[Beefy]

 
Quote:

square waves into different loads (including real headphones and real headpones + capacitor)

Correct me if I am wrong, but the real headphone load was Sennheiser CX300 IEM's. I really don't think that such a sensitive load is a good indication of what happens with your a full size, lower sensitivity dynamic headphone, which requires much larger voltage and current swing, has frequency dependent resonances, etc.

 

[SpaceTimeMorph]

With as low an output impedance as this amp has measure though, the voltage swing across the output stage (due to varying headphone impedances) should be minimized.  Also, back EMF from the headphone would be better dissipated with a low impedance output from the amp (better damping factor).  Both points making this less of an issue.  That being said, running a test using a reactive load would be good from an empirical standpoint, just to prove that theory and execution are as close as possible.

 

[Armaegis]

Quote:

 
Someone should double check this to make sure I didn't make a mistake but:
 
Sensitivity @1mW=sensitivity @1V - 10*log(P(1V in mW))
 
if you want to substitute in the formula to get power from 1 volt you get
Sensitivity @1mW=sensitivity @1V - 10*log(1/R*1000) = sensitivity @1V - (10*log(1/R)+30)
 
logs are base 10 logarithm

 
 

Seeing as I asked the original question... but yeah now that I've crunched through the formulae myself that looks right.

 

[digger945]

 from a thread here:
http://www.head-fi.org/forum/thread/168037/db-per-milliwhat-efficiency-vs-sensitivity-vs-how-loud-do-they-really-go/15
 
 
 

Convert sensitivity in dB/V to sensitivity in dB/mW.

Another good sensitivity thread.

Quote:

Originally Posted by T_Schmidt  hey, sorry to bringing to life a very dead thread, but I have been searching for hours and have not been able to find the SPL dB / mW rating for the AKG K701's (or an actual conversion formula, either). I just bought a pair and would like to know the sensitivity to pair it up with a respective amplifier.

I just replied to a thread (see post #37) where I put "an actual conversion formula" to convert sensitivity in dB/V and impedance to sensitivity in dB/mW.

The formual gets the same results as post #18 above. I'll repeat the equation here for reference.


[size=medium]==== Extra Credit =============
[size=medium]For the geeks out there, the equation to convert dB/V to dB/mW is[/size]

[size=medium]dB/mW = dB/V + 10*log(R*P/V^2)[/size]

[size=medium]where[/size]
[size=medium]dB/V is the sensitivity in dB (SPL) at 1 Vrms of voltage into impedance R, 103 dB/V into 300 ohms in this case[/size]
[size=medium]R is the nominal impedance, 300 ohms in this case[/size]
[size=medium]V is the reference voltage, 1 Vrms in this case[/size]
[size=medium]P is the reference power, 0.001 watt in this case[/size]
[size=medium]dB/mW is the sensitivity in dB (SPL) at 0.001 watt of power (that is 1 milliwatt)[/size]
[size=medium]log is the logarithm base 10[/size]

[size=medium]In the above example we have:[/size]

[size=medium]dB/mW = 103+10*log(300*0.001/1^2) = 98.7[/size][/size]



 
 

 

[Steve Eddy]

 
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[SpaceTimeMorph]

It's easier than that though
 
Smw = Srms + 10*log(Z/1000)
 
Where
 
Srms = sensitivity in dB/Vrms
Smw = sensitivity in dB/mW
Z = headphone impedance in ohms
 
This of course assumes 1Vrms is the reference...

 

[SpaceTimeMorph]

Quote:

Originally Posted by Steve Eddy /img/forum/go_quote.gif
 
se
 

 
Ha, didn't know that's how you felt

 

[Steve Eddy]

Quote:

Ha, didn't know that's how you felt

Not feeling much of anything now that the morphine's kicked in.

 
se
 
 

 

[DingoSmuggler]

Quote:

It's easier than that though
 
Smw = Srms + 10*log(Z/1000)
 
Where
 
Srms = sensitivity in dB/Vrms
Smw = sensitivity in dB/mW
Z = headphone impedance in ohms
 
This of course assumes 1Vrms is the reference...

