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Topic: 1/8 Source Impedance Rule of Thumb (Read 4419 times) previous topic - next topic

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  • mzil
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1/8 Source Impedance Rule of Thumb
Agreed. However a FIIO E3 plugged into a UCA 202 gets you there for less than half of that.

If I am understanding you correctly, you are suggesting bypassing the built-in headphone out already found on the UCA-202 and instead using its line out to the Fiio E3 to drive some headphones?


BTW I made a typgraphic error above. I meant FIIO E5. I had to look at the one on this computer to refresh my memory! ;-)

Yes.  The headphone jack on the UCA 202 has a source impedance of about 50 ohms which can lead to audible frequency response changes with some headphones.

The Fiio e5 is said to provide a source impedance of less than 1 ohm:

http://nwavguy.blogspot.com/2011/03/fiio-e...dphone-amp.html

^There is (or was) an E3, but it lacks a volume control and is not well thought of by nwavguy, so I wasn't sure what exactly you were suggesting. Thanks for the clarification.

Regarding the Behringer UCA202's output impedance spec of 50 ohm for the UCA202 and driving low impedance headphones, I suspect the importance of strictly adhering to the "1/8th rule" is a bit exaggerated. He only shows a worst case scenario, his Ultimate Ears SuperFi 5 Pro, and from there scares everyone into accepting this rule as gospel. I really wish I could see more curves than just that. For all we know the amount of deviation plummets with a conventional, non-balanced armature design with a similarly low impedance, or if one accepts deviations of say +/- 1 dB as being relatively benign (considering the much larger, comparatively rollercoaster curves of different headphones in general, one model to the next), his "rule" becomes more like "1/2" or "1/4" , for all we know. Your thoughts?
  • Last Edit: 13 March, 2013, 08:53:48 PM by greynol

1/8 Source Impedance Rule of Thumb
Reply #1
^There is (or was) an E3, but it lacks a volume control and is not well thought of by nwavguy, so I wasn't sure what exactly you were suggesting. Thanks for the clarification.

Regarding the Behringer UCA202's output impedance spec of 50 ohm for the UCA202 and driving low impedance headphones, I suspect the importance of strictly adhering to the "1/8th rule" is a bit exaggerated. He only shows a worst case scenario, his Ultimate Ears SuperFi 5 Pro, and from there scares everyone into accepting this rule as gospel. I really wish I could see more curves than just that. For all we know the amount of deviation plummets with a conventional, non-balanced armature design with a similarly low impedance, or if one accepts deviations of say +/- 1 dB as being relatively benign (considering the much larger, comparatively rollercoaster curves of different headphones in general, one model to the next), his "rule" becomes more like "1/2" or "1/4" , for all we know. Your thoughts?


50 ohms is about 3 times 16 ohms and 16 ohms is a pretty common impedance for headphones and earphones, so I don't think NWAVguy is exaggerating the source impedance issue.

When the source impedance is >> the impedance of the headphones, even just a broad 10% change in impedance can be audible.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #2
I really wish I could see more curves than just that.


Heres another 30 or 40 devices and a bunch of headphones:

http://rmaa.elektrokrishna.com/index.php?dir=Comparisons
http://rmaa.elektrokrishna.com/index.php?dir=Single_Players

...
I'd say hes quite conservative.  The impact of output impedance is enormous, and it factors into more then just frequency response (see THD at 1/2 vs. 1/8 for instance).



Thanks, but might you do me a favor and point out a particular example within those 100 files [all of devices with output impedances that are unknown to me and to the best of my knowledge not stated within the file itself], which demonstrates that breaking the 1/8th rule by just a little, (say closer to 1/4, 1/5th, 1/6th or 1/7th) causes a response variation one  would expect to be audible? Thanks.


