Q: Why on Earth would you get two big hulking boxes to do the job that can be achieved by one much smaller box?
A: Because they sound so much better!
Once upon a time, a real hi-fi system did its amplification in two boxes. You had a pre-amplifier to switch inputs, control volume, and to boost the voltage so it was just right, before passing it on to the power amplifier, which would convert that signal voltage to a massive high-current output to make a pair of loudspeakers do their thing as best they can.
These days, we tend to see a single box; what’s referred to as an integrated amplifier. This is a device that, surprise surprise, integrates the preamp and the power amp into one box. This is a great solution for convenience and cost. An integrated amp comes in one box, it takes up less room on the shelf. You’ll only need to pay for one chassis, one shared power supply, there’s fewer cables in the spaghetti behind your AV cabinet, etc., etc.
As always, with convenience comes compromise. The same way we as consumers chose compressed streaming over higher quality CDs. We lost quality, but it was so much easier.
Before I get too far up the ladder to my high horse, I will qualify by saying that there are some amazing integrated amplifiers out there. The RA-1592 for example, is a fantastic bit of kit if you can only deal with a single box, and it goes a long way to solving many of the shortcomings with cramming two devices into one box.
Rotel RA-1592 Integrated Amplifier
But, as with everything, there is a better way. An option for higher-fi.
When we ask an integrated amp to do the job of the pre-amplifier, and the power amplifier, we are giving it two tasks that are pretty close to polar opposites. It has to handle delicate low level signals, right next to a great hulking power transformer of the power amplifier stage. These different tasks share many components, that would otherwise be suited to specialisation. So, let’s specialise!
Enter the Separates
The pre-amplifier is where we start. It has to take very low signal inputs from your source unit, your CD player, your streamer, or even from a turntable, with a signal that is orders of magnitude lower again. It has to route those signals, and reinforce them, run them through a volume selection structure, it may have some tone controls so you can make your room sound just so. All of these jobs are operating at a low level. These are quite delicate signals, and if some electromagnetic noise is introduced into the signal, it can account for a large percentage of what the signal is when it gets amplified again by the power amp. Good pre-amplifiers also include additional features. A phono stage for your turntable, a high quality DAC for your digital sources, and other such things. These all are subject to interference too.
Rotel RA-1570 Pre-amplifier with RB-1582 MKII Power Amplifier
So, we keep it separate. We give it it’s own power supply, so that it remains un-interfered with by any other device sharing the voltage taps. We give it its own chassis to shield from external interference. In that chassis we have space. Space to lay out all the various and complex componentry so that the different parts of a pre-amp don’t interfere with one another. Most importantly, you put the massive, electrically noisy transformers of the power amp, in another box.
This all goes together to make your music and your movies, cleaner, clearer, more detailed. You’ll hear the bows hit the strings. The gentle reverb off the drum skin after the beat. The grass crunching as our hero on screen walks through a field.
The job of a power amp is to take the voltage coming in from the pre, and add just masses of power. At reasonable listening volume a power amp has 20 to 30 times as much electricity coming out of it. In its own chassis, with its own dedicated power supply, a power amp can be free to do what it wants to do! It doesn’t have to worry about big electromagnetic interference from its transformers being too near the dainty signals from a preamp. It doesn't have to share a power supply with anything, it gets all the watts all to itself! There’s more room for more and bigger capacitors to hold more energy so those big notes hit hard. There is more space for more efficient heat distribution, so not only can we run more power, we can run it more safely.
The control over your speakers means that you can hear every different horn in the brass section. A kick drum goes off like a grenade. A grenade goes off like the world is coming to an end.
How Much Power?
A big, powerful 200 watt (or more!) power amp, sounds ridiculous on paper. The output of a speaker could be absolutely deafening. It isn’t always about loud though. A high power amplifier means even when you’re only feeding a handful of watts to your speakers, your amplifier has more control over them. A song at low volume will still have all the presence and dynamics of a loud song, without sounding thin and bassless. Of course, if you do want it loud, there’s lots of power for that too!
When you’re really climbing up the ladder to more and more power we can separate the power amp even further. We step up to monoblock amplifiers. One amp for each speaker. All the points of separating out your pre and power taken even further. This is where dumping half a kilowatt into a speaker becomes wonderfully possible. Your speakers will have never sounded so good, and your neighbours never complained so much. It won’t matter. You won’t be able to hear them over the music.