You've got that a bit mixed up.
using your same units it would be
 
Smw = Srms - 10*log(1000/Z)
 

 

[b0ck3n]

Quote:

 
For 1Vrms into 32 ohms, the power being delivered is a bit over 31mW.  To get 1mW into 32 ohms, you need only 0.18Vrms. Doing a conversion from one to the other. the PX100's sensitivity becomes 99dB/mW.  I mention all this to show that it's not the most efficient portable headphones compared to many others.  Yet it is an immensely popular model so it's appropriate to use as an example.
 
Where do you get 0.18 V from? You need 1 volt RMS at 32 ohms to get 31 mW.
 
How much gain is needed?  You must take into consideration the sensitivity of the headphones, the output level of the source, and add enough headroom to account for recordings that are low in volume so that you don't run out of volume pot travel.  Many recordings are almost constantly pegged near the 0dBFS line and dynamically compressed.  These recordings will sound very loud (and bad).  But there are also recordings that are recorded very low in level (some from the late 70s and 80s for example), maybe 10-15dB below 0dBFS.  Thus, it is prudent to select an amp gain level above and beyond what's needed to amplify a 0dBFS signal to compensate.
 
The target number for the O2 is 110 dB peaks which is seriously loud. The PX100 needs 4 dB less than 1 V RMS to hit 110 db SPL. The math is antilog(-4/20) = 0.63. 1 V RMS * 0.63 = 0.63 V  RMS.
 
With an 0.5 V RMS source the O2 would need at least .63/.5 = 1.3X gain to achieve 110 dB SPL peaks. A gain of 2X would provide some extra volume control range.
 
To give an example of how loud 110 dB really is that's about the Mini3's limit driving the HD600.
 
Let's say we want to have 15dB of additional headroom.  And we assume we want to be able to have unclipped peaks of 114dB (just take Sennheiser's spec as an example because it's a reasonable number and happens to require a nice clean 1Vrms).  To add 15dB extra headroom we need to swing 5.6Vrms.  Note that we're not actually going to run 5.6Vrms into the headphones, we would set the volume control to whatever listening level we want.  Let's then assume a y2 DAC as the source with 1.4Vrms output @0dBFS.  The gain we want is then 5.6V/1.4V = 4x.  If we increase the peak dB SPL requirement to 120dB then we need a gain of 8x.
 
Going by that value as a rule headphone amps would have so much gain most would never get the volume control past 20%. The Mini3 doesn't have that much headroom with typical full size cans. It sure doesn't with the HD600/650.
 
This illustrates that a "Mini³ with gain of 5x driving the PX100" case I mentioned is not outside "real world" usage, and is in fact quite a nice setting.  On average recordings (not too loud, not too soft), my volume pot position is usually a little over half way, but on really soft recordings I can turn it up to satisfactory levels without running out of pot travel.
 
Even with a 0.5 V RMS source 5X gain is ridiculous with the PX100.
 

 
Quote:

 
i would echo the above response from Ti and heed the concerns about having a very limited input source V and gain to drive multiple headphones.  having some headroom for differences in recording volume is not about placebo or expectation bias.  it's real.
 
If you look at commercial desktop amps, including the high-end models, most have a low gain setting in the 2X - 4X range to use with home sources. The HeadAmp GS-1, for example, is 3X.
 
this amp at 2Vrms/3.1x gain would not be able to drive my AKG 88dB/600 with any of my older Steely Dan albums to an adequate volume - 80-85dB.  also, 2Vrms is not a hot source.
 
What AKGs are those, link to specs please? Doing the math, 1 mW at 600 ohms is SQRT(0.001 * 600) = 0.77 V RMS. The O2 would hit the 6.2 V RMS with 2V in and 3.1X gain. So you have 20*log(6.2/.77) = 18 dB. 88 dB + 18 dB = 106 dB, not the 80-85 dB you claim. To put this in perspective, the Mini3 at 2.4 V RMS before clipping on battery power, and about 3 V RMS on AC power, would manage 99 dB on AC.
 
Those are still some seriously ineffective, worst case type headphones. You'd need about 10 V RMS to hit 110 db SPL which is more than most desktop amps can provide, let alone portable amps.
 