I don't doubt  there will be visual problems with the graphs at anything less than his 1/8th rule, sure, however I've read some stuff he's written which suggests to me that he's concerned with tiny visual deviations of such plots which I would not expect to be audible in real world use. That's what I'm trying to gauge: what he considers a problem vs. what I consider a problem.
  • Last Edit: 25 February, 2013, 06:32:30 PM by mzil

  • saratoga
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1/8 Source Impedance Rule of Thumb
Reply #3
Thanks, but might you do me a favor and point out a particular example within those 100 files [all of devices with output impedances that are unknown to me and to the best of my knowledge not stated within the file itself], which demonstrates that breaking the 1/8th rule by just a little, (say closer to 1/4, 1/5th, 1/6th or 1/7th) causes a response variation one  would expect to be audible? Thanks.


Ugh no, I have no idea what those values are.  I don't actually own all those devices.  Other then that some devices are obviously too high and others obviously low enough.

I don't doubt  there will be visual problems with the graphs at anything less than his 1/8th rule, sure, however I've read some stuff he's written which suggests to me that he's concerned with tiny visual deviations of such plots which I would not expect to be audible in real world use.


1/8 is just a rule of thumb.  Its not a "strict" rule like you're suggesting because these aren't linear devices.  It depends on the impedance vs. frequency relationship of both the amp and headphones as well as the nonlinear behavior of both devices.  Trying to come up with a precise rule isn't going to work.  Some devices will need much less then 1/8, others will be relatively fine with more. 

So while you can measure the output impedance to see how "good" an amp is, you still need to actually test the device with a given load to see exactly how it will perform.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #4
I really wish I could see more curves than just that.


Heres another 30 or 40 devices and a bunch of headphones:
[links]

Having to research what the published output impedance is of those 30 or 40 devices and then opening those 100 files to then calculate which ones deviate from the 1/8 rule grossly, to then compare such curves against  the ones which deviate more subtly, like 1/4, 1/5th, 1/6th, etc., is not what I meant when I wrote that, but thanks anyways.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #5
Perhaps it is useful to you since you have the output impedances of these various devices commited to memory, I don't, so I have no idea by how much they break the 1/8th rule of thumb. Remember, I never questioned if impedance can have consequences on the frequency response. That's not the topic.

Quote
The purple line on this graph looks pretty audible to me. I'll leave evaluation of the rest of the parameters to you.


I'm not interested in what you deem audible, I'm interested in what nwavguy deems audible. EEs aren't always the best psychoacousticians, I've found.

I'd ideally like to see (the curves of) an example of a combination of devices (headphone and amp) which just barely breaks his 1/8 rule (say for example 1/5th, 1/6th, or 1/7th). I would like to know why he goes with 1/8th as his rule of thumb and not, say for example, 1/6th, 1/7th or 1/9th.

It all comes down to the fact that I once saw him comment on the deviation of some curve, possibly completely unrelated to impedance, I can't recall, which he deemed "an audible problem". I, on the other hand, looked at the greatly expanded dB scale on the left edge and noted that this "problem" amounted to an overall frequency response deviation of just +/-1 dB (or so, IIRC) from 20-20kHz, with relatively smooth transitions from octave to octave, nothing abrupt, and thinking to myself "He calls that an audible problem?".

Sorry, I don't have the time to re-examine everything he has ever written to find it again, but it established in my mind, at the time, that what he deems audible and what I deem audible might not align.

If I happen to stumble upon it in the future, I will report back.
  • Last Edit: 26 February, 2013, 01:07:03 PM by mzil

  • saratoga
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1/8 Source Impedance Rule of Thumb
Reply #6
I'd ideally like to see (the curves of) an example of a combination of devices (headphone and amp) which just barely breaks his 1/8 rule (say for example 1/5th, 1/6th, or 1/7th). I would like to know why he goes with 1/8th as his rule of thumb and not, say for example, 1/6th, 1/7th or 1/9th.


