How to calculate the correct power for your amp? (part1)

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How to calculate the correct power for your amp? (part1)

This is a question that we get on a regular basis and as much as we would like to give you simple one-liner answer, things require a little bit more of an explanation. The fact that things are not as clean cut also explains why audio power requirement is such a controversial topic. Time to dedicate a blog or two on the much debated issue.

Amplifier power is important for an enjoyable listening experience as well as the safety of your speakers. People often directly associate amplifying power to sound loudness, but that is a fairly limited way of seeing things. Your speakers and the environment in which you are enjoying a movie or listening to music are equally important and should be taken into account when deciding on the correct power for your amplifier.

But let’s begin with some basics for this week.

Let’s get some misconception out of the way first: It may come as a surprise that both insufficient as too much amplifying power may damage your speakers.

Yes, insufficient amplifying power may also cause speaker failure, called ‘clipping’. Clipping is a harsh distortion generated when the speaker is pushed outside it’s frequency response.

The natural response, if your (too) small amplifier is unable to reach a sufficient loudness level, is to turn the amplifier volume to the maximum level. Any electrical energy within the amplifier output signal falling outside the frequency range of your speaker would not be converted into sound and ends up as heat. Sustain this for a sufficient length of time, and voila… you have speaker thermal failure.

So, if your system is not able to play as loud as you would like it to be without the signal distorting, then you need a bigger, more powerful audio amplifier.

That bodes the question: how much amplifier power do you need?

Let’s get a little bit technical, but we will attempt to keep it clear.

Remember that sound is a wave and that Power is expressed as Watts. You will often find amplifier power expressed as ‘RMS Watts’, which stands for average watts obtained by the RMS (root mean square) method using a clean and tidy well-behaved frequency sine wave signal.

In a real life situation, the audio signal driving a speaker consists of a multitude of frequency sine waves all playing at the same time (all interfering with each other, and therefore also causing much higher peaks), rather than a pure single frequency sine wave signal. As a result, in real life, an amplifier will deliver less than its rated power. Therefore, your audio amplifier power needs to be larger than the speaker’s rated power.

‘The Practical Home Theatre Guide’ advises to allow for a 30%, even 50% increase in amplifier power over the rated speaker’s average, or ‘RMS Power’. Thus if your speakers are rated at 100W, the amplifier should be capable of delivering close to 150W when rated for a sine wave signal.

But there is more to consider when deciding how much power your amplifier needs to have than ‘RMS Watt ratings’.

There is a direct link between the RMS Power rating your amplifier produces and the sound level produced by your speakers. The more power an amplifier generates, the louder the speakers will sound.

FTC and IEA are two common power rating standards that make it easy to compare amplifier output ratings. We highly recommend you to refer to those standards for a realistic representation of what an amplifier can do.

Sound levels are expressed in decibels or dB’s. One decibel represents the minimum perceptible change in volume by human hearing. Normal speaking sits at around 60dB while a concert climax may reach 105dB. In a home theatre, the THX reference level for loudness is 85dB for each of the audio channels.

In addition, if you are after a high-end audio system for your home theatre, ‘The Practical Home Theatre Guide’ advises to allow for an additional 20dB dynamic headroom (‘dynamic headroom’ refers to excess power reserves or capabilities available from the amplifier for use during those loudness peaks to avoid clipping).

But truth be told, we have very individual preferences in what is the correct loudness to watch a move or listen to music. The THX 85dB is therefore only a guideline and you may want to subtract about 3dB if you prefer to listen to lower levels, or add 3dB if you like your music or movie really loud.

And with this info under your belt, you will have all the ammunition to tackle next weeks’ topics.

As we started off by saying, sound loudness not only depends on the audio amplifier power output, but also on both speaker efficiency and the environment you are in. None of these three should be ignored as they are all equally important in the set-up of your home theater or audio room to ensure that the resulting sound loudness will match your desired volume level.

Next week we continue….

In the meantime, give us a call or send us an email if you need help making decisions. We love to help.

The Rapallo Team