Amplifier power ratings can be baffling with all the technical jargon from manufacturers. I still get confused by all the numbers sometimes, and I worked as a sound engineer for many years.
So consider this your handy guide to understanding amplifier power ratings so you can pick gear with confidence. I’ll explain critical power differences like continuous vs. peak, 6-ohm vs. 8-ohm loads, and how much wattage your speakers really need.
You don’t need an engineering degree to learn enough to match an amp and speakers correctly.
Key Points
- Power ratings can be misleading – consider specs like watts per channel, impedance, and RMS power.
- More power doesn’t mean much louder. Doubling the watts only increases volume by 3dB. A 50-100 watt amp will be plenty loud for most homes.
- Match your amp and speakers. Focus on pairing components properly rather than maximizing power. An efficient speaker may not need a super-powerful amp.
- Sound quality matters more than power. A more expensive, better-made amp will likely sound better than a cheaper high-power amp.
Main Topics
What’s a Watt?
You measure the power output of an amplifier in watts, which measures the amount of energy it creates.
And you calculate wattage by multiplying voltage by current:
watt = voltage x amps
… which is an oversimplification – but will do for our purposes. Once you know this, you can see how it relates to an amplifier.
The amplifier provides the voltage to ‘push’ an electrical current (amps) to the speaker. The speaker then turns this current into sound (and heat).
If you know the power an amplifier can generate, it gives 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.
If you want more detailed technical information on this subject, this is an excellent article to check out.
However, the good news is that you don’t 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 think. And anyway, there may be more important things.
So, here are the main issues to consider when you are researching amplifiers.
1. Watts Per Channel
The main rating for amplifiers 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!
Do you want to work out how loud your amplifier and speakers will be in your room? Check out the SPL calculator.
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-ohm 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 – also called PMPO (Peak Music Power Output) or PMP (Peak Music Output).
You 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 it by the number of channels, and say this is the power the system can output. It’s not.
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 you want the data to reflect a real-world scenario when using your 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 you 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. Do You 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.
Wrapping Up
When looking for a new amplifier or receiver, you must look past the simple power ratings and dig deeper to ensure you compare apples to apples.
Don’t assume more watts automatically means better performance. In most cases, you don’t need an overly powerful amp – just enough clean power to properly drive your speakers.
Finally, match your amplifier and speakers appropriately to get the best sound. Focus on finding components that work well together rather than simply maximizing wattage.
My guide to matching an amplifier to speakers is a helpful 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.
