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What spec of an amp tells you how well they will power a certain ohm?

post #1 of 6
Thread Starter 

I currently have a sound card that I mostly use for speakers. The front 3.5 jack has this info


1.9 VRMS max into 10 Ohms @ 
< 0.02% THD (360 mW)
2.25 VRMS max into 47 Ohms @ 
< 0.015% THD (100 mW)
2.25 VRMS max into 100 Ohms @ 
< 0.01% THD (50 mW)
2.25 VRMS max into 300 Ohms @ 
< 0.01% THD (16 mW)
2.25 VRMS max into 600 Ohms @ 
< 0.01% THD (8 mW)


I'm assuming milliwatt is the amount of power being sent to the headphone. With that said what is an idea mW to power a headphone at a decent level ?

post #2 of 6

Almost all headphones only take mWs to reach 90db and above.

post #3 of 6
Thread Starter 

So only 8 mW would be enough to listen to pair of headphones @ a decent listening level ?

post #4 of 6

It depends completely on the headphone if that will be enough or not. I personally like to think of it in 3 general categories of headphone types when looking at amps to match for them. Type 1 would be normal dynamic driver headphones that have low impedance's, like all the 32 ohm and below headphones. Type 2 can be dynamic driver headphones that are 80 ohm and above with the average type in the 250-600 ohm range. Type 3 would be anything that requires a good deal of power, so any of the planar dynamic driver headphones or the electrostatic ones.

So in terms of what type needs how much power, type 1 needs the least amount, meaning your soundcard will be fine in terms of getting good volume levels, just have a good enough dac as a source. type 1's usually are what can be powered well out of ipods and phones too.

Type 2 are in my opinion the trickiest to match up well because their impedance can fluctuate through the frequency range, meaning if you have a pair of headphones that are type 2 then you will have to do some good research on finding the right match, generally those type need some kind of an amp, but not always will they need a lot of power, but some might. so in this case, just research for the specific headphone what are a few good matches people recommend.

Type 3 is easier to find an amp for, but depending on which specific headphone (since the selection in this grouping is relatively small compared to the other types) you would use, you just simply look for an amp that can at minimum put out 1 watt into each channel. If it can do that then with the exclusion of the electrostatics and he-6, that power output will be fine for them. If you have an electrostatic then you need an electrostatic amp, I'm not super familiar with them, but from what I've seen so far, there isn't too much variety and they will get the job done at least. Oh and the he-6 is just special lol, it needs at least 6 watts into each channel.

So using my set up as an example, I use LCD 2's with an amp that puts out 2 watts at 16 ohms, and according to the company (burson) when I emailed them asking about power output into 50 ohms (which is what the LCD 2's are rated at), my amp would be doing just slightly over 1 watt for them, overall with the clean power the amp provides it works very well. While my previous main uage pair of headphones, the denon's, were a normal dymanic driver 25 ohm headphone and sound loud out of any device they get plugged into. Amps in that case only slightly improve the sound.

I hope this helps, theres other ways to explain all this, and theres also the science about impedance of headphones etc which I am not very knowledgeable on, I just understand what does what lol. I think using the grouping system I laid out here should make picking amps for headphones a little easier for you and anyone else if you are coming at it with no idea about the subject.

post #5 of 6
Thread Starter 

I just need tangible numbers on dynamic headphones.  Even if they are vague I need a rough estimate of how many mw or from reading over your post watts I'm gonna need to power headphones. I understand headphones are different and you can't group them altogether but I really need to gauge what ohm range I'm looking for. As mentioned before I bought the audio card purely for the speakers and it has worked out absolutely perfect.


All I need is someone to say @ 80,150,250,300 and 600 ohms you are gonna need X mW or X watts to power them.  I only plan to use dynamic headphones so that should lessen the power needed vs other driver types.  I was looking @ sennheiser 600/650 and beyerdynamic 250/600ohm 880/990's 

post #6 of 6

I recently learned of a good way to figure this out, learned this after I made my previous post, look at the sensitivity of your headphone. this is the number that is stated as: 90dB/1 mW. thats what my audeze lcd 2 headphones are rated at. what that means is if you put 1milliwatt of power into them, they will be able to reproduce 90 decibels of sound volume (assuming you were at full volume). and since the loudest you will ever be able to listen to music before your ears start hurting and getting damaged is only about 100-105 decibels (and thats pretty loud), then you need to figure out how much more power is needed for the headphones to get that level. I dont remember the math equation for this, but I remember the chart:

for 90dB/1mW at 50 ohms, you need about 540 mW to make them play close to 120 dB (thats like louder or on par with a metal concert/acoustic drum kit played loudly. this will be painful to your ears). for my denon headphones which are 25 ohms and like 106dB/1mW it would only take a little over 1mW to make them sound loud and dynamic. prob more like 5-10mW. not very much.

The confusing part comes in for high ohm headphones because if you look at any amp, they all output at most like 100-400mW at 300-600 ohms, but if your headphone like I know the hd650 is something like 96dB/1mW then 100-400mW will totally power them, which is why amps dont ever get made with more output at that level. the high ohm rating is the resistance of current (totally paraphrasing actual technical terms, but still gets the point across), so thats why you see ppl using tube amps with lots of voltage but not much current, since they only need 100-400 mW to be powered but lots of volts to gets past the resistance.

In general for you request of an easy chart, just look at the headphones sensitivity, in general if it is 92dB/1mW or above, then you will never need more than 500mW to drive them properly. pretty much any amp/soundcard that can output 500mW will be able to power almost every headphone on the market, except a select few. if it can output 700-1W then you will for sure have the extra headroom for any headphone. Like I said considering the headphones you mentioned, just make sure it has between 300-700 mW of output power, then you will be good. the rest comes down to synergy.

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