Hi all, I'm currently trying to understand, fully, the relationship between headphone impedance and HAmp output impedance.

My understanding when we are talking about amplifiers and passive speakers is that the resistance (in ohms) should be matched.

What I'm trying to understand is how this relates to headphones and they way they are loaded. I recently purchased a pair of AKG K702 headphones for monitoring while location tracking and to initially test them out I pugged them into my iPod classic which gave a really low volume output. I was surprised when I put them into my MacBook Pro HP output and it was driving them really nicely and output level was not on full before I had enough volume.

I did some research and apparently lower rated (in Ohms) headphones are easier to drive and so give a louder signal when plugged into the same device.

When I am recording on location I will either be driving them with a Liquid Saffire 56 or a Mackie Big 'Knob'!

The liquid saffire has these specs for headphone output:
Output Impedance <70 Ohms
Load Impedence >24 Ohms

The Mackie:
Headphone Output: 150 Ohms

K702 Headphones Rated Impedance: 62 Ohms

My questions are: which output is more suited to driving the headphones properly and why?

I feel like it should be obvious but I cant puzzle it out other than smaller headphone impedance means easier to drive.

Thanks for any help you can give.

Dave

Have a read of that and see if it helps. Headphones are most of the way down the article.

http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp

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http://soundcloud.com/richie-royale
http://www.mixcrate.com/richieroyale

A few thoughts...

High impedance headphones need more voltage but less current. Low impedance headphones need low voltage but high current. I wouldn't say one is easier to drive than another as some headphone amps could possibly be damaged by driving low impedance headphones at high volumes while high impedance headphones are unlikely to damage a headphone output - they'll just be too quiet if the headphone amp doesn't have a high enough voltage output.

Don't mix up output impedance with recommended impedance - they're different but if you want to transfer the maximum possible power to your headphones then the output impedance and the headphone impedance should be the same. However, we very rarely need to transfer maximum power in any application unless you are working for the electricity supply company or you have a very weedy headphone amp.

Your headphones have a medium impedance so they'll work with most things. I would try the different options and use whichever sounds best.

James.

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JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

Hi Dave,
It occurs that you might be worried about "damping"? Don't be. It is true that a loudspeaker should "see" a very low resistance in order to control the LF cone resonances. This Damping Factor is the ratio of speaker impedance to amplifier output Z and whatever bllx the amp makers might tell you. a DF of 10 or more is quite adequate indicating an amp Zout of 0.8Ohms for an 8R nominal speaker Z and since the speaker will have a DC resistance of some 6Ohms there is little point in trying for lower! (it IS possible to build amplifiers with a negative opZ but that is a whole other can of worms!).

Headphones on the other hand have a dc R much the same as the rated impedance. My Sennheiser HR 448s measure 33.4 and 34.0 Ohms left/right so a very low amp Zout will make diddly difference.
This damping idea reached the zenith of stupidity when "hi fi" headphones first appeared and since the were intended for the new fangle sstate amps they "logically" had an impedance of 8 Ohms. Wrong! The amps of the day used attenuators in the HP feed and then terminated it with an 8 Ohm resistor..Totally pointless!

"Proper" studio phones were 400-600 Ohms and would work happily across a transistor amp output but some attenuation was needed to keep noise low. I remember being astounded by the sound quality of the first pair of AKG K50s I heard!

Dave.

Quote ef37a:

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Hi Dave,
It occurs that you might be worried about "damping"? Don't be.

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This might be an interesting read for both Daves - from people who 'ought' to know what they are talking about

www.benchmarkmedia.com 0-ohm-headphone-amplifier-sonic-advantages-low-impedance-headphone-amp


Martin

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YewTreeMagic

Thank you Martin.

Very interesting. Whilst I have neither the resources nor qualifications to question the fndings of such reputable people as Benchmark the work is just their own, no peer review is mentioned.

I also have a problem with the DC resistance of headphones. I know this is distributed along the winding but the conceptual model of "lumped" resistance in series with a perfect generator is pretty well accepted and such a model does not allow claims of damping factors of several 1000.

It is also odd that this is the first I at least have ever heard of such low Z drive amps, one would think all upmarket AI's and mixers would have caught on and fitted them and trumpeted their virtues from the rooftops?

I await to be convinced.

Dave.

I smell an elephantine odoriferous rodent!

Now why do you suppose they chose a series resistance equal to the phones impedance? You don't suppose it could have anything to do with the fact that matched impedance gives maximum energy transfer do you? So now they make sure that any electro-mechanical effects are at their absolute worst. As a matter of interest increase the source impedance more and you move into firmly into current drive mode which has all sorts of implications.

Also, by reducing the source impedance to near zero, in the first place you are 'shorting out' any voltage fluctuations, so of course it looks better. You don't get much voltage developed over zero ohms no matter how much energy you pump into it. Also in real terms you haven't increased the damping factor by the figure given. Due to the high phones impedance still being there you've really only doubled it.

I notice they made no attempt to monitor the acoustic output from the phones. I feel confident this would tell a very different story. Bung a 500Hz square wave in and see how much difference to overshoot is made by their supposedly massive damping. I can tell you now, it won't be a lot.

Finally, they are using their own 'research' to promote their own product. Can you say conflict of interest?

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It wasn't me!
(Well, actually, it probably was)

"Also, by reducing the source impedance to near zero, in the first place you are 'shorting out' any voltage fluctuations, so of course it looks better."

DOH! Din't see that coming Will and I bet (if I am generous and they are not sharlatans)neither did Benchmark!

Echos of Cold Fusion mayhap?

Dave.

Surely headphone designers would take high source impedance into account when they design their headphones as this is a feature of most headphone outputs. I find it hard to see how a low output impedance amplifier could be made that would work at acceptable volume with both 8 ohm and 600 ohm headphones.

James.

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JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net