I think you're missing the point here.  This isn't a strict rule, its a design guideline intended to help engineers design well performing systems subject to a number of assumptions about how typical systems work.  You can't "break" a design guideline since its not universally applicable.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #7
When I wrote "rule of thumb" that's what I meant, i.e. not a strict rule but rather a rough generalization. But he didn't pick 1/4 and he didn't pick 1/10th or 1/20th, and I'd like to see the "why", rather than blindly accepting his arbitrary number [from a guy I don't trust to make good judgment calls about what is or isn't transparent, in the first place], that's all.
  • Last Edit: 26 February, 2013, 02:42:12 PM by mzil

  • saratoga
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1/8 Source Impedance Rule of Thumb
Reply #8
When I wrote "rule of thumb" that's what I meant, i.e. not a strict rule but rather a rough generalization. But he didn't pick 1/4 and he didn't pick 1/10th or 1/20th,


He didn't pick anything because he wasn't a 17th century mathematician.  This is all textbook stuff passed down from people studying mechanical oscillators to early EE a hundred years ago.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #9
I'm of the mind that the impedance match issue is continuously variable. It is not like at 1/7th there is a problem, but then, BAM, at 1/8th the problem is completely gone. I suspect that with an expanded scale we would still see alterations to the frequency response even at the 1/8th rule of thumb level in some instances. He, or whoever picked that number, found that as a rough generalization, any deviations observed at that point were pretty minuscule and deemed "inconsequential". I, however, might deem them inconsequential at, oh I don't know, maybe the 1/4 mark (a randomly picked level I grabbed out of nowhere, it also might be in the opposite direction for all I know).

[I also realize that headphones don't have a fixed impedance number, even though we often talk about them as if they do, and that in truth it varies by frequency.]

As per your claim that the correct number used, not picked by him you say, has always been "1/8th", well before the invention of headphones, as it applied to other mechanisms, might you please tell me the exact terminology for this 1/8th rule of thumb principal, so I might further my own research on the matter, or better yet, provide a link speaking to this 1/8th number being used outside the context of audio headphones/speakers/amps? Thanks.
  • Last Edit: 26 February, 2013, 03:43:38 PM by mzil

  • greynol
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1/8 Source Impedance Rule of Thumb
Reply #10
I hope you aren't being serious.

It's a general guideline, FFS!  I'm betting you've never studied analog circuit design under someone who knows a thing or two about it.  Perhaps control systems?  The math between electrical and mechanical control systems is the same, just the devices and units are different.
  • Last Edit: 26 February, 2013, 03:53:52 PM by greynol
13 February 2016: The world was blessed with the passing of a truly vile and wretched person.

Your eyes cannot hear.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #11
Yes, the "1/8th rule of thumb" is indeed a general guideline which was not picked by nwavguy, so says his above post. My research suggests otherwise, or at least I can't find any references to this 1/8th figure pre-dating him, and I'd like some backing evidence about the "17th century mathematicians" and that it is "textbook stuff passed down" claim:

Quote
He didn't pick anything because he wasn't a 17th century mathematician. This is all textbook stuff passed down from people studying mechanical oscillators to early EE a hundred years ago.
[emphasis mine]

Seems like a simple request to me.
  • Last Edit: 26 February, 2013, 04:07:01 PM by mzil

  • greynol
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1/8 Source Impedance Rule of Thumb
Reply #12
Feel free to enroll yourself into a course instead of bogging this discussion down.  Maybe you can contact NwAvGuy directly.
  • Last Edit: 26 February, 2013, 04:48:14 PM by greynol
13 February 2016: The world was blessed with the passing of a truly vile and wretched person.

Your eyes cannot hear.

  • saratoga
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1/8 Source Impedance Rule of Thumb
Reply #13
Yes, the "1/8th rule of thumb" is indeed a general guideline which was not picked by nwavguy, so says his above post. My research suggests otherwise,


I don't see how you can do research on this without having any understanding of what you're talking about.

or at least I can't find any references to this 1/8th figure pre-dating him,


Well the wikipedia page on this cites an article from the 1950s that suggests a little less then 10, while wikipedia itself says about 10.


