[oggdude]

Sorry to make a new thread dudes,
 
I am just curious as i am currently using a Presonus HP4 (50 ohm output impedance) and an Apogee One (30 ohm output impedance).
 
I am currently using Sennheiser hd 650's (300 ohm load impedance) K702 (62 ohms) and KRK KNS 8400 (36 ohms).
 
After reading the rule of 8 (the load impedance should be 8 times greater than the output impedance) it seems the HP4 has too much output for all 3, the apogee is fine for the 650's only.
 
According to what i have been reading the KRK's and the AKG's would work best with the output impedance of something more similar to an iPhone 4 of around a 1 ohm output.
 
Can anyone help explain weather or not I'm barking up the wrong tree and looking at this entirely wrong or am i correct in thinking the HP4's output is just too hight?
 
The sensitivity of all the headphones is roughly around the 100 dB mark for all 3 so i know they are loud enough on all sources, i'm just concerned about the impedances changing frequency responses.

 

[Jd007]

Why not listen to all of them and decide with your ears?

 

[oggdude]

It's what i'm currently doing.
 
I'm just curious about the science behind it all and i am interested on other peoples perspective on that.
 
To me the iPhone can power all three headphones ok just maybe not enough volume for the HD 650 but only just, the K702 with volume being a few notches under the 650's, the KRK's work fine.
 
With the Apogee all work very well but the volume for the K702 has to be slightly higher than the 650's.
 
Volume wise the HP4 can be very loud, so no problems their.
 
Quality wise i would say the sound was the same with the HP4 running from a motu 8 pre and the Apogee One. With the iPhone from the brief tests i have done today the K702 sound the same with the HP4 setup and the Apogee One but the iPhone sounds to have a little less impact and depth in the bass frequencies and the sound is a little less lively, like I'm listening to the K702's while having my hood up with the headphones on the outside of the hood.
 
I can do a subjective test but i don't have the equipment to check if frequency responses change with differing output impedances in relation to differing load impedances.

 

[tomb]

Quote:

It's what i'm currently doing.
 
I'm just curious about the science behind it all and i am interested on other peoples perspective on that.
 
To me the iPhone can power all three headphones ok just maybe not enough volume for the HD 650 but only just, the K702 with volume being a few notches under the 650's, the KRK's work fine.
 
With the Apogee all work very well but the volume for the K702 has to be slightly higher than the 650's.
 
Volume wise the HP4 can be very loud, so no problems their.
 
Quality wise i would say the sound was the same with the HP4 running from a motu 8 pre and the Apogee One. With the iPhone from the brief tests i have done today the K702 sound the same with the HP4 setup and the Apogee One but the iPhone sounds to have a little less impact and depth in the bass frequencies and the sound is a little less lively, like I'm listening to the K702's while having my hood up with the headphones on the outside of the hood.
 
I can do a subjective test but i don't have the equipment to check if frequency responses change with differing output impedances in relation to differing load impedances.

The science has to do with damping factor, but this theory is far from absolute regarding headphones.  It's a fairly well known factor in matching speakers to power amplifiers, but it's far from proven that it applies to headphones in the same way.
 
A well known, Summit-Fi mfr has done extensive empirical testing with Grados and claims that high damping factors lead to the harshness that many experience with Grados.  There may be something to this when looking at the difficulty that people have in pairing the best amp with Sennheiser HD800's, for instance.
 
Tyll seems to confirm this in his latest review article on the Abyss headphones:
http://www.innerfidelity.com/content/jps-labs-abyss-ab-1266-planar-magnetic-headphones
Of course, most of the article is on the Abyss, but he compares them to HD800's and Stax.  He has some very interesting things to state about the amplifier powering the HD800's (ECP Audio).  Not mentioned, but it uses transformer outputs that are much, much higher impedance (probably 300 ohms as set by the high-low Z switch) than one would normally find with a typical top-of-the-line solid-state amp.
 
The L-2 is a tube, transformer-output amp, but it's not the tubes that tame the HD800, it's the transformers and perhaps higher output impedance.
 
That said, I doubt seriously - double doubt, actually - that your "iPhone can power all three headphones OK ..."  Amplifiers do much more than provide volume.  There are transients that are only audible with very fast slew rates and ample power reserves.  At the same time, an amplifier needs the authority to control the drivers through complex passages without getting congested (also requiring sufficient power reserves, among other things).  That's a decidedly non-technical description of a very technical subject, but suffice to say that I don't agree with your claim.
 
Also, if you are connecting the Apogee as a source, its output impedance is essentially cancelled by the amplifier.  Once the phones are connected to the output of an amplifier, the only thing the phones care about (in terms of impedance) is the amplifier's output impedance, not the source.
 
Finally, recognize that the Presonus appears to be a headphone distribution amplifier.  Its design and application are much different than a dedicated, audiophile-quality amplifier.  I'm not sure you really have quality amplification in any case - Apogee (source), iPhone (paltry portable), or Presonus (studio multiple headphone distribution).  Just MHO.

 

[PleasantSounds]

So far a lot of words but not enough science in this thread....
 
Let’s consider the impedance separately for the amp and for the headphone. I’ll start with the latter as it is more intuitive. But first let’s make another simplification and assume that impedance can be interpreted just like resistance, as they are somewhat similar.
So what practical information can you derive from the fact that headphone A has impedance X and headphone B – Y ?
 
