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, and it can be challenging to understand all the specifications given to us by the manufacturers.
So, if you’re looking for a simple guide to amplifier power ratings, you’ve come to the right place.
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, so I will try to provide the main points 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:
watt = voltage x amps
… which is an oversimplification – but will do for our purposes
The amplifier provides the voltage to ‘push’ an electrical current (amps) to the speaker. The speaker then turns this current into sound (and heat).
It is helpful 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 to – and how well it will drive specific speakers.
The aim of this site is to give simple, straightforward information without getting too distracted by the technical details – which can be difficult at times.
If you want more detailed technical information on this subject, this is an excellent 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 Rating Checklist
Some amplifiers create more power than others. And everybody gets very excited about the power that their amplifier can deliver.
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. 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 summarize a few issues that 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).
Power output figures for home theater amplifiers can range from around 20 watts to 200 watts per channel. 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.
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. So, don’t feel the need to buy a more powerful amplifier for the sake of it.
You might need to think about a powerful amplifier if you have a huge room – and you want it very loud in that huge room for long periods!
2. Check the Frequency
When manufacturers specify power ratings for amplifiers, they like to mess with our heads – the rascals!
You will see a whole host of figures for the power output of an amplifier. But, the only ones you should be concerned with are the figures measured across the full audio spectrum – 20Hz to 20kHz.
Power specs measured on limited frequencies don’t tell us enough about how the amplifier will perform in the real world.
They will give a figure for power that is much higher than the actual average power output.
You cannot compare the power output of two amplifiers 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 compare power between two different amps, the readings need to be for the same impedance load.
And, what provides the primary impedance load? A speaker.
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-ohm load, they are not the same and cannot be compared.
Go check my 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.
They may also be called 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, which is calculated from RMS voltage and amps.
This is what is important when you are using your amplifier from day to day.
Peak values reflect the maximum power the amplifier can create. But this will only happen for fractions of a second while playing loud parts of a music track or a sound effect.
So, peak power values look great as they give a bigger wattage – but this isn’t a good reflection on how powerful the amplifier is for everyday use.
You will find that most budget sound systems will usually quote the Peak Music Output figure to make them appear much more powerful than they really are.
They take the peak power for one channel, multiply by the number of channels, and say this is the power the system can output.
You’re not going to fall for that old trick, now are you?
5. How Many Channels?
For stereo systems, the watts per channel rating should be measured when powering both speakers simultaneously.
The same goes for multi-channel systems – powering all outputs at the same time.
If the rating is for one channel only, then the power output will appear to be higher than it is.
Again, this is important because we want the data to reflect a real-world scenario when using our amp at home.
In practice, for AV receivers, you will often find the best you will see is the power ratings for 2-channels driven. That’s OK.
The front stereo pair will do most of the work anyway.
Just make sure you compare like-with-like if you are comparing two different receivers.
6. No Distortion
Power specs should be measured when the amplifier is delivering a clean audio signal without distortion or clipping.
Any figures with a high amount of distortion aren’t going to mean much in the real world.
If the power level quoted creates a distorted sound, why would we want the amplifier to get to that level?
A typical specification for an AV receiver is:
80 watts (8 ohms, 20 Hz – 20 kHz, 0.08% THD, 2ch driven)
The level of distortion is referred to by the ‘0.08% THD’. THD stands for Total Harmonic Distortion.
If the distortion quoted is less than 1%, that is probably acceptable as that will barely be audible.
For audiophile amplifiers, the distortion will be more like 0.02 or 0.03%.
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 significant increase in watts won’t give you the same large increase in volume.
The main advantages of an amplifier with more power are:
- It has extra headroom to play music/soundtracks with a large dynamic range. This means it can comfortably play short bursts of powerful sound. The loud bits!
- You will be able to drive your speakers harder. This will mean you get the best performance from your speakers, and they will sound better. However, this will only make a difference if you have high-quality speakers, and will make little difference to most people with budget or even mid-range speakers.
- It will also drive inefficient speakers more easily. 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 sound quality.
The sound quality of an amplifier should be a more important consideration than the power it can produce.
Unfortunately, there isn’t a measurement of sound quality.
Well, there is. They are called ears!
However, you will find that what sounds good to one person may not sound great to another.
If you want to assess an amplifier’s sound quality, the best way is to hear it for yourself.
See if you can go to a retailer that has a listening room. Or, you can go by any reviews you can find.
Also, bear in mind that an amplifier’s sound will vary depending on the speakers connected to it – and the room it is in.
So it’s not an exact science.
Another way of judging sound quality is just by going by the price. As a rough rule of thumb, a more expensive amplifier will have better quality components and should sound better.
My guide to the best AV amps under $1000 highlights some AV receivers with higher quality components.
These are designed to sound better for audiophiles compared to their cheaper alternatives.
Hopefully, this guide to amplifier power ratings will be helpful when researching your next amplifier or AV receiver.
The most important thing when 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 must 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.
You will be able to comfortably drive most speakers designed for your home with relatively modest power outputs.
It is a good idea to look for an amplifier and speakers that are compatible with each other. Don’t just look at the numbers for the amplifier.
My guide to matching an amplifier to speakers is a helpful article if you are unsure about what you need to consider.
My guide to matching an amplifier to speakers is a useful article if you are unsure about the things you need to consider.
About The Author
Paul started the Home Cinema Guide to help less-experienced users get the most out of today's audio-visual technology. He has been a sound, lighting and audio-visual engineer for around 20 years. At home, he has spent more time than is probably healthy installing, configuring, testing, de-rigging, fixing, tweaking, re-installing again (and sometimes using) various pieces of hi-fi and home cinema equipment. You can find out more here.