk to anyone not into electronics, so we shall skip it here in favour of a more ‘working man's’ approach !
The standard Mosfet power amplifier. Tried and tested technology that's been around for decades. Solid, reliable, lots of choice on the market so you can get the specific features and inputs you want. ‘Analogue’ style solid state amplification with large power transformers and heatsinking / fan cooling. The good ones are heavy, and they run hot enough to cook an egg on, but they have vast amounts of power on tap across the frequency range, and are especially good at low frequency reproduction.
Class D - The newer ‘switch mode’ type amplifiers. The technology has been around for years, but only for industrial use in measuring equipment and motor control. It found its way into the audio market a few years back, but prices were always high and power output limited. This is no longer the case, as design changes now means their power output often significantly exceeds their older brother, and at less cost due to them not requiring the enormous and expensive transformers and heat sinking of a mosfet unit. They are extremely energy efficient designs, producing very little heat during operation, meaning they can be made much smaller, with most rack mount Class D units only requiring a 1U space.
These factors have cemented their use in virtually all active speakers, as the high power and low weight, combined with low heat output make them perfect for fixing onto the back of a speaker cabinet.
Can i just use my old Hi-fi amp ?
For actual measurable quality, the Hi-Fi guys will always stick with traditional Mosfet or even valve powered designs, as the way it deals with the audio signal is considered a lot more ‘organic’.
Which is nice, but most Dj’s playing out are looking for power, reliability, and simplicity in use. They are also feeding source audio that's often far from ‘Hi-Fi’ , having being programmed to be bass heavy or to have massive punch, and being mixed with other tracks of equal or varied sound. It's a totally different set of dynamics that's required, and its delivery is way beyond what any home amplifier can achieve.
A power amplifier, be it Mosfet or Digital, stand alone or built into a speaker cabinet will provide the solid current and frequency range that Dj and PA designed speaker units require to produce the larger sound pressure levels needed for live performance. It's not about subtlety at that level, and more about frequency control and separation. A Hi-fi sounds great at normal volume, but try and fill a hall with music using one and you will quickly realise it's not up to the task by any means, as it will quickly distort into sonic mush. It simply doesn't have the guts required, or the correct signal filtering and noise reduction to deal with that level of output.
Choosing between the old or new styles of power amplifier is for most people going to come down to cost, and if it has the features you need. You can't go wrong choosing either type, as there are obvious drawbacks and benefits to either, so it all depends on what will suit you and the level of performance you require from it.