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Old 20th February 2011, 01:52 PM   #11
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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.
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Old 11th March 2011, 02:58 AM   #12
Bonsai is offline Bonsai  Taiwan
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I think the correct answer here is 'keep it as low as possible'.

Very nice graphs BTW Ashok - thanks for posting.
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Old 13th March 2011, 09:31 PM   #13
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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!

Last edited by DualTriode; 13th March 2011 at 09:54 PM.
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Old 13th March 2011, 10:08 PM   #14
sandyK is offline sandyK  Australia
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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.
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Old 30th March 2011, 11:38 PM   #15
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Quote:
Originally Posted by Bonsai View Post
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:

Click the image to open in full size.

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
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Old 30th March 2011, 11:49 PM   #16
sandyK is offline sandyK  Australia
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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.

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Old 31st March 2011, 12:02 AM   #17
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Originally Posted by sandyK View Post
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.
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Old 31st March 2011, 12:33 AM   #18
sandyK is offline sandyK  Australia
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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.
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Old 31st March 2011, 12:56 AM   #19
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Quote:
Originally Posted by sandyK View Post
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.

Quote:
Originally Posted by sandyK View Post
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.
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Old 31st March 2011, 01:14 AM   #20
sandyK is offline sandyK  Australia
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Quote:
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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