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Power Specs - Understanding Amplifier And Receiver Power Ratings

Understanding the power specs of amplifiers can be important when we are looking to buy a new model.

Unfortunately that is often easier said than done, as it can be difficult to understand all the specifications that are given to us by the manufacturers.

In fact, as we will see, this issue is made even harder by the fact that often the power ratings are presented in a way that makes it very hard to compare different amplifiers.

Now, this issue can be very complex, and it is easy to get lost in a maze of numbers and terminology.

However, this site isn't the place for long technical discussions, and so I will try to provide the main points that you need to know - without getting too involved in mathematical equations!

Having said that, let's start with a quick physics lesson for dummies (like me)...

What's a watt?

The power output of an amplifier is measured in watts - and it is a measure of the amount of energy it creates.

If you've got a tiny bit of geek hidden inside you somewhere, you'll be interested to know that a watt = voltage x amps (which is an oversimplification - but will do for our purposes).

The voltage is provided by the amplifier to 'push' an electrical current (amps) to the speaker. The speaker then turns this current into sound (and heat).

It is useful to know the power an amplifier can generate as it gives us an idea of how loud it will be (or what size room it will be suited too), and how well it will drive certain speakers.

Electrical multimeter

The aim of this site is to give simple, clear information without getting too distracted by the technical details (which can be difficult at times), but if you want to know a bit more detailed technical information on this subject, then this is a good article to check out.

However, the good news is that we don't really need to know too much about the maths involved - so let's move on.

Power ratings checklist

Some amplifiers are capable of creating more power than others, and everybody gets very excited about the power that their amplifier can create - because more power means it's going to be louder right?

Well, sort of, but maybe not as loud as you may think - and anyway, that may not be the most important thing.

Now, this is a very complex area, and as mentioned at the start of this article, this website isn't the place for long, dull technical discussions on power specs (I'll stick to writing long, dull articles on other things ).

So what I will do is summarise a few issues that I think are important to be aware of when you are researching amplifiers.

1. Watts per channel

The rating for amplifiers that we are interested in is the watts per channel.

This tells you the amount of power an amplifier can provide to the speakers on each output channel (with one speaker attached to each channel).

Common power figures for home cinema amplifiers can range from around 20 watts to 200 watts per channel (and more) - and the greater the power rating, the louder the amplifier will be.

However, is a 100 watt amplifier twice as loud as a 50 watt amplifier?

No, not even close.

Amplifier power specifications

Doubling the power only increases the sound level by 3 dB, which isn't a great deal (10 dB is about 'twice as loud'). So don't get too carried away by the numbers when looking at power specifications.

A 50-100 watt per channel amplifier will provide more than enough volume for most home applications - and so don't feel the need to buy a more powerful amplifier for the sake of it.

You might need to think about a really powerful amplifier if you have a very large room - and you want it very loud in that very large room for long periods!

2. Check the frequency

When manufacturers specify power ratings for amplifiers, they like to mess with our heads - the rascals!

While you may see a whole host of figures for the power output of an amplifier, the only ones you want to be concerned with are the figures measured across the full audio spectrum (usually 20Hz to 20kHz).

AV Receiver power specifications

Power specs that are measured on limited frequency ranges (often just one frequency) don't tell us enough about how the amplifier will perform in the real world - and they will give a figure for power which is much higher than the actual average power output.

The power output of two amplifiers cannot be compared unless they are measured in the same way - and the bigger number doesn't necessarily mean it is better (or louder)!

3. Check the impedance

Power output should be measured against an impedance value.

If you are going to compare power between two different amps, then the readings need to be for the same impedance load (a speaker provides the impedance load).

The impedance will affect the amount of power the amplifier will output - so if one amplifier is rated with an 8 ohms load and another with a 6 ohms load, then they are not the same and cannot be compared.

Go and check our our guide to speaker impedance if you're feeling brave - and really have nothing better to do.

4. Root mean square values - not peak

Don't pay too much attention to peak power ratings if they are specified.

These are often referred to as PMPO (Peak Music Power Output) or PMP (Peak Music Output).

We should be more concerned with the average power an amplifier can produce over time. This is calculated from RMS voltage and amps - and this is what is important when you are using your amplifier day to day.

Peak power values sound great as they give a bigger wattage - but this isn't a good reflection on how powerful the amplifier is for everyday use.

5. How many channels?

For stereo systems, the watts per channel rating should be measured when powering both speakers at the same time.

The same for multi-channel systems - powering all outputs at the same time.

Amplifier power specifications with one channel driven

If the rating is for one channel only, then the power output will appear to be higher than it actually is.

Again, this is important because we want the data to reflect a real world scenario when we are using our amp at home.

6. No distortion

Power specs should be measured when the amplifier is delivering a clean audio signal without distortion or clipping.

Any figures which are quoted with a high amount of distortion aren't really going to mean much in the real world.

If the power level quoted creates a distorted sound - then why would we want the amplifier to get to that level?

If the distortion quoted is less than 1% then that is probably acceptable as that would hardly be audible.

7. Why do I need more power?

Getting more volume isn't necessarily the best reason to buy a more powerful amplifier.

Firstly, as stated above, a large increase in watts isn't going to give you the same large increase in volume.

Speakers with sound level meters in the background

The main advantage of more power in an amplifier is it has extra headroom to play back music/soundtracks with a large dynamic range i.e. it can play the short bursts of loud sound easily.

It will also drive inefficient speakers more easily - whereas more efficient speakers won't require extra power.

So, unless you have a very large room to fill with sound, you may not need as much power as you think.

8. Power output and sound quality

The power that an amplifier can generate has very little to do with the sound quality.

The sound quality of an amplifier should be a more important consideration than the power it can produce.

Amplifier Power Specs Summary

Hopefully this guide to amplifier power ratings will be useful when you are researching your next amplifier or receiver.

The most important thing when you are deciding between different amplifiers, is that you make sure you are comparing like with like. It is so easy to look at the power specs and assume the one with the biggest number is the best - which is why the manufacturers like doing it!

Also, don't be fooled into thinking you have to get an amplifier with the highest wattage that you can afford.

There isn't a great deal to be gained from increasing the power output of your amplifier - certainly not in terms of pure volume which is what many people assume - and you will be able to comfortably drive most speakers designed for your home with relatively modest power outputs.

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