Thursday, 13 June 2013

Getting Amped Part 2


Power Play

The amount of power your amplifier can deliver depends on the load. The higher the load, the more power will be delivered and often doubling the load roughly doubles the power that the amp can deliver.
Think of it like a car again, it would be really hard to overheat a car engine by sitting in the driveway and just revving it at full revs. However it is much easier to overheat it when you are towing a big load in a trailer. Everyone is no doubt familiar with the hot stink of a car that has just climbed a hill. Climbing a hill is harder than sitting in the driveway revving. Even though the car might be capable of delivering 200 horsepower, you can't actually use all that power until you put a load on the car. Amplifiers are exactly the same.

When purchasing an amp, the first thing you have to work out is what your load is. Generally manufacturers promote the power of their amps at its 4 Ohm or 2 Ohm load. So if an amp can deliver 200 Watts at 8 Ohms, 400 Watts at 4 Ohms and 600 Watts at 2 Ohms they will probably call it a 600 watt amp, sometimes even a 1200 Watt amp, if it has two channels.  This is where people trip up and buy the wrong amp.
If you have two, 8 Ohm Speakers rated at 500 Watts each and you bought this amp, then you are really only powering your speakers with 200 Watts not 1200 Watts  as you might think.
Thats a lot of extra power your speakers could handle that you aren't giving them.

In reality, you probably need a 2000 Watt amp to give your speakers the full 500 Watts that they could handle.

When powering speakers, it pays to OVERPOWER them not under power them.
This is because speakers can handle brief spikes over their rated power but if you under power them, there is a risk you might drive the amp too hard and cause square wave distortion that can actually destroy your speakers. That's right, you read that correctly, you have a greater chance of blowing up your speakers by using too small a power amp compared to using too large a power amp.

The other reason overpowering is good is to do with headroom, if you have a big grunty amp that is idling then its going to sound a lot better than a tiny little amp thrashed to within an inch of its life. Think of it like a Mini Cooper towing a trailer, sure you can do it, but you will probably damage the mini. A V8 Range Rover is going to do a much better job of towing the trailer than a Mini. Towing a small trailer with a Range Rover does not do any damage to the  trailer and it gives you a nice smooth ride. Whenever possible I overpower speakers by a factor of two, so if you have 500 Watt speakers, I power them with 1000 Watts.

I mentioned earlier that amps store power, this is because the frequency that power comes out of the wall socket is only 50Hz (50 cycles per second), that is the equivalent of a very low bass note. Now if we are trying to produce a higher note that is 500 Hz (a medium note on a piano) and we need to make it loud in a hurry, then the mains power from the wall is moving too slowly to deliver the the power needed at the instant we need it. This would sound like a 'sag' where the initial note was quiet then gradually got louder as the power supply caught up. In some poor quality amps you can actually hear this with a good loud bass note. You hit a note really loud and all of a sudden and it sounds like it has been sucked empty, then gradually gets louder, then gets quieter again as the amp 'runs out of guts'.

To get around this issue, amps need to store a lot of power. They take a constant stream of slowly oscillating power coming from the wall socket and store it up so that a large amount of power can be delivered very quickly. Think of it like a Water Blaster, the low pressure from the water main fills a temporary tank. Then the water blaster sucks this out and makes it high pressure. If the tank on your Water Blaster is too small then it constantly runs dry and you have to stop water blasting and wait for it to fill up again. If the power storage capacity of your amplifier is too small, then it will constantly run out of enough power. Amplifiers are just the electronic equivalent of a high pressure Water Blaster.

Different classes of amp have different ways of converting and storing power. Old amps use large coils of copper and giant capacitors which is why they weigh so much. Modern amps convert the power into a very high frequency using a Switch Mode power supply very similar to the one inside your computer (only much bigger) allowing them to be lighter and more efficient.