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Topic: Output impedance on portable mp3 players? (Read 27576 times) previous topic - next topic
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Output impedance on portable mp3 players?

As far as I know, amplifiers must have an output impedance less than 0.1Ω (damping factor at 20 at least) to drive speakers/headphones properly. How about portable mp3 players, such as Apple iPod or Creative Zen??

Wikipedia states that:

http://en.wikipedia.org/wiki/Headphone_amplifier

Quote
For example, a 32 Ω headphone (such as the Grado line) driven by a popular DIY headphone amp with a <1 Ω output impedance (the Gilmore Dynamic) would have a damping factor of >32, whereas the same headphone driven with an iPod (5 Ω output impedance) would have a damping factor of just 6.4.

iPod has an output impedance of 5Ω, thus its damping factor is only 6.4 (32Ω). Is this a valid measurement? Is this the capacitive resistance of iPod's coupling capacitors? (220uF for 3th, 4th Generation)

Output impedance on portable mp3 players?

Reply #1 –
Quote from: udauda on
As far as I know, amplifiers must have an output impedance less than 0.1? (damping factor at 20 at least) to drive speakers/headphones properly. How about portable mp3 players, such as Apple iPod or Creative Zen??

Wikipedia states that:

http://en.wikipedia.org/wiki/Headphone_amplifier

Quote


For example, a 32 ? headphone (such as the Grado line) driven by a popular DIY headphone amp with a <1 ? output impedance (the Gilmore Dynamic) would have a damping factor of >32, whereas the same headphone driven with an iPod (5 ? output impedance) would have a damping factor of just 6.4.

iPod has an output impedance of 5?, thus its damping factor is only 6.4 (32?). Is this a valid measurement? Is this the capacitive resistance of iPod's coupling capacitors? (220uF for 3th, 4th Generation)

As far as I recall, the output impedance of an amplifier is given by its entire design. I recall that some amplifier designs used coil transformers in order to achieve a lower output impedance under their design constraints. I believe they wouldn't use them if it was a matter of single capacitor.

And there is also the question of damping factor importance for headphones. I understand the concern for a bass loudspeaker having a heavy moving mass but headphone membranes are typically very light, so there may be much less of a concern about electric damping.
Some high-end headphones even have their impedance in hundreds of ohms which severely hinders their electric damping yet they deliver superb sound.

Output impedance on portable mp3 players?

Reply #2 –
So the wikipedia's claim on iPod having 5 Ohm output impedance can't really be proved...?? I think 5 Ohm has something to do with iPod's couping capacitor. Do you know how to measure the nominal value of capacitive resistance of 220uF?? I believe capacitive resistance is frequency-dependent, but is there anyway to measure it other than Xc = 1 / (2pfC) ?

BTW, electric damping does play an important factor in driving headphones as well. (regardless of the size of headphones' impedance)

According to J. Meier, the founder of Meier-Audio:
http://www.meier-audio.homepage.t-online.de/tipstricks.htm
Quote
|Zphone(f) / (Zphone(f) + Ro)| < 1
Generally Zphone(f) has a maximum at the lower frequencies and therefore the attenuation of the lower frequencies is less than that of the higher frequencies. The figure shows the attenuation at various frequencies for the Sennheiser HD600 and the Beyerdynamic DT931 at output impedances of 150 and 470 Ohm. It clearly shows that the lower frequencies are less attenuated than the higher frequencies.

If the damping factor decreases, the frequency response of headphones flunctuates along their impedance curves.

Output impedance on portable mp3 players?

Reply #3 –
The output impedance can be verified quite easilly. Just play a file containing a simple sinus waveform (say 500Hz), attach a 5 ohm resistor to the output and measure the RMS power delivered to it. Then swap it for a 10 ohm one then a 2 ohm one. If the output power was the biggest for the 5 ohm one, then the output impedance is likely to be 5 ohms at that frequency.
Note, you should not carry out the experiment if you don't know what you're doing. You should not have the restistors attached for longer than necessary and you should have the volume turned down.
Quote from: udauda on
BTW, electric damping does play an important factor in driving headphones as well. (regardless of the size of headphones' impedance)

You have to realize one thing - the damping depends on the sum of headphones' impedance and the output impedance of the amplifier. If the latter is of order ~ohms and the former of order ~10 or ~100 ohms, then the latter plays very little role and you shouldn't be concerned about it.
If you think that iPod's 5 ohm output impedance is too high for your 32 ohm headphones then you should try if the pod can drive headphones with impedance of ~100 ohms where the damping factor would be much higher. If it can, just choose those high-impedance headphones if the damping factor is something that bothers you.