It just describes how this headphone delivers power. Power translates to loudness, but is also affected by efficiency: the more efficient the headphone is, the less power it needs to produce the same loudness (or sound pressure). So back to the impedance: what does it have to do with power delivery? It tells you how much current is the headphone going to draw when the output is set to certain voltage. The higher the impedance, the less current it will draw. So assuming the same efficiency of our headphones A and B, the one with lower impedance will get louder as it will draw more current (or get more power). Higher impedance headphone requires higher output voltage to consume the same amount of power.
 
You can use this info as a hint for the amp selection: low impedance headphones don’t need high output voltage amps, but may require higher currents. High impedance need more voltage swing, but they consume less current. Many amp designs specialise in one area or the other. For example mobile phones running on battery power have better synergy with headphones that don’t need high voltage. On the other hand some tube amps may not produce enough current to satisfy more power hungry low impedance headphones.
 
So how does the amp’s output impedance come to play? Tomb has mentioned damping factor and he’s right – here’s the explanation.
 
Assume that headphone receives a single impulse. As a result the membrane is pushed away from the neutral position, generating a sound wave. But straight after that the opposite process happens: the mechanical properties of the membrane make it move back to the neutral position (we’ll disregard the oscillations that may accompany this process). During this phase, the headphone itself actually generates an electric impulse – after all it’s a coil moving along a magnet. This impulse gets combined with the ones produced by the amp, and we get distortions.
 
Now for the amp’s output impedance: the lower it is, the better these impulses generated by headphones get absorbed, reducing these distortions.
 
So why don’t we always go for the lowest possible output impedance? Because it creates load on the headphones – i.e. the lower it is, the harder it becomes for the membrane to return to the neutral state. This also causes distortions... In other words damned if you do and damned if you don’t. Fortunately there is a middle ground where both distortions types are low enough to be nearly unperceptibe (that’s the 8x - 10x rule of thumb). And for those who are still bothered with them there are electrostats that use a different principle and don’t suffer from this problem (that’s why they sound so good).

 

[oggdude]

Quote:

So far a lot of words but not enough science in this thread....
 
Let’s consider the impedance separately for the amp and for the headphone. I’ll start with the latter as it is more intuitive. But first let’s make another simplification and assume that impedance can be interpreted just like resistance, as they are somewhat similar.
So what practical information can you derive from the fact that headphone A has impedance X and headphone B – Y ?
 
It just describes how this headphone delivers power. Power translates to loudness, but is also affected by efficiency: the more efficient the headphone is, the less power it needs to produce the same loudness (or sound pressure). So back to the impedance: what does it have to do with power delivery? It tells you how much current is the headphone going to draw when the output is set to certain voltage. The higher the impedance, the less current it will draw. So assuming the same efficiency of our headphones A and B, the one with lower impedance will get louder as it will draw more current (or get more power). Higher impedance headphone requires higher output voltage to consume the same amount of power.
 
You can use this info as a hint for the amp selection: low impedance headphones don’t need high output voltage amps, but may require higher currents. High impedance need more voltage swing, but they consume less current. Many amp designs specialise in one area or the other. For example mobile phones running on battery power have better synergy with headphones that don’t need high voltage. On the other hand some tube amps may not produce enough current to satisfy more power hungry low impedance headphones.
 
So how does the amp’s output impedance come to play? Tomb has mentioned damping factor and he’s right – here’s the explanation.
 
Assume that headphone receives a single impulse. As a result the membrane is pushed away from the neutral position, generating a sound wave. But straight after that the opposite process happens: the mechanical properties of the membrane make it move back to the neutral position (we’ll disregard the oscillations that may accompany this process). During this phase, the headphone itself actually generates an electric impulse – after all it’s a coil moving along a magnet. This impulse gets combined with the ones produced by the amp, and we get distortions.
 
Now for the amp’s output impedance: the lower it is, the better these impulses generated by headphones get absorbed, reducing these distortions.
 
So why don’t we always go for the lowest possible output impedance? Because it creates load on the headphones – i.e. the lower it is, the harder it becomes for the membrane to return to the neutral state. This also causes distortions... In other words damned if you do and damned if you don’t. Fortunately there is a middle ground where both distortions types are low enough to be nearly unperceptibe (that’s the 8x - 10x rule of thumb). And for those who are still bothered with them there are electrostats that use a different principle and don’t suffer from this problem (that’s why they sound so good).

 
Thats a fantastic explanation thank you.
 
I am aware i need a much better amp, just trying to save the coin to get one. I already had the presonus for tracking on location. Since i have yet to hear an audiophile grade headphone amp i am only used to hearing headphones from studio equipment such as interfaces and mixing desks. I have no real reference as to what anything beyond that sounds like.
 
I've been swatting up on a few books including Douglas Selfs Audio power amplifier design handbook but you have filled in some of the gaps in my knowledge with a very concise post.

 

[tomb]

Quote:

<snip>  
Now for the amp’s output impedance: the lower it is, the better these impulses generated by headphones get absorbed, reducing these distortions.
 
So why don’t we always go for the lowest possible output impedance? Because it creates load on the headphones – i.e. the lower it is, the harder it becomes for the membrane to return to the neutral state. This also causes distortions... In other words damned if you do and damned if you don’t. Fortunately there is a middle ground where both distortions types are low enough to be nearly unperceptibe (that’s the 8x - 10x rule of thumb). And for those who are still bothered with them there are electrostats that use a different principle and don’t suffer from this problem (that’s why they sound so good).

A very excellent explanation - especially the last paragraph.  Thanks!