[lejaz]

If the OP wants the AKG sound sig. there's other models that are only 55ohms that will sound decent out of an ipod....k240mkII for one. Maybe even the k271, though I have no personal experience with them

 

[Kevin Brown]

OK, I am clearly not understanding this.  Lejaz: you say the K240 Mk ii's should be easier to drive than the K601's, but:
 
AKG K240 Mk II  91 dB, 55 ohm
  K601 101 dB, 120 ohm
 
The 601's have a higher sensitivity (which means easier to drive) and a higher impedance (which means easier to drive).
 
What am I missing?
 
It's certainly easier to drive 8 ohm speakers than 4 ohm speakers.  The 4 ohm speakers "pull" or demand more current out out of an amplifier, and if the amp can't deliver that, sound quality suffers.  But it seems as though everyone here thinks that higher impedance headphones are harder to drive than lower.  But that's the opposite to speakers.
 
??

 

[mikeaj]

For headphones, as opposed to speakers, it seems like voltage is usually the more demanding factor when looking at being able to drive the output at loud volumes.  The maximum peak-to-peak voltage output is limited by the power supply rails (or lower than that) in most configurations, and the rails are narrow for many portable amplifiers since they run on batteries.  It takes more voltage to drive higher impedance loads to the same volume if the sensitivity is the same.  Granted, lower impedance loads are harder to drive from other perspectives, such as keeping crosstalk low, keeping other distortions low, and requiring more current.
 
The K240 Mk II is nominally 91 dB/mW SPL, which equates to 104 dB/V.
The K601 is nominally 101 dB/V, not dB/mW!  This equates to around 92 dB/mW.
 
Also, the impedance of both headphones vary over frequency.  I think the impedance as well as the sensitivity are both quoted at 1 KHz, but I'm not sure.  In any case the K601 has higher impedance than you might expect for a headphone listed at 120 ohms.
 

 
edit: actually, based on the impedance and sensitivity, none of the K240 Mk II, K242, K271 Mk II, K272, etc. are particularly easy to drive compared to most headphones in that price range, but I have never listened to any of those.

 

[sanakaku]

Use a portable amp it really needs it to get that potential.. I'm having mine next week eeek im quite excited

 

[Kevin Brown]

For headphones, as opposed to speakers, it seems like voltage is usually the more demanding factor when looking at being able to drive the output at loud volumes.  The maximum peak-to-peak voltage output is limited by the power supply rails (or lower than that) in most configurations, and the rails are narrow for many portable amplifiers since they run on batteries.  It takes more voltage to drive higher impedance loads to the same volume if the sensitivity is the same.  Granted, lower impedance loads are harder to drive from other perspectives, such as keeping crosstalk low, keeping other distortions low, and requiring more current.
 
The K240 Mk II is nominally 91 dB/mW SPL, which equates to 104 dB/V.
The K601 is nominally 101 dB/V, not dB/mW!  This equates to around 92 dB/mW.
 
Also, the impedance of both headphones vary over frequency.  I think the impedance as well as the sensitivity are both quoted at 1 KHz, but I'm not sure.  In any case the K601 has higher impedance than you might expect for a headphone listed at 120 ohms.
 

 
edit: actually, based on the impedance and sensitivity, none of the K240 Mk II, K242, K271 Mk II, K272, etc. are particularly easy to drive compared to most headphones in that price range, but I have never listened to any of those.

 
Mike- This is not how I understand transducers being driven, whether big "speakers" or the little ones in headphones. 

 
V = I * R.
 
Let's say I don't have any headphones hooked up to a source.  There is still a voltage drop across negative and positive, the resistance is infinite, so no current flows.  Now I hook up a load (headphones).  The voltage drop doesn't change.  For higher impedance, less current is demanded.  Now I change the volume, the voltage drop increases to also result in a current increase, but that voltage drop would be independent of the load.
 