I believe there is hundred more relevant factors than the damping factor when deciding what headphones/player to buy.

And also, your article mentions output resistance of ~100 ohms which is completely out of the range of your 5 ohm iPod.

Output impedance on portable mp3 players?

Reply #4 –
Rod Eliot has some pretty interesting articles on the effects of source impedence:

http://sound.westhost.com/z-effects.htm
http://sound.westhost.com/impedanc.htm
http://sound.westhost.com/project56.htm

Quote from: udauda on
So the wikipedia's claim on iPod having 5 Ohm output impedance can't really be proved...?? I think 5 Ohm has something to do with iPod's couping capacitor. Do you know how to measure the nominal value of capacitive resistance of 220uF?? I believe capacitive resistance is frequency-dependent, but is there anyway to measure it other than Xc = 1 / (2pfC) ?
That's pretty much the best way - and obviously indicates that the effective impedance of the output will be frequency dependent. If you calculate the complex impedance (Z) of the a capacitor coupled amp, and plot |Z| (the magnitude) against frequency, you will find that it's pretty flat across the entire audio band - presuming that your amplifier is well designed. Speakers are generally much more variable.

Output impedance on portable mp3 players?

Reply #5 –
Quote from: Martel on
The output impedance can be verified quite easilly. Just play a file containing a simple sinus waveform (say 500Hz), attach a 5 ohm resistor to the output and measure the RMS power delivered to it. Then swap it for a 10 ohm one then a 2 ohm one. If the output power was the biggest for the 5 ohm one, then the output impedance is likely to be 5 ohms at that frequency.
Note, you should not carry out the experiment if you don't know what you're doing. You should not have the restistors attached for longer than necessary and you should have the volume turned down.

This actually has nothing to do with output impedance but rather with matched impedance, which is the load impedance for which the amplifier is most efficient.

The output impedance has more to do with the use of negative feedback in the amplifier circuit. Without the use of negative feedback the output impedance of a typical audio amplifier would be on the order of a few ohms. Use of, for example, 10:1 negative feedback would reduce the output impedance by a factor of ten and would also improve linearity by this much.

Output impedance on portable mp3 players?

Reply #6 –
Quote from: pdq on
Quote from: Martel on

The output impedance can be verified quite easilly. Just play a file containing a simple sinus waveform (say 500Hz), attach a 5 ohm resistor to the output and measure the RMS power delivered to it. Then swap it for a 10 ohm one then a 2 ohm one. If the output power was the biggest for the 5 ohm one, then the output impedance is likely to be 5 ohms at that frequency.
Note, you should not carry out the experiment if you don't know what you're doing. You should not have the restistors attached for longer than necessary and you should have the volume turned down.

This actually has nothing to do with output impedance but rather with matched impedance, which is the load impedance for which the amplifier is most efficient.

The output impedance has more to do with the use of negative feedback in the amplifier circuit. Without the use of negative feedback the output impedance of a typical audio amplifier would be on the order of a few ohms. Use of, for example, 10:1 negative feedback would reduce the output impedance by a factor of ten and would also improve linearity by this much.


Since the damping factor(Zl/Zs) considers only pure resistive impedance (resistance), you may estimate Zs by fiddling with Zl until maximum power is delivered into it (which is exactly at the point where Zs=Zl). And this is exactly what I proposed.
 

Output impedance on portable mp3 players?

Reply #7 –
Quote from: Martel on

Since the damping factor(Zl/Zs) considers only pure resistive impedance (resistance), you may estimate Zs by fiddling with Zl until maximum power is delivered into it (which is exactly at the point where Zs=Zl). And this is exactly what I proposed.

