Help me understand output power, what is needed for what headphones
Feb 20, 2018 at 9:49 AM Thread Starter

#### carter840

Hi All,

I am currently shopping for headphones along with a DAC/Amp and one of my biggest issues is that I read reviews about some amps having less power than others. I also read about varying efficiency of headphones.

Can someone help walk me through how to calculate what power is needed given a specific pair of headphones, and then how to understand an Amps specs to see if it meets the need?

For instance, if I use this site http://www.digizoid.com/headphones-power.html and take the specs of the Fostex T50RP (50ohm, 92db/mW) I see I need ~200mW of power to drive these very loud.

But when I look at amps I see that the power rating is based on the resistance of the headphones in question. Specs read things like 300mW or more (32ohms). How am I supposed to relate this to a given pair of headphones at a different resistance?

Feb 20, 2018 at 6:14 PM
To find the relation between amp watts, headphone resistance and amp voltage, Ohm's law will determine any one factor from the other two. i.e to first calculate voltage out an amp from its listed X mW@Y Ohm spec for some given ohm, then plug in that voltage to compute the mW rating of the amp that will go to the 50 or whatever ohm headphone.

More generally if you want to know about an amp's specs and how it relates to a particular headphone performance (volume, bass damping, distortion) you can check out a few great older posts and contributions on the topic:

http://nwavguy.blogspot.ca/2011/09/more-power.html

Feb 21, 2018 at 1:56 AM
I am currently shopping for headphones along with a DAC/Amp and one of my biggest issues is that I read reviews about some amps having less power than others. I also read about varying efficiency of headphones.

Can someone help walk me through how to calculate what power is needed given a specific pair of headphones, and then how to understand an Amps specs to see if it meets the need?

Look at the sensitivity of the headphone, which is given in xxdB at 1mW. As a very rough estimate, for every +3dB, you need to double the output. So if you start out with an HD600 which has 97dB at 1mW, that means you can hit 100dB with 2mW, 103dB with 4mW, 106dB with 8mW, and so forth, until you hit 120dB or slightly over just to make the estimate easier, which in this case will be hitting 121dB with roughly 256mW.

Note that you don't literally get to 97dB the instant you move the volume knob since gain is a factor, so if you have too low gain, 256mW isn't necessarily going to hit 120dB. Not that you'd need to since the pain threshold is around 100dB. 120dB is just a very rough target so you can safely assume that the amp has enough reserve power so that when you're listening to properly recorded music you'd have no problems hitting dynamic peaks. At the same time, since distortion and noise get piled on with more output, chances are a more powerful amp will be farther off from audible distortion and noise, so basically you're picking a powerful amp just to stay farther from its limits, not for actually using an amp at its limits, especially clipping. If you manage to clip an amp at that output level you're getting hearing damage already.

Note also that that is just something you will just have to assume unless complete measurements are out there somewhere (or you can get them yourself). You can have an amp that starts with a rating of 0.0001% THD+N at 250mW at 32ohms, but it might be at 1% THD+N already when you're at 500mW at 32ohms or 200mW at 300ohms, while an amp can start out at 0.0005% and end up at 0.01% before it clips. Kind of like how you have the First Watt amps that max out at 15watts or 25watts, but at that output level still stays at much lower THD+N than other amps rated for 100watts per channel despite being at the same output level.

But when I look at amps I see that the power rating is based on the resistance of the headphones in question. Specs read things like 300mW or more (32ohms). How am I supposed to relate this to a given pair of headphones at a different resistance?

You can't at face value. Typically you can estimate that when the impedance doubles the power is halved, but that's not always the case. The Schiit Asgard2 for example has around 1000mW at 32ohms and 250mW at 300ohms, but the Schiit Lyr spits out 6000mW at 32ohms and drops to 350mW at 300ohms. Still more than the Asgard, but looking at it based on percentage vs 32ohm output, its output profile is biased for spitting out a lot of power at 32ohms, since there are a lot of low impedance headphones with markedly lower sensitivity, hence they need that power more than higher sensitivity, high impedance headphones (not that you need the full 6watts).

You'll just have to guesstimate based on the given output level nearest to the rated load impedance you'll put on it.

Feb 27, 2018 at 7:30 AM
PortegeManic,

As always, well said, well said.

More people need to understand this, maybe one day I suggest doing a presentation or podcast.

Hope you have a great day !