But at the same time, I'm trying to remember what I had learned a long time ago about voltage rails and power amps.  The voltage drop between those rails does have to be higher to be able to deliver more current ... to lower impedance loads though.
 
From the specs I had dug up, I did think the K601's were easier to drive than the K240 mk II's.  I see that isn't the case.
 
Ok, just for comparison if you can bear with me ...  I also have K240DF's here right now.  They can't be driven at all from my PC headphone jack.  These are the specs I dug up:
 
88 dB SPL/mW (600 ohms)
 
They would be harder to drive than either, the 240 mk II's or the 601's?  (I'll know soon enough in that I have K601's on the way.  I don't have a problem getting an amp if I need one, but I'd prefer headphones that I can use with or without an amp.)
 
Last one. 

  Grados.  I have SR80i's here at home and SR125's at work.  Easy shmeezy to drive out of a PC.  Their specs:
 
98 dB, 32 ohm
 
OK, much lower impedance, which should be harder to drive, along with a pretty high sensitivity though.  And they are definately much easier to drive than the K240DF's in practice.
 
And ... if they say just "98dB" that is assumed to be 98dB/mV?  But if it's /mW, they would always say mW?  But sometimes I see dB/mV.  That's not a typo?  dB/mV is *different* than dB/V?  And how do you convert between dB/V and dB/mW?  (I obviously didn't know those 2 were different.)  Not a simple multiplier, right?
 
(Confusing ...)
 
Sorry for all the questions, but I want to understand the "why" here rather than just reading that "600 ohms is harder to drive than 120 ohms" for example, when sensitivity has to play a role too. 

 
Thanks !!

 

[mikeaj]

With a certain idealized source with zero output impedance, the output voltage shouldn't change if the circuit is loaded or not.  Sources are not ideal, so there may be some differences.  For our discussions it's probably best to ignore the finer details, however.

The reason why voltage is often the limiting factor for headphones is because the power (and thus current) they require is relatively low.  Loudspeakers require on the order of watts, while headphones require on the order of milliwatts.  Generally, the maximum voltage an amplifier can put on its output is less than the positive voltage supply rail; the minimum voltage on the output is likewise more than the negative supply rail.  Some rail-to-rail op amp designs can output voltages very close to the rails, so they are frequently used in portable amplifiers.

The peak-to-peak voltage possible and therefore the maximum output signal that can be delivered without clipping, is going to be less than the difference between the power supply rails.  Some portable amps have charge-pump circuits to essentially create virtual rails for the amplifier sections to use, extending the difference in the rails, but the point remains that they are a limiting factor.

Let's do a back-of-the-envelope calculation:  If the power supply rails are 7.4 V apart, let's say that the amplifier can only output a signal with 7 V peak-to-peak.  7 V ptp = 2.47 V rms, as there's a 2 * sqrt(2) factor in the conversion for a sine wave.

P = I * V = I^2 * R = V^2 / R.  (rms values appropriate here)

With a 120 ohm load, the limit of the output is thus (2.47)^2 / 120 = 0.0508 W = 50.8 mW.  Of course, there may be other factors at play.  Many portable players have something more along the lines of 10 mW output or even less with a 120 ohm load.*

Headphone sensitivity seems to be given only in dB/mW SPL or dB/V SPL, not dB/mV.  What the manufacturers really mean here is the volume in dB referenced to 0 dB being the human auditory threshold, given 1 mW of power input for dB/mW.  The volume in dB given 1 V rms of signal input is the dB/V.

Converting between dB/mW and dB/V is not difficult given the impedance of the load.  Ignoring the fact that the K601 has an impedance that changes over frequency, let's just say it's a 120 ohm load.  101 dB at 1 V rms corresponds to P = (1)^2 / 120 = 0.008333 W = 8.3333 mW output.  So equivalently, it is 101 dB if you put 8.3333 mW of power into it.  At 1 mW output power, you have 8.3333 times less power than when you are putting in 1 V rms.  10 * log(8.3333) = 9.21 dB difference.  Therefore, the headphones are (101 - 9.21) dB/mW nominally.

So the "conversion" between dB/mW and dB/V depends on the impedance.  The value in dB/mV would just be 60 dB less than the value quoted in dB/V, if you run the numbers (assuming that the SPL vs. power has a linear relationship, which I wouldn't necessarily assume is true).  It's just a convention to use dB/mW or dB/V for headphones.  Anyhow, I think people writing "dB/mV" are making a typo.

The current required for 101 dB SPL at 1V rms input for the K601 is I = P / V = 8.333 mW / 1 V = 8.333 mA.  That's pretty low.  Even the current needed for Grados and IEMs is fairly low, so it's not usually a concern.

The K240DF would be much harder to drive than the K601, correct.  You're also right that sensitivity has a large role to play--blanket statements that X ohms is harder to drive (in terms of volume without clipping) than Y ohms are ignoring this point.  Granted, a lot of the high-impedenace headphones also have low sensitivity, but you should look at both impedance and sensitivity.

Of course, some people recommend amplifiers to upgrade the quality of sound or to color the sound.  Those considerations are outside of the scope of looking at volume levels and potential clipping.

*But keep in mind that about 85 dB is the long-term (8 hours/day) listening limit to avoid hearing damage.  The limit is higher for shorter-term exposure.  Therefore, if you listen at "reasonable" levels, you shouldn't be sending more than 1 mW to your headphones anyway, except for at peaks in the music.

If I've missed something or am off the mark, please correct me here.

 

[Kevin Brown]

dB/mV a typo: see here:  http://www.gradolabs.com/product_pages/sr125.htm  ??  dB/V or dB/mW do you think?
 
OK, so perhaps in a nutshell, the difference between speaker impedance and headphone impedance is that for headphones, it's the voltage that gates the power than an amp can deliver to a headphone, and for speakers, it's the current output capability of the amplifier.  ??
 
Thanks very much for the detailed response Mike. 

  And then I made my own spreadsheet from the example in this thread too to try and understand this:  http://www.hydrogenaudio.org/forums/lofiversion/index.php/t72730.html
 
 It is *slowly* sinking in.
 
Oh, one more question:  Sensitivity is dB/mW, and I can use that number plus the impedance to give me efficiency which is dB/V.
 
Is it true that I can directly compare the efficiency between headphones as a measure of "driveability" and then pretty much ignore impedance?  Keeping in mind whether an amp or PC headphone output or iPod output is *rated* for that impedance.  (Some amps aren't recommended for 600 ohm headphones, for example.)
 
(If you look at the A/B/C/D headphone comparison in the link about 1/2 way down the page, and then just looking at the last column, "SPL @ 1V", which some manufacturers directly give to us.)
 
Using AKG headphones, I got these as the efficiencies (dB/V) from the sensitivities (dB/mW) and impedances:
 
 

K240DF	90.22
K240 mk II	104
K501	101
K601	101
K701	105
K702	105

 
AKG actually gives 91dB/mW and 104 dB/V in the specs for the K240 mk II, but you can calculate the dB/V from the dB/mW with the impedance !!  Neat.
 
And then sometimes it's difficult figuring out when a company just says "dB" whether it's /mW or /V ...

 

[mikeaj]

You're welcome.  Glad to see you're getting it.
 
About the Grado spec, you got me!  I would guess they mean 98 SPL @ 1 mW, since Grados are known for being fairly easy to drive.
 
In my understanding, current, voltage, or power output can be the limiting factor for an amplifier depending on the load, signal, and internal characteristics.  I may have accidentally misled you into thinking that voltage was usually the limiting factor for headphones--really, it's that voltage is more of a concern than current.  The physical reason why in most cases is unfortunately beyond my understanding.  Maybe I should have taken more than the minimum number of electronics and power courses when getting my degree.

  As far as I know, your summary of the speaker vs. headphones issue is usually correct.
 
I think the sensitivity (dB/mW) may be the more useful measurement in looking at amplifiers because most give some kind of max output power spec.  Of course, max output power will depend on the load, and only sometimes will they give you the max power at impedance A, impedance B, and impedance C, etc.  Also they tend to quote output power in different conditions, or with different levels of distortion...Really, the moral of the story is that there should be more standardization, with output power, SPL (/mW or /mV), frequency response specs, and the like.
 
To answer your question more directly, you still need to consider the impedance of the headphones; just efficiency or just sensitivity are not enough.  Whether or not an amp is rated for a certain impedance or not isn't important I think.  You can drive 600 ohm headphones with amps not spec'ed for that--it just might not be loud enough.  An amp spec'ed for 300 ohms may not be enough for some 250 ohm headphones.  Many op amps and the like aren't spec'ed below 600 ohms, yet people use them to drive IEMs.
 
Maybe the best illustration is a graph, so look at this graph I took from the datasheet for the Texas Instruments TPA6130.  That chip is used as the power op amp in the output stage of the Fiio E5 and E7 amps.  V_DD is the positive supply rail, while the other rail is 0 V.  The device seems to be current limited at around 10 to 20 ohms load for that gain, but not so for higher loads.
 

 

[Kevin Brown]

Am I reading that graph right?  That op amp chip would be better for 32 ohm headphones than for say 300 or 600 ohm headphones?
 
OK, I need to more better absorb what you say about sensitivity and impedance.  And I do understand why different manufacturers might want to manipulate their "specs" so that their headphones can't be directly compared to others too, unfortunately. 

 

[sanakaku]

nvm 

 just opened multiple tab and post multiple thread

 

[Henzwill]

You're welcome. Glad to see you're getting it.

About the Grado spec, you got me! I would guess they mean 98 SPL @ 1 mW, since Grados are known for being fairly easy to drive.

In my understanding, current, voltage, or power output can be the limiting factor for an amplifier depending on the load, signal, and internal characteristics. I may have accidentally misled you into thinking that voltage was usually the limiting factor for headphones--really, it's that voltage is more of a concern than current. The physical reason why in most cases is unfortunately beyond my understanding. Maybe I should have taken more than the minimum number of electronics and power courses when getting my degree.

As far as I know, your summary of the speaker vs. headphones issue is usually correct.

I think the sensitivity (dB/mW) may be the more useful measurement in looking at amplifiers because most give some kind of max output power spec. Of course, max output power will depend on the load, and only sometimes will they give you the max power at impedance A, impedance B, and impedance C, etc. Also they tend to quote output power in different conditions, or with different levels of distortion...Really, the moral of the story is that there should be more standardization, with output power, SPL (/mW or /mV), frequency response specs, and the like.

To answer your question more directly, you still need to consider the impedance of the headphones; just efficiency or just sensitivity are not enough. Whether or not an amp is rated for a certain impedance or not isn't important I think. You can drive 600 ohm headphones with amps not spec'ed for that--it just might not be loud enough. An amp spec'ed for 300 ohms may not be enough for some 250 ohm headphones. Many op amps and the like aren't spec'ed below 600 ohms, yet people use them to drive IEMs.

Maybe the best illustration is a graph, so look at this graph I took from the datasheet for the Texas Instruments TPA6130. That chip is used as the power op amp in the output stage of the Fiio E5 and E7 amps. V_DD is the positive supply rail, while the other rail is 0 V. The device seems to be current limited at around 10 to 20 ohms load for that gain, but not so for higher loads.

It's not just the impedance that count, we should also consider sensitivity.
Low impedance with low sensitivity will be harder to drive than high impedance with high sensitivity.
My Sony NW-A45 with Mr Walkman Project Z Plus did a commendable job powering my AKG K501 and K601, both have 120 Ohm impedance but low 89dB/mW sensitivity. K501 needs volume level at around 80-90 on A45 while K601 needs around 90-100 for my comfortable listening level.
The sound quality is around 80% of what it can produce with proper amping.