You forced me to get some books out to disprove your claims, and I don't appreciate that at all. I've enough on my plate as it is without having to devout hours of my time to this forum thread. In the future I'd suggest people look at claims that seem backed by math and physics with suspicion - unless you can verify the numbers for yourself, don't trust them to be accurate.

 

[b0ck3n]

The O2 is as suitable as a desktop amp as it is as a portable/transportable because of it's performance - low output impedance (0.58 ohms), gobs of power (relative to it's size) and very respectable distortion levels. You can't do better within the audible range.
 
Discussion on how to avoid the input/gain issue at your computer rose on another forum. You can just put the O2 volume at max and use the audio player's volume controls.
 
Like I've said the O2 was designed around the measurements. Given that a particular pair of headphones was designed for use with an output impedance as close to 0 as possible, the O2 (paired with an equally transparent source) will let you hear your pair of headphones the way they were intended to sound. The amp in itself has no sound signature. This might relate to "shrill and mechanic" with some headphones - if you don't like your headphones, try another pair.
 
 
Quote:

 
Well, since this isn't the sound science forum...  I forgot we had some flat earthers here.
 

Humans believed the earth was flat because that's how we perceived it while standing on it. If all the evidence suggests that the O2 is "transparent" and "sounds like nothing", and you suggest that's not true, then in this case you're the flat earther. That's not to say that different people won't perceive different things when listening to the O2, perception is still very much real, it's just pointless talking about it in absolute terms as one's perception doesn't always relate to the same as another's.
 
 
 

 

[kwkarth]

Quote:

The O2 is as suitable as a desktop amp as it is as a portable/transportable because of it's performance - low output impedance (0.58 ohms), gobs of power (relative to it's size) and very respectable distortion levels. You can't do better within the audible range.
 
Discussion on how to avoid the input/gain issue at your computer rose on another forum. You can just put the O2 volume at max and use the audio player's volume controls.
 
Like I've said the O2 was designed around the measurements. Given that a particular pair of headphones was designed for use with an output impedance as close to 0 as possible, the O2 (paired with an equally transparent source) will let you hear your pair of headphones the way they were intended to sound. The amp in itself has no sound signature. This might relate to "shrill and mechanic" with some headphones - if you don't like your headphones, try another pair.
 
 
Quote:


Humans believed the earth was flat because that's how we perceived it while standing on it. If all the evidence suggests that the O2 is "transparent" and "sounds like nothing", and you suggest that's not true, then in this case you're the flat earther. That's not to say that different people won't perceive different things when listening to the O2, perception is still very much real, it's just pointless talking about it in absolute terms as one's perception doesn't always relate to the same as another's.

It really seems to me that a great many people are talking about how this amp should sound based upon the measured and predicted measurements.  I would very much rather see/read reports of how it actually sounds in real life, not just on discworld.  I suspect Pratchett would agree.
 

 

[n3rdling]

Gonna have to wait until the boards start getting into people's hands for that.  In the meantime I think the objective discussion is very entertaining/insightful.

 

[Steve Eddy]

Quote:

Originally Posted by kwkarth /img/forum/go_quote.gif
 
I would very much rather see/read reports of how it actually sounds in real life, not just on discworld.
 

 
But how do you know how something "actually sounds" given that how something sounds is a highly subjective thing and can vary dramatically among listeners?
 
Some think Julia Roberts is absolutely gorgeous. Others think she's rather ugly. So how does Julia Roberts "actually look"?
 
From the O2's designer's perspective, its objective performance is such that what alterations it does make to the signal are so far below known audible thresholds that by all evidence, it should not impart any sound of its own.
 
se
 
 
 
 

 

[Head Injury]

Quote:

But how do you know how something "actually sounds" given that how something sounds is a highly subjective thing and can vary dramatically among listeners?
 
Some think Julia Roberts is absolutely gorgeous. Others think she's rather ugly. So how does Julia Roberts "actually look"?
 
From the O2's designer's perspective, its objective performance is such that what alterations it does make to the signal are so far below known audible thresholds that by all evidence, it should not impart any sound of its own.
 
se

Don't take the bait, Steve!