This would be true for a perfect voltage source with unlimited voltage and current, but think about it.

If you assume a 0.1 ohm output impedance (which is not uncommon) then for an amplifier that delivers 100 watts into 8 ohms, it is capable of driving 3.54 amps into the combined 8.1 ohms impedance, which is 28.6 volts.

If the same amplifier delivered its maximum power into 0.1 ohms (0.2 ohms total) then the 28.6 volts would deliver 143 amps, which is over 2000 watts!

The reality is that practical audio amplifiers have a relatively low output impedance in their normal operating range of voltage and current, but the load needs to be matched to the amplifier's voltage and current limits for maximum efficiency, independent of the output impedance.

Output impedance on portable mp3 players?

Reply #8 –
I guess I'll throw-in my 2-cents worth of speculation....   

Quote
This would be true for a perfect voltage source with unlimited voltage and current, but think about it....  The reality is that practical audio amplifiers have a relatively low output impedance...
  The whole point of the original question is that iPod's may not have fractional-ohm output impedence.    I have no idea...  PDQ's test would help to answer that question. 

The effects of damping factor are worst at the speaker's/headphone's resonant frequency.  For speakers, this is usually in the low-bass range.  For headphones, it's typically the midrange.

The capacitive reactance will be worse (higher reactance/impedence) at low frequencies (of course).    At the mid-frequency headphone resonance, the output-capacitor will have little effect on the damping factor. 

I'd be more concerned with the high-pass filter effect of a series capacitor.  If I've done the calculations correctly (they say never do math in public), you'll get about a 6dB drop at 20Hz, given the above assumptions (220uF and 32 ohm headphones).  But 32 ohms is low for headphones, and the low-frequency roll-off would be much less with more-normal headphones.

I haven't seen any iPod schematics...  But, there is a good possibility that there is a ~5 ohm series resistor at the headphone output.  This would be to protect (current limit) the output amplifier from damage, in case a shorted pair of headphones (or a 4 ohm speaker) was attached.  And if I remember correctly, a regular headphone jack gets shorted-out in normal use whenever the headphones are plugged in or out...

Output impedance on portable mp3 players?

Reply #9 –
Quote from: pdq on
Quote from: Martel on

The output impedance can be verified quite easilly. Just play a file containing a simple sinus waveform (say 500Hz), attach a 5 ohm resistor to the output and measure the RMS power delivered to it. Then swap it for a 10 ohm one then a 2 ohm one. If the output power was the biggest for the 5 ohm one, then the output impedance is likely to be 5 ohms at that frequency.
Note, you should not carry out the experiment if you don't know what you're doing. You should not have the restistors attached for longer than necessary and you should have the volume turned down.

This actually has nothing to do with output impedance but rather with matched impedance, which is the load impedance for which the amplifier is most efficient.


The amplifier is most efficient at zero output impedance, not matched.  At matched its no more then 50% efficient, which is quite poor.

Quote from: pdq on
If the same amplifier delivered its maximum power into 0.1 ohms (0.2 ohms total) then the 28.6 volts would deliver 143 amps, which is over 2000 watts!


If the 100w amplifier delivers it's maximum power, then its delivering 100w, not 2000w.  Your logic here doesn't make any sense.  You can't calculate the voltage for one load, switch it, and assume the voltage is unchanged.

Quote from: pdq on
The reality is that practical audio amplifiers have a relatively low output impedance in their normal operating range of voltage and current, but the load needs to be matched to the amplifier's voltage and current limits for maximum efficiency, independent of the output impedance.


No it needs to be matched for maximum power transfer (which is rarely the goal).  For maximum efficiency (which is nearly always the goal) it should be as low as possible.  This is why an ideal amplifier has zero output impedance regardless of the load impedance.

Output impedance on portable mp3 players?

Reply #10 –
It seems like some people have a confusion between Impedance Matching and Impedance Bridging:
http://en.wikipedia.org/wiki/Impedance_matching
http://en.wikipedia.org/wiki/Impedance_bridging


Anyway, I just found out the resistance of KSC-75's voice coil:
http://i95.photobucket.com/albums/l149/lowyat/DSCN2978.jpg
Quote
my multimeter has an error or +0.3 ohm
so the driver voice coil impedance is 58.3 ohm to be exact.

http://www.head-fi.org/forums/3079201-post2530.html

Since KSC-75's nominal resistance is 60Ω, if you drive KSC-75 with an amplifier with 0.1Ω, the resulting Damping Factor is: 60Ω / 0.1Ω+0.1Ω(cable)+58.3Ω(voice coil) = 1.0. Then what if you drive the headphone with 5Ω amplifier? 60Ω / 5Ω+0.1Ω(cable)+58.3Ω(voice coil) = 0.9

I guess headphones don't really get affected by the damping factor as much as speakers, due to the feather-weight diaphragm and the amount of air resistance it deals with.  Of course, as Mr. Meier stated, if you add an impedance adapter to the amplifier's output, you may alter the acoustic signature of headphones, regardless of their resistance values:

Quote
> > when you drive the low-impedance headphones (like apple earbud, Grado, Sony, etc.) with an external headphone amplifier, does the frequency response get affected by the increased output impedance, just like the high-impedance headphones??

Yes, the same principles apply.

Cheers
Jan


I think my overwhelming curiosity has been resolved!! Thnx for your inputs, guys.

Output impedance on portable mp3 players?

Reply #11 –
Quote from: udauda on
Anyway, I just found out the resistance of KSC-75's voice coil:
http://i95.photobucket.com/albums/l149/lowyat/DSCN2978.jpg
Quote
my multimeter has an error or +0.3 ohm
so the driver voice coil impedance is 58.3 ohm to be exact.

http://www.head-fi.org/forums/3079201-post2530.html

Since KSC-75's nominal resistance is 60?, if you drive KSC-75 with an amplifier with 0.1?, the resulting Damping Factor is: 60? / 0.1?+0.1?(cable)+58.3?(voice coil) = 1.0. Then what if you drive the headphone with 5? amplifier? 60? / 5?+0.1?(cable)+58.3?(voice coil) = 0.9


Resistance != Impedance

-brendan

Output impedance on portable mp3 players?

Reply #12 –
Quote
Resistance != Impedance
-brendan


Yes, resistance + reactance = impedance. Voice coils's resistance is frequency dependent. Is my DF calculation incorrect??

Output impedance on portable mp3 players?

Reply #13 –
Quote from: pdq on
If the same amplifier delivered its maximum power into 0.1 ohms (0.2 ohms total) then the 28.6 volts would deliver 143 amps, which is over 2000 watts!
No, it wouldn't. Please, learn the Ohm's law.
If the Zl would be 0.1 ohms, then voltage delivered to it would be minimum and most of the total voltage would lie on the internal resistance of the amplifier. This means all the energy would dissipate inside the amplifier, potentionally damaging it. The higher the impedance (resistance) of the connected load, the more effective is the energy delivery to it but due to limited output voltage, it limits the total output power. Regardless of efficiency, the greatest output power is delivered when load and source resistance are equal and then 50% of the energy is consumed by load and 50% inside the amplifier.

And, if we're talking strictly about damping factor, it considers only real impedance with no imaginary (capacitive/inductive) component.

Output impedance on portable mp3 players?

Reply #14 –
Quote from: udauda on
Since KSC-75's nominal resistance is 60?, if you drive KSC-75 with an amplifier with 0.1?, the resulting Damping Factor is: 60? / 0.1?+0.1?(cable)+58.3?(voice coil) = 1.0. Then what if you drive the headphone with 5? amplifier? 60? / 5?+0.1?(cable)+58.3?(voice coil) = 0.9

Your calculation is a little bit off. Damping factor is the ratio of load impedance to source impedance. For a 60 ohm load and a 0.1 ohm source impedance the damping factor is 600, and for a 5 ohm source impedance it is 12.


Quote from: Martel on
Quote from: pdq on

If the same amplifier delivered its maximum power into 0.1 ohms (0.2 ohms total) then the 28.6 volts would deliver 143 amps, which is over 2000 watts!
No, it wouldn't. Please, learn the Ohm's law.
If the Zl would be 0.1 ohms, then voltage delivered to it would be minimum and most of the total voltage would lie on the internal resistance of the amplifier. This means all the energy would dissipate inside the amplifier, potentionally damaging it. The higher the impedance (resistance) of the connected load, the more effective is the energy delivery to it but due to limited output voltage, it limits the total output power. Regardless of efficiency, the greatest output power is delivered when load and source resistance are equal and then 50% of the energy is consumed by load and 50% inside the amplifier.

Any real amplifier has an upper limit on output voltage and an upper limit on output current. If the amplifier is unable to supply either maximum voltage or maximum current then it is operating below its maximum efficiency. The only way for it to operate at its maximum efficiency is for its output current to be its maximum value at the same time that its output voltage is at its maximum value, and the ony way this can happen is if the load impedance is exactly equal to the ratio of maximum voltage to maximum current. This is the optimum load impedance, and if this were also the output impedance of the amplifier then the damping factor would be 1.0, and I assure you that any good amplifier has a damping factor much higher than 1.0.
Quote from: Martel on
And, if we're talking strictly about damping factor, it considers only real impedance with no imaginary (capacitive/inductive) component.

Says who?

Output impedance on portable mp3 players?

Reply #15 –
Quote from: pdq on
Says who?
Says wikipedia. 
http://en.wikipedia.org/wiki/Damping_factor

Because if you didn't have this clause, you would have to express the damping factor as a complex number ( like |DF|*e^(i*DFphi)), which wouldn't make much sense for majority of people. I guess the imaginary part of the damping factor does not contribute to actual damping since idle current doesn't do any "work" (other than heating up the cables).

Output impedance on portable mp3 players?

Reply #16 –
Quote from: Martel on
Quote from: pdq on
Says who?
Says wikipedia. 
http://en.wikipedia.org/wiki/Damping_factor

Because if you didn't have this clause, you would have to express the damping factor as a complex number ( like |DF|*e^(i*DFphi)), which wouldn't make much sense for majority of people. I guess the imaginary part of the damping factor does not contribute to actual damping since idle current doesn't do any "work" (other than heating up the cables).

I guess this is a terminology issue. To me the term "real impedance with no imaginary (capacitive/inductive) component" implied just the resistive component. If you interpret it as the impedance including the effects of resistances, inductances and capacitances but just ignoring the phase angle then I would agree with it.

Output impedance on portable mp3 players?

Reply #17 –
Well, I don't think that any portable's output is good enough to consider the damping factor a major issue (compared to all the issues portables have). We already gave it more attention than it deserves, IMHO.

Output impedance on portable mp3 players?

Reply #18 –
Quote from: Martel on
Well, I don't think that any portable's output is good enough to consider the damping factor a major issue (compared to all the issues portables have). We already gave it more attention than it deserves, IMHO.


We sure did! LOL..

Anyway, I found this article from AVholics.com:

http://www.audioholics.com/education/ampli...response-page-2

which explains 'the real life' damping factor issue.

Quote
Several things are apparent from this table. First and foremost, any notion of severe overhang or extended "time amplitude envelopes) resulting from low damping factors simple does not exist. We see, at most, a doubling of decay time (this doubling is true no matter what criteria is selected for decay time). The figure we see here of 70 milliseconds is well over an order of magnitude lower than that suggested by one person, and this represents what I think we all agree is an absolute worst-case scenario of a damping factor of 1.

Secondly, the effects of this loss of damping on system frequency response is non-existent in most cases, and minimal in all but the worst case scenario. Using the criteria that 0.1 dB is the smallest audible peak, the data in the table suggests that any damping factor over 10 is going to result in inaudible differences between that and one equal to infinity. It's highly doubtful that a response peak of 1/3 dB is going to be identifiable reliably, thus extending the limit another factor of two lower to a damping factor of 5.

All this is well and good, but the argument suggesting that these minute changes may be audible suffers from even more fatal flaws. The differences that we see in