The MEASURED resistance of the 280s
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acs236

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This is strange. Motivated in part by the divergent opinions of these phones, particularly whether an amp is necessary to drive them properly, I used my multimeter to measure the resistance of the phones. I don't know if I'm doing it right, but the method I used yielded the results I expected from the Koss Portapros (about 64 ohms). I touched one lead to the tip of the connected, and the other lead near the base. The result: 89 ohms, not 64.

Interesting or am I missing something?
 
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mkmelt

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Headphones, like dynamic loudspeakers, present a complex impedance load to an amplifier. Depending on the frequency of the signal being passed, the instantaneous impedance can be either resistive or reactive.

A simple impedance graph will typically have one or more sets of peaks and dips representing maximum and minimum impedance values for the driver across the audio spectrum. The maximum impedance usually occurs at the driver's resonant frequency(s). The minimum can occur at any frequency.

The same data displayed as a complex impedance graph would look like a series of concentric circles, often with tight little loops. Where the circles cross and touch each other corresponds to the resonant frequency of the driver. The X-axis (measuring frequency from 0 - 20,000 Hz) separates the resistive component from the reactive one. Any value above the X-axis is positive and is a resistive load, any value below the X-axis is negative and is a reactive load.

A few loudspeakers, and headphones, present mostly a purely resistive load across the audio band. If you go to the website of HeadRoom Corp. at www.headphone.com, they have simple impedance curves for many popular headphones that you can display.

Generally, speaker drivers that present mostly resistive loads are easier for an amplifier to handle as there is no back-EMF signal present at the output terminals for the ampflifer circuits to deal with. This may not be a real problem with headphones, but it definitely affects the sound of certain speaker/amplifier combinations.

Your multimeter presents a DC signal to the speaker, something which would never happen when connected to an amplifier. The nominal impedance value you read is probably somewhere between the minimum and average value, but not the maximum impedance value.

Low level DC current such as your meter provides probably won't hurt your phones. Higher current levels from a larger battery like a 6 volt lantern battery or a brute like a 12 volt car battery would probably overextend the driver and quickly overheat the voice coil, ruining the phones.
 
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acs236

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So are saying is that my 280s, which are rated by Sennheiser at 64 ohms, and which I measured with my meter at 89 ohms, could in truth have an impedence rating of over 89 ohms?

Quote:

Originally posted by mkmelt

Your multimeter presents a DC signal to the speaker, something which would never happen when connected to an amplifier. The nominal impedance value you read is probably somewhere between the minimum and average value, but not the maximum impedance value.


 
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That 89-ohm reading is the DC resistance of your HD 280 Pros, and that DC resistance has little to do with the rated impedance of those 'phones. I've also measured many other headphones with your method, and I almost always came up with higher numbers than the manufacturers' ratings (in fact, one of my cheap crappy Sony headphones actually measured 86 ohms in DC resistance despite their 24-ohm rated impedance).
 
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acs236

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I guess it's just conincidence that the Portapros, and my other sennheisers came out reasonably close to their rated impedence. Is there an accurate way to measure?

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Originally posted by Eagle_Driver
That 89-ohm reading is the DC resistance of your HD 280 Pros, and that DC resistance has little to do with the rated impedance of those 'phones. I've also measured many other headphones with your method, and I almost always came up with higher numbers than the manufacturers' ratings.


 
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mkmelt

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It is possible that over some narrow frequency range your 280s will have a maximum impedance that is higher than what you measured with your multimeter. If it did, it does not mean a whole lot, except to note that your particular brand and model of headphone has an impedance curve with one or more peaks in the curve.

If you go to the HeadRoom web site at www.headphone.com, you can compare the impedance curves of a number of popular headphones. I looked for the model HD-280 there but they did not show any data for that model of Sennheiser, however they do have data for the HD-270 as well as other Sennheiser models and most of the Grado headphones.

For example, if you compare the impedance curves of the Senn HD-270, HD-580, and the Grado SR-80, the Grado is pretty constant at around 30 ohms, the HD-270 is fairly constant at around 60 ohms, with a slight rise above 10KHz. The HD-580 has an impedance of a bit more than 300 ohms except for a very steep peak centered around 100Hz that rises almost to 600 ohms, before dropping back to the nominal 300 ohm range and then starting a gradual rise above 5KHz that reaches almost 400 ohms by 20KHz. The manufacturer's specification sheet correctly shows the impedance of the HD-580s as being 300 ohms. The peak is not relevant.

By comparison, the AKG-240DF headphones have an impedance of more than 600 ohms across the entire audio spectrum, and actually rises above the nominal 600 ohm measure in the range above 5KHz. The AKG-240DF headphones are known to be be hard to drive using a portable player without a headphone amp. This is due in part to the relatively high impedance, and may be due to their relatively low sensitivity.
 
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acs236

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Headroom unfortunately doesn't have an impedence graph of the 280. I remember a thread sometime back comparing the impedence of the Sony v6 (63 ohms?) with the 280, comparing them with regard to how easy they are to drive. If the the rated impedence of the headphones is normally measured at the 1khz frequency range, there is a lot of information that we don't have . In other words, two phones rated at 64 ohms could be remarkably different over a larger spectrum, and therefore could have very different amplification requirements.
 
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acs236

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For the fun of it, I tested a few more phones including my Ety 4s -- it measured between 107 and 108 ohms, exactly what fixup.net said (100 ohm resistors, + 2 ohms (cable) + 5 ohms (transducers)).

I'm still finding it strange that every single pair of phones I test besides the 280s are within one or two ohms of their rated value.
 
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well, isnt DC 0 ohms??.... so wouldnt you be measuring the impedance of the phones at 0hz? as opposed to the whole audio spectrum?
 
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acs236

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I think the impedence rating most phones are given represents the the measurement at 1khz anyway, not the entire spectrum.
 
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Quote:

Originally posted by acs236
So are saying is that my 280s, which are rated by Sennheiser at 64 ohms, and which I measured with my meter at 89 ohms, could in truth have an impedence rating of over 89 ohms?



I maybe talking out my bottom, but I used to know all the formulas.

I think, the DC resistance of a driver is always a SMALLER
value than the impedence. Like an 8 ohm speaker would measure about 6 ohms with an ohmmeter.

I am saying, sigh, the minimum resistance/reactance of the headphone element will be the DC resistance.

The stated impedance of a headphone element should be a factor of inductive and capacitive reactance at a certain frequency,
and the minimum value is it's dc resistance. DC is really 0hz, and 10 hz is not far away
.

My guess is the spec for the headphones is not accurate, or the measuring or measuring instument is incorrect.

If the DC resistance is 89 ohms, I would put the headphones at about 120ohm impedence.

My aging brain could be wrong, but I can still analyze a tube circuit.
 
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acs236

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This is what I suspected, and while I tried measuring in the first place. Do you know of an accurate way to test the impedence of headphones? I wonder if the folks at Headroom could help....

Quote:

Originally posted by fredpb


My guess is the spec for the headphones is not accurate, or the measuring or measuring instument is incorrect.



 
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Sorry, don't know how to test the impedence of headphones. You need special equipment that I don't know how to use or have.

Only Headroom gurus can help you.
 
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