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Questions about amping :D

post #1 of 12
Thread Starter 
Hello all!

So instead of turning around, going up and down I decided to straight off ask the question although this is completely, not my style XD

So my question is, let's say a "X" headphone maximum power capacity is 100mw and 32 Ohm. But the amp output is 300mw at 32Ohm. Will the headphone take in only 100mw or 300mw? Will it damage anything inside?

Thanks!
Billson biggrin.gif
post #2 of 12

Power delivered depends on the source level, amp gain, any volume controls anywhere including the amp, any amp output impedance, headphone impedance, etc.

 

The more power the headphones receive, the louder the sound they produce.  Max power capacity for pretty much anything except maybe some really cheap sets, is generally way too loud for actual listening.  As in, you'd get hearing damage really quickly if you put it on your head.  Furthermore, distortion levels would be way up there.  The max power rating is I think a continuous rating and not a peak rating though, usually.

 

If you're really maxing out the amp and putting 300 mW into the headphones, then yeah, the headphones would receive 300 mW.  The coils may overheat and otherwise be damaged (if it can only handle 100 mW).  Driver could run out of excursion.  I think.  I'm not entirely sure on the actual power ratings set and their typical levels relative to the physical limitations.


Edited by mikeaj - 4/20/13 at 10:19pm
post #3 of 12
Thread Starter 
Seems legit~ thanks! But what if let's say headphone like HD600 requires at least 100mw (example only, not according to specs) and I drive it with 10mw, will the driver weaken? I know it will result in lower volume but other than that, anything else will happen? XD
post #4 of 12

"require at least 100 mW" for what? Let's assume to reach 100 dB SPL.

 

If your amp only supplies 10 mW then you will only reach 90 dB SPL. Nothing else will happen. Well, assuming all else being equal, the headphone drivers will produce a bit less distortion at lower volume.

 

 

I think your idea from "underpowering" headphones stems from clueless people that will tell you that you need lots of headroom (excess gain) for an amp to drive a headphone properly. That's absolute nonsense.
Just like anything else in audio, excess gain is also a tradeoff. Too much and you reduce the usable volume control range (look at the mag ni thread where some people are stuck at 8:00 to 9:00 o'clock) which goes hand in hand with channel balance problems. High gain also produces more noise, distortion ..

 

Unless you listen at high SPLs, most of the time most headphones only need a fraction of a milliwatt of power. And ideally, your volume control should allow you to go up to 14:00 o'clock.


Edited by xnor - 4/21/13 at 4:09am
post #5 of 12
Thread Starter 
Quote:
Originally Posted by xnor View Post

"require at least 100 mW" for what? Let's assume to reach 100 dB SPL.

If your amp only supplies 10 mW then you will only reach 90 dB SPL. Nothing else will happen. Well, assuming all else being equal, the headphone drivers will produce a bit less distortion at lower volume.


I think your idea from "underpowering" headphones stems from clueless people that will tell you that you need lots of headroom (excess gain) for an amp to drive a headphone properly. That's absolute nonsense.

Just like anything else in audio, excess gain is also a tradeoff. Too much and you reduce the usable volume control range (look at the mag ni thread where some people are stuck at 8:00 to 9:00 o'clock) which goes hand in hand with channel balance problems. High gain also produces more noise, distortion ..

Unless you listen at high SPLs, most of the time most headphones only need a fraction of a milliwatt of power. And ideally, your volume control should allow you to go up to 14:00 o'clock.

So you mean "underpowering" is not true? I trust you! Your answer is always very logical XD

How do you calculate how much mW to get how many dB? Curiosity comes**

Billson ^_^
post #6 of 12

Check the sensitivity of your headphones, usually specified as X dB SPL @ 1 mW or @ 1 V.

 

To get X+10 dB you need 10 times the power (10 mW) or about 3.2 times the voltage (3.2 V) and 1/10 or 1/3.2 for -10 dB.

 

HD600 is about 102 dB @ 1V or 97 dB @ 1 mW.

87 dB = 0.1 mW

77 dB = 0.01 mW

...

 

 

edit: corrected sensitivity, see below


Edited by xnor - 4/21/13 at 10:47am
post #7 of 12

wow. so really you barely need ANY power if your a quite listener? 

 

and you say "headroom" is a lie? i was under the impression that you need some extra, unused, power to allow for voltage swings and big, or sudden climaxes in the music?

post #8 of 12

You, with most headphones you don't need much power at all. Most are around 100 dB @ 1 mW but some IEMs are crazy efficient reaching almost 120 dB @ 1mW.

 

I don't say headroom is a lie. What I say is that you do not need lots of headroom and it also doesn't guarantee that the amp is any good else we'd all be using monoblocks with our headphones...

As said before, a rule of thumb is that the volume control should be between 10:00 and 14:00 (2 pm) most of the time. That way you still have some excess gain for quiet songs. If you normalize tracks using something like ReplayGain you need even less excess gain because loud and quiet songs will be approximately equally loud.

post #9 of 12
Quote:
Originally Posted by xnor View Post

[...]

HD600 is about 112 dB @ 1V or 107 dB @ 1 mW.

 

I'm pretty sure they aren't that sensitive, and you probably looked up the wrong phones or typoed a 0 into a 1.

 

I think listed is 102 dB @ 1V or 97 dB @ 1 mW.  These lower numbers are also very close to what InnerFidelity got.

 

So

 

97 dB = 1 mW

87 dB = 0.1 mW

77 dB = 0.01 mW


Overall points still stand of course.


Edited by mikeaj - 4/21/13 at 8:55am
post #10 of 12

Let's take a simple example:

 

The DAC outputs 2 V for full-scale signals. The amps gain is 5x (about 14 dB). The headphone sensitivity is 95 dB @ 1 mW and nominal impedance 300 ohms. We want to listen at 75 dB SPL.

 

So the volume control needs to be set so that the amp outputs about 0.01 mW. To keep things simple let's take a full-scale pure tone (sine wave) as signal.

 

P = V*V/R

--> V = sqrt(P*R) = sqrt(0.00001 * 300) = 0.055 V

 

full-scale output: 2 V * 5 = 10 V

 

20*log10(0.055 / 10) = -45 dB ... or a voltage gain of 0.0055x and on a normal volume control that is somewhere around 8 o'clock so very close to the off position. That sucks.
 

 

Now let's assume you're listening to real music and the average* RMS amplitude of a track is -20 dB relative to a full-scale sine wave. Then we're at -25 dB (0.056x) which leaves plenty headroom for all the peaks that come close to full-scale. The volume control is still somewhere below 10 o'clock!

*) this needs to be a pretty dynamic song and the RMS amplitude can shortly hit -5 dB from time to time, which would be about 90 dB SPL. True peaks of course could reach 0 dB or even surpass that.

Compressed metal songs can reach an average of only -7 dB or even louder..

 

edit: lowered headphone sensitivity


Edited by xnor - 4/21/13 at 10:58am
post #11 of 12

thank you very much!

post #12 of 12
Quote:
Originally Posted by mikeaj View Post

I'm pretty sure they aren't that sensitive, and you probably looked up the wrong phones or typoed a 0 into a 1.

You are of course right, it's 102 not 112 dB/V. Thanks.

 

Made some corrections and changes above.


Edited by xnor - 4/21/13 at 10:52am
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