  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #14
Is it really that hard for anyone to simply tell me the "1/8th rule of thumb"  principle's proper name as it applies in general, and not specifically headphones? So I might use the correct terminology to google it on my own, which is all I ever asked for in the first place:

Quote
might you please tell me the exact terminology for this 1/8th rule of thumb principal, so I might further my own research on the matter,
?

I never asked for a tutorial, just the correct name for it, so I can research it on my own.

P.S. Wikipedia also cites nwavguy, specifically, with the 1/8th rule as it applies to headphones.
  • Last Edit: 26 February, 2013, 04:51:30 PM by mzil

  • saratoga
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1/8 Source Impedance Rule of Thumb
Reply #15
Is it really that hard for anyone to simply tell me the "1/8th rule of thumb"  principle's proper name as it applies in general


I don't think it has a proper name.  Its an obvious consequence of how dampening works. 

People generally don't name things that are intuitively obvious.  Like the rule of thumb that says if you walk into a door it'll hurt.  Theres no door walking into principle because its obvious to people who can walk that they shouldn't walk into doors. 

P.S. Wikipedia also cites nwavguy with the 1/8th rule as it applies to headphones.


I know, I wrote that article.

  • greynol
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1/8 Source Impedance Rule of Thumb
Reply #16
Ok, any additional posts on the matter will go in the Recycle Bin.  I recommend starting a new topic or using the PM system.  If the former, please first ask (again, via PM) that this portion of the discussion be split.
13 February 2016: The world was blessed with the passing of a truly vile and wretched person.

Your eyes cannot hear.

1/8 Source Impedance Rule of Thumb
Reply #17
Perhaps it is useful to you since you have the output impedances of these various devices commited to memory, I don't, so I have no idea by how much they break the 1/8th rule of thumb. Remember, I never questioned if impedance can have consequences on the frequency response. That's not the topic.

Quote
The purple line on this graph looks pretty audible to me. I'll leave evaluation of the rest of the parameters to you.


I'm not interested in what you deem audible, I'm interested in what nwavguy deems audible. EEs aren't always the best psychoacousticians, I've found.


I'll excuse the slur as you obviously have no idea who I am or what I've studied or done. 

Given that NWAVguy is spinning his wheels with an overkill headphone amplifier your prejudices seem odd to me.
  • Last Edit: 26 February, 2013, 05:24:45 PM by Arnold B. Krueger

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #18
No slur intended. My bad. I worded it poorly. I was speaking in general terms and didn't mean to imply that the EE I was refering to was you, if it seemed that way due to my poor wording. I respect your judgement and consider your comments on what is and isn't audible to be authoritative, it just wasn't what I was seeking at the time.
  • Last Edit: 26 February, 2013, 05:50:28 PM by mzil

  • greynol
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1/8 Source Impedance Rule of Thumb
Reply #19
Quote
The purple line on this graph looks pretty audible to me. I'll leave evaluation of the rest of the parameters to you.


I'm not interested in what you deem audible, I'm interested in what nwavguy deems audible.

How about what you deem audible?
ABX would be one example, but others I would be interested in include measurements showing they have been EQ'd by an amount we all would generally expect to be audible. [many dB, not tenths of a dB, for example]

It is not at all unreasonable to describe the sweeps Arny is calling into question as varying from flat by "many dB."
  • Last Edit: 26 February, 2013, 05:48:30 PM by greynol
13 February 2016: The world was blessed with the passing of a truly vile and wretched person.

Your eyes cannot hear.

  • mzil
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1/8 Source Impedance Rule of Thumb
Reply #20
^Based on their very close time stamps, I assume you hadn't read my previous post at the time you composed yours, and now no longer request any response to the above. If I have misconstrued that, please let me know.