'Ohm' ratings and why they're important

[thelastpolarebear]

Its beginning to confuse me alot, I think I am putting in too much thought into it. So if I am right, ohms just refer to electrical resistance? So a 250 ohm headphone would be more difficult to drive than an 80ohm?
 
The reason Im asking is, Im looking to get a pair of of headphones for music/gaming and Im a bass head but I dont want TOO much bass. Im looking at the Beyer DT770's which are pