Amplifier Power Ratings and Picking The Right Power Amp

Luckily, calculating how much power you require is a logarithmic calculation. As logarithms are a pain in the ass, I recommend using a calculator, or even a website, to help you figure out how much power you need for your application.

How much power you need depends upon a few different things:

  1. Whether your application is home theater or music based
  2. How far your listening position is from the speakers
  3. The efficiency/sensitivity of the speakers
Honorable mention: speaker impedance and the amplifier's relative continuous power output

1. If your goal is a home theater, or your focus is on movies and a cinema like experience, then you will want to be able to hit 105dB SPL cleanly. If you don't know what this means, visit my page on dB SPL and volume levels.

For music, being able to hit 105dB SPL is excessive. Music is generally mixed at 85dB as a reference level and even then, I find listening at these kind of levels to be fatiguing and far from enjoyable. I do recommend though that your system is capable of hitting 95dB SPL to ensure that you have plenty of headroom and that you are never pushing your amplifier (whether standalone or internal to a receiver) too hard.

If you plan to do music and movies, ensure your system can hit 105dB SPL.

2. Being able to achieve these kinds of sound pressure levels are directly related to how far your listening position is from the speakers (this can also be read about further on my page about dB SPL and volume levels).

If doing a multi-channel home theater system, you will use the distance, in feet, from your furthest sitting position to your front three speakers. This is because your surround, surround backs, and if you have them, height channels are used for ambiance, reverb, and general effects - they are not the main speakers providing a majority of the audio content.

If running a stereo system, you would obviously use the distance from listening position to the front left and front right speaker (don't ask, what if they are different? They should not be as it will impact your imaging. Your sitting position relative to your speakers should form an equilateral triangle).

3. Your speakers' sensitivity lets you know how much sound pressure they play from ~3.3ft away when provided 1 watt of power. Lets say John's speakers are 95dB efficient and your speakers are 85dB efficient, then your speakers will require twice the power John's do to play at the same volume. Note: this does not mean your speakers are bad, or sound poor, it simply tells you how efficient the speaker is and how much power it requires; efficiency does not correlate to sound quality!

Now that you have all of your parameters, plug them into this calculator:

In this case, you will be trying to solve for either 105dB SPL or 95dB SPL. Plug in the distance from your speakers, efficiency of the speakers, and estimated power. To be conservative, I recommend selecting away from walls/boundaries; this way you are not relying on assumed gain from having your speakers close to boundaries. Once you calculate out what is needed to hit your recommend sound pressure levels, you'll know what you need for power.

Now that you know what your situation dictates for required power, you can start looking for amplifiers. You will want to ensure your amplifier can hit these power levels without being pushed too hard. The way to determine this is by ensuring your power amplifier can hit these levels with low harmonic distortion and through out the full audible band; this means the amplifier should be capable of these power levels from 20Hz - 20KHz with 0.1%THD or less. For an 8 ohm load, the amplifier should be able to deliver these ratings with all channels driven.

As an amplifier is pushed to its limits, the total harmonic distortion (THD) will start rising drastically. When you see manufacturers using power output ratings at 1%THD, you know that they are giving you close to peak output of the system rather than giving you a real world, continuous rating. Because of this, I recommend sticking with manufacturers that provide ratings below 0.1%THD as you know they are reliable and are not inflating their specifications for marketing purposes.

As noted above, the impedance of your speakers and the output of the amplifier into this load needs to be taken into account. We know that impedance is the electrical resistance of the speaker that changes depending upon frequency. Because of this, you will simply use the nominal impedance of the speaker; this is provided by the manufacturer.

This matters because if a speaker is less resistant to the flow of electricity, the amplifier is able to provide more power. For example, a 4 ohm speaker will receive more power from a power amplifier than an 8 ohm speaker because it provides less resistance.

The down side of this is that it will cause the power amplifier to run hotter. If your speakers are below an 8 ohm load, check with the power amp manufacturer to ensure the amplifier can support a xΩ load.