How to read amplifier specs? - Rapallo New Zealand :: Home Theatre & Hifi | Design & Installation
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Amplifiers are complicated machines that require a fair amount of engineering. What goes on inside the bowels of your amplifier is important and determines a lot about what your amplifier can or cannot do, what set-up it will suit  and last but not least what budget you are looking at. When deciding on which new amplifier you should buy, specs are a good place to start. That said, it is rather easy to get lost in a maze of numbers and terminology. So we’ll try to break it down.

Considering that the first and foremost job of an amplifier is to ‘amplify’ a power signal before it gets fed to your speakers (who in turn will transform the current into sound), power output might be an obvious starting point.

Clearly, it is pretty useful to know the power an amplifier can generate. The general idea out there is that amplifier power specs will give you an idea of how loud your set-up will be. This is not untrue, but things are not as clear-cut as some people seem to think.

On top of how loud you can push it, the power output of your amp will also determine what size room your amp can work and how well it will drive certain speakers.  (Just on a side note: the power output of an amplifier is measured in Watt (W))

So how do you make sense of power output?


Our example


Let’s take the example of one of our most popular AV receivers, The Yamaha RX-A780. It’s a good quality mid-range 7.2 channel AV-receiver that we like to recommend to our customers.

These are the specs that are listed:


Amplifier Section Channel 7.2
Rated Output Power (1kHz, 2ch driven) 110 W (8 ohms, 0.9% THD)
Rated Output Power (20Hz-20kHz, 2ch driven) 95 W (8 ohms, 0.06% THD)
Maximum Effective Output Power (1kHz, 1ch driven) (JEITA) 160 W (8 ohms, 10% THD)
Dynamic Power / Ch (Front L/R, 8/6/4/2 ohms) 130 / 170 / 195 / 240 W


Clearly, we haven’t just got one figure available to us when it comes to amplifier power. The thing to remember is that amplifier brands like to reign by causing confusion. They’ll throw many specs at you; some useful, some a lot less relevant to you as a consumer. The amplifiers brand’s idea is that the higher figure they can throw at you, the more impressed you will be.


Our biggest piece of advise


So here’s a big piece of advice when you’re in the market for a new amplifier:

What you really want to know is how an amplifier will perform in a real life situation, rather than in a lab situation that will never (and I mean NEVER) occur.

Which of these figures is telling us what?


Power Output


One of the things that will determine how your amplifier will perform is how many speakers (channels) your amplifier will drive. Nobody drives just one speaker, which makes the ‘Maximum Effective Output Power’ immediately a blown up figure that does not reflect reality.

Another real life requirement is that your amplifier will play music or a movie. This implies it will play the full frequency range (20hz-20kHz), rather than a test tone (1kHz). In other words: The ‘Rated Output Power (20Hz-20kHz, 2ch driven)’ tells you how this amplifier will perform with 2 channel driven, playing the full frequency range. Fair to say it’s a much more realistic picture than some of the other power specs.

Between brackets you notice the following details: 8Ohm.  8Ohm refers to the impedance of the speakers used for this figure. Many budget and mid-range speakers will be 8 Ohm speakers. But some speakers will be 6Ohm speakers (like the ELAC Debuts or the Q Acoustics 3000i series) or even 4Ohm (like the ELAC Uni-Fi’s and the ELAC Vela’s).

Clearly, demands on an amplifier from a 8Ohm speaker will be very different than a 4Ohm speaker. We’ll get back to this a little later.


 Dynamic Power (Peak Power) vs Continuous Power (RMS)


So what does the Dynamic power figure mean?

‘Dynamic Power’ or ‘Peak power’ tells us the power the amplifier can handle for a very, very short time period (we’re talking milliseconds) when fed a 1000hz signal and volume turned up until the amplifier reaches the clipping point. Dynamic power is usually measured into 8 ohms, but specifications like the one for our Yamaha RX-A780 are published for 2, 4, and 6 Ohm loads as well.

Is this a figure that tells you how your amplifier will perform in your living room? Not really. It provides a large figure, which exactly why manufacturers like to mention it. At best, it tells you how your amplifier will handle certain bursts (like a bang of cymbals for instance).

What you really want to find out is the continuous power of an amplifier or RMS ( short for ‘Root mean square’ ) Like Dynamic Power it is expressed in Watt, but it’s a much more realistic figure as it refers to how the amplifier handles continuous power rather than bursts. In our above specs , that is not explicitly mentioned, but we can assume that the amplifier specs called ‘rated out power’ are measured as continuous power.


 Total Harmonic Distortion ( THD)


Which leaves us to decipher the THD percentage figure that is stated between brackets.

THD stands for ‘Total Harmonic Distortion’. Total Harmonic Distortion is a measure of just how much effect the amplifier has on the sound output. More distortion generally means more colouration to the sound. The lower this figure, the closer the output of the amplifier will sound to the original recording. Of course, speakers will also have effect on sound, but the bottom-line is that when comparing amplifiers, a lower number is better. It’s a sign of a high quality amp when it has extremely low distortion ratings. But to put things in perspective, the human ear cannot hear anything under 2%.


Amplifier power and speakers

So now that we know which specs to look at when it comes to  amplifier power output, what do they mean?

A 100W amplifier will be double as loud as a 50W amplifier, right? Wrong! The difference between 100W and 50W per channel is about 3dB and that is not a lot at all.  So when it comes to how loud you can play, please take power output with a large pinch of salt!

The main advantage of more power in an amplifier is that it has extra headroom to play back music/soundtracks with a large dynamic range. This means it can comfortably play short bursts of sound.

It will also drive inefficient speakers more easily – whereas more efficient speakers won’t require extra power. This brings us to the topic of matching an amplifier to your speakers. We have written a separate blog on this topic before.  It’s a very important consideration, so you definitely want to look into it, but it would make today’s blog enormous. So we’re just going to refer you to have a read here.

In short:  8 Ohm speakers are very easy to drive and you could expect any amplifier to be able to handle those. However, if the amplifier isn’t designed to drive speakers with a lower impedance (4 Ohm for example), then the amplifier may overheat if you turn the volume up very loud.  This is because it will draw more current than the power supply is designed to deliver. All going well, the amplifier will shut itself down before it causes too much damage. But we’ve seen speakers damaged before that happened.

So that basically handles the power output specs of an amplifier. But it’s certainly not the one and only specs you want to look at when comparing amplifier specs.

A few more to look at are:


Signal to noise ratio. (SNR)


If you stand in a quiet room, in the countryside, away from the hustle and bustle, you might notice noises you’ve never noticed before. The humming of a radiator or cars travelling down a nearby road for example. None of this is apparent when the kids are home with the TV blasting away, but that background noise is still there. An amplifier is the same, there is always a very small amount of noise from the electrons whizzing around inside. The goal is to make this background noise unnoticeable. You want to hear more of the music and less of the noise. The measure of this is the signal to noise ratio.  This is a spec that you will find in audiophile amps rather than AV receivers. It’s important to note that higher numbers mean a better specification, since there is more useful information (the signal) than there is unwanted data (the noise).




Can you connect up everything you want to? You’ll want to make sure you have enough inputs for everything you’re plugging in! Ensure you have the type of connections you want/need, like 3.5mm for iPods, Phono for turntables and USB for laptops and home theatre PC’s.  Similarly, you may want Bluetooth or Wi-Fi connectivity. Or you may insist on balanced inputs.


Bells and whistles and the other ‘details’


You may have plans to play music in different zones around the house.  You may have read our blog about bi-amping  and decide to go there.  It may be important to you what amplifier class are we talking about and what DAC can be found inside?

All these things can be considerations in your checklist of which one is the right amplifier for you. And specs are the best place to start…. But never forget to listen as well.


So to sum it up:


  1. Look for the realistic specs that reflect a real life situation rather than inflated figures from a meaningless lab test.
  2. Compare like for like. In our example when comparing the Yamaha RX-A780 with another amplifier you want the same test situation: full frequency, continuous power driven into 2 8Ohm speakers.
  3. Compare THD
  4. Compare SNR
  5. Check inputs and connections. Only what’s relevant to you counts.
  6. Ensure any other features or specs that are important to you are there
  7. Check measurements



Source: Yamaha, Cambridge Audio, The home cinema guide,







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