How much power do i need for my speakers?

I am often asked questions like "is this amp powerful enough for my speakers?" so i thought i would take a moment to write up a definitive answer. 

To begin answering the question, it's important to know how loud you want your system to be at your listening position. This can be different for everyone, but using the THX reference level of 85db with 20db of headroom (max 105db) as a starting point is pretty safe. 


Without getting into all the math... Doubling amplifier power boosts output by 3db. Using 2 speakers in a stereo setup will boost output by 3db. Positioning speakers close to a wall will boost output by around 3db positioning speakers in a corner will boost output by around 6db. And output will decrease by 6db for each doubling of distance between the listener and the speaker. 

Its kind of a lot to remember, but fortunately there are tools, like this one that do all the work for you: 

Using the popular KEF LS50 speakers as an example, with their ~85db sensitivity, a distance to the speakers of 10 feet, the speakers being placed close to a wall, and a target max loudness of 105db (per THX), we see that in an ideal world we would be using an amplifier capable of at least 235 watts per channel with low distortion... With that said, remember that cutting power in half, to say ~120wpc, will only result in a drop of 3db... Yes, there is a reason why KEF uses 280W for the low frequency driver, and 100W for the concentric tweeter in the powered version of these speakers. 

KEF also published a great article about the 60-110 rule of thumb, you can read it here:





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