Maximum allowable headphone amp output impedance.

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Looking at some headphone impedance plots I find that a reasonably high output impedance in the headphone amp doesn't cause too much error in the ( electrical ) frequency response at the terminals of some headphones. But does it matter practically from damping point of view ?
Some headphones have a pretty flat impedance across the band and some are quite wavy with a lift at upper bass and at HF. So for the latter the Zout would matter a bit.
Are headphones designed to work with ideal voltage sources or are they designed not to be affected much by high Zout ( say 50 ohms or 100 ohms!).
 
there is a euro standard for 100 Ohm output resistance but I doubt many current headphones are really designed to that anymore

as you say a flat impedance curve means the frequency response of the headphone isn't very sensitive to the output Z of the amp

certainly the sensitive iem and ear buds for portable players are usually driven with very low Z source so as to use the most of their low 2-4V battery supply

although good sealing iems can be so efficient that they really need 10-20 dB step-down from even 1 Vrms drive

some headphone amp manufacturers have a output Z switch for people to tweak the response if they believe it makes a difference with their headphones
 
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there is a euro standard for 100 Ohm output resistance

Actually 120 Ohm.

Reading between the lines it seems as if the 'standard' was introduced because there was such a wide range of 'phones impedances available. Setting the output Z to 120 Ohms means that the majority of 'phones could be driven by the majority of amplifiers to a reasonable level of loudness. In other words the 'standard' was set for bland harmonisation and not 'to set the standard for high performance'.

Ignore it is my recommendation, because that is what most headphone manufacturers seem to do.

My experience suggests that most headphones are not critical of low damping, but I've obviously not tried all combinations!

: )
 
I find even 10 ohms and 40 ohms makes a noticeable difference to the response at the headphones. 100 ohms affects most headphones. Some with almost 1.5db lift in the mid bass end and also some at the extreme HF end. Will surely alter the sound. A 2.2 ohm output impedance seems to be acceptable for most phones I have !

If anyone wants to see the response plots I can put them up later. Too much effort right now. I'm turning in for the night !
 
Some interesting response plots.

I used a Sennheiser HD-580 ( 300 ohms ) and a Philips-805 ( 32 ohms).
I used a Zout of 2.2 Ohms, 10 ohms ,40 ohms, 100 ohms to check variations in the electrical response at the headphone terminals.The files starting with 580 are for the HD-580. Those without the phones number are for the SHP-805 . The HD-580-Z is the impedance of the Sennheiser and the SHP805-Z is the impedance of the Philips phones.
Both show visible response changes when the Zout is 10 ohms and gets worse as the Zout goes up. 2.2ohms Zout seems to be quite acceptable.
Maybe we can say that the Zout should not be higher than about 10 ohms (?). It might generally be hard to notice a 0.5 dB response change though on some tracks it may be audible.
 

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Hi all,

I also vaguely recollect the 120 Ohms value as being some sort of standard value. It makes sense, because in many (cheaper) amplifiers, the headphone output is derived from the main speaker output by a voltage divider, which always presents a higher than zero impedance to the headphone jack, and 120 ohms is easily achievable with a wide range of division rations. See for example Headphone Amplifier

However, I don't think that this is still the case. I find no reference at all to this in datasheets of modern headphone amplifier chips. Websites of headphone manufacturers are also silent on this issue. I have posted the question to Sennheiser, hopefully they'll find some time to answer it. When I get any answer, Ill post it here.
 
Sennheiser's comments on output resistance


My interest in headphone listening began three years ago with a gift of Sennheiser HD650. I built a ss Class A headphone amplifier based on John Linsley Hood circuit provided by Geoff Moss. The results were great and I really loved headphone listening despite (or because of) a rich and over-warm sound. I had built the amps with resistors in the output to the headphone socket and realised through reading forums that I could tune the sound by varying the resistor. I find with the HD650 that no resistor in the output is best.

I now use a very pure copper replacement cable and the HD650 is fast, detailed and punchy. Vocals in particular are very realistic. I also have a Borbely all FET amp which is very fast and clean, the HD650 is terrific with this.


I asked Sennheiser for their views on resistance and this is the reply from Germany:


If a real headphone amplifier is used, the output impedance should be as low as possible. Therefore it should be 0 Ohms. If you add additional resistors at the AF it is more and more getting to go to current source mode and you are influencing the sound as experienced. In some cases this may be preferred - but the sound of the headphone with its frequency response gets influenced"
 



If a real headphone amplifier is used, the output impedance should be as low as possible. Therefore it should be 0 Ohms. If you add additional resistors at the AF it is more and more getting to go to current source mode and you are influencing the sound as experienced. In some cases this may be preferred - but the sound of the headphone with its frequency response gets influenced"

Not a very informative answer. They don't say if the 120 Ohm standard still has any meaning. I doubt it. If it had, one would expect manufacturers to design their headphones to provide the best sound quality at this impedance. Sennheiser at least seems to indicate that their products are optimized to be driven with a zero Ohm voltage source as provided by most dedicated headphone amps. This also means that Sennheiser phones are not sounding their best on the average integrated amp which provides an approx. 120 Ohm source. This, however, could be easily modded.

It would be interesting to know the positions of AKG, Beyerdynamic, Koss and others.

The 120 Ohm doesn't make sense at all for low impedance earphones in the 16 Ohm region, as are mostly used in portable gear. Most of the output power would just be wasted.
 
If you look at my post #6 you will see how the output impedance affects the electrical response at the headphone terminals.
The Philips SHP-805 headphone is a 32 ohms unit with an bump at the resonance point and increasing HF impedance.
You can see that a 2.2 ohms output impedance doesn't affect it much. At 10 ohms you get a +/- 0.25 db variation and at 100 ohms there is a huge variation. The effect would be less for headphones with reasonably flat impedance over the bandwidth and those with higher impedance say like 300 or 600 ohms.
Some phones have a very flat impedance over the whole bandwidth and these will be less sensitive to output impedance of the amp.
Bottom line is that a lower output impedance is preferable as it does reduce the 'chance' of a varying response on the headphones.
How much Zout is acceptable depends on how flat ( uniform ) the impedance of the headphone is over the bandwidth. That would involve some research on the Net or making one's own measurements.
Look at Learning Center - Build a Headphone Graph | HeadRoom Audio
I think they have impedance plots for various headphones.
 
Hello All,
Excellent thread and posts guys.
The available headphones and amplifiers are all over the map. I have a pair of Sennheiser HD 600’s that I use for late night stress management.
I have used every manor of amplifier (mostly diy home brew) the variable with the greatest and most controllable impact to the sound quality has been amplifier output impedance; the greater the Zo the fatter and looser the bass response.
The cheapest and the best bang for the buck was a BUF636 in the feedback loop of a LM3562 Op-Amp powered by 12 volt gel cells.
With this home brew Op-Amp/Buffer I attached a 6 position 2 poll rotary switch with the ability to switch in any series resistance between 0 and 120 Ohms. My preferred output impedance ended up being somewhere between 5 and 15 Ohms.
With a target of ~ 10 Ohms Zo I set about building several tube headphone amplifiers. The best tube amplifier performance/$ goes to the Aikido Cathode follower with 6BQ7’s. This amplifier has a Zo of ~ 160 Ohms. Using a capacitor and this transformer EDCOR Electronics Corporation. WSM15K/600 at the output results in ~ 6 Ohms being seen by the headphones. You can pay much more for less performance. This amplifier is kick but with the 300 Ohm Sennheisers.
DT
All just for fun!
 
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IEC61938 is the relevant standard. It states that headphones should be driven by a 120 ohm source regardless of headphone impedance.
Headphones such as the AKG K701 and Audio Technica ATH W1000 are designed to comply with that standard, however for best performance they need an amplifier with much higher than average supply rails. e.g. +-15V or higher. They sound fabulous when correctly driven by such an amplifier, especially if in Class A biased to around 100mA per channel.
These headphones have a great low end when correctly driven, and sibilance is also negligible compared when driven from low impedance sources.
I believe the high end 800s are also designed to be driven from a 120 ohm source impedance too.
SandyK
 
I think the correct answer here is 'keep it as low as possible'.
I want to second the above. The 1996 IEC "120 ohm" standard has obviously not been well adhered to. Stereophile did an interesting article a while back on headphone measurements and showed the problems higher output impedances cause. And balanced armature IEMs (i.e. Etymotic, Shure, Ultimate Ears, Westone, and many more) are an even bigger problem for higher output impedances. Here's a frequency plot I did comparing the factory 50 ohm headphone output of the Behringer UCA202 DAC with a modified version that had a 2 ohm output impedance:

Modified%20UCA202%20vs%20Original%20Frequency%20Response%20Various%20Loads%20-3%20dBFS%20%28see%20legend%29%20with%20comments_thumb%5B1%5D.png


The yellow trace at the top is no load, the green trace is the 2 ohm version with Ultimate Ears SuperFi IEMs, and the light blue trace, showing a huge 14 dB of total variation, is the same headphones with the 50 ohm output version.

And frequency response isn't the only problem. There's also the total "Q" of the headphones. The Q, and hence the bass quality, depends heavily on the source impedance. A 50 ohm source is very different than a < 1 ohm source--especially with low impedance 16 - 32 ohm headphones. So not only can you end up with wild frequency response problems, but the bass quality might be awful as well because the Q ends up too high (i.e. if the designer assumed a zero ohm source and you don't use one) or too low (i.e. if the designer assumed a high impedance source and yours is zero ohms).

If someone back in 70's had established say a < 2 ohm standard we would have nearly all headphones today designed for such a source. And many headphones would likely perform better as well. But, instead, we have the electronic designers compromising their designs with non-zero output impedances, and we have some of the headphone designers compromising their designs because they have no idea what source impedance their headphones will get plugged into. Especially for high-end audiophile products, significant compromises like these are simply bad.

I suppose some headphone enthusiasts may enjoy the endless variations they get due to impedance mismatches when they swap gear around. But surely there has to be a better way to fine tune the sound than random impedance match problems?

Speakers have been designed since the 70's to work properly with a zero ohm source. There's absolutely no reason the same thing can't be done with headphones. It would make for much more consistent results. Personally, I hope we continue to head in that direction.

I have to wonder if the 120 ohm standard is partly because cheap op amps can't drive much less? Or perhaps it's left over from when headphone jacks were connected to the speaker terminals through large value resistors? I'm surprised this topic doesn't get more discussion as it's really a significant issue.

For anyone who's curious to know more, including more plots and a link to the Stereophile story, there's an article on my blog:

Headphone & Amp Impedance
 
RocketScientist
There is a vast difference between the requirements of most IEMs and typical heaphones such as the AKG K701 etc, and others like the Audio Technica AT W1000, and also the new HD800s as well as many others.
IEMs are more often used where the players have only a 3V battery supply,
so they are often much lower impedance.

SandyK
 
RocketScientist
There is a vast difference between the requirements of most IEMs and typical heaphones such as the AKG K701 etc, and others like the Audio Technica AT W1000, and also the new HD800s as well as many others.
IEMs are more often used where the players have only a 3V battery supply,
so they are often much lower impedance.
I don't entirely agree. Several companies make IEMs that are designed for home or studio use. Some cost thousands of dollars (like JH Audio, the flagship Ultimate Ears, Westone, etc.). I don't think many people are buying $2000 IEM's to plug them into their 3 volt portable.

And companies like Etymotic make low impedance IEMs for portable use and higher impedance IEMs for home/studio use. But even the higher versions still have huge impedance variations with frequency. Multi-driver IEMs are typically even worse. A lot of people are using IEMs with home or computer audio gear because they like the sound and/or the isolation.

And the Stereophile article used AKG K530's and still found 5 dB of response variation. So it's not just a problem with IEMs.
 
RocketScientist

In November 2005, Silicon Chip magazine published their Studio Series Headphone amplifier.
They went into the subject of the IEC standard and produced graphs like you are talking about, then designed the HA to have low output impedance, and using a Zobel network at the output.
More than 120 of these headphone amplifiers were constructed worldwide by members of another forum, however, virtually without exception, they all preferred a modified version using typically either 68 ohm or 120 ohm output resistors.
The modified version also used a John Linsley Hood designed PSU add on which gave great gains at the low end especially, and further improvements overall.

SandyK

P.S.
I seriously doubt that $2,000 IEMs are being used by too many people.
My understanding is that very few people use IEMs at home with their headphone amplifiers.
 
In November 2005, Silicon Chip magazine published their Studio Series Headphone amplifier.
They went into the subject of the IEC standard and produced graphs like you are talking about, then designed the HA to have low output impedance, and using a Zobel network at the output.
More than 120 of these headphone amplifiers were constructed worldwide by members of another forum, however, virtually without exception, they all preferred a modified version using typically either 68 ohm or 120 ohm output resistors.
Do you have a reference on the "virtually without exception" part? Just look at this thread for proof of people who prefer lower impedances. There's a lot more evidence of the same. From what I've seen, here and on the dedicated headphone forums, the amps the serious headphone nuts prefer most tend to have low output impedances. And that's true for commercial designs and DIY designs like those from Kevin Gilmore.

I seriously doubt that $2,000 IEMs are being used by too many people.
My understanding is that very few people use IEMs at home with their headphone amplifiers.
JH Audio headphones start at $400 and go up from there. There are some 3000+ threads on Head-Fi for JH Audio alone, many with hundreds or even thousands of post in each thread. I'd say the majority of JH Audio owners are using an amp or headphone DAC. You do the math. Here's just one thread for the $1200 JH-13 Pro's with 8600 posts:

JH Audio JH-13 PRO appreciation thread - Head-Fi.org Community

I know people who listen to IEMs at work with a headphone amp or DAC to block out the annoying person in the cubical next door. Musicians widely use IEMs when recording or performing as monitors. On every headphone forum I know of IEMs get roughly equal billing with full size cans. And it's mostly the balanced armature variety (with wild impedance variations) that are most popular.

There's obviously some debate over output impedance. But, as I said, you don't have to use IEM's to have wild frequency response changes. Sure, some might like to trade the tighter, deeper, low frequency extension you get with a low output impedance for the more boomy, peaky, less controlled, less deep bass you get by raising the Q with a higher output impedance. But I think it's quite a stretch to claim "virtually without exception" people prefer that sort of sound.
 
Do you have a reference on the "virtually without exception" part?

RocketScientist
There are several threads in the Jaycar HA area in Rock Grotto, with around 2,500 replies approximately, and >100,000 views.(total)
Remember too, that the total membership is only a fraction of that of HeadFi.
It's all pretty heavy going reading though.
SandyK

P.S.
Let's agree to disagree on this issue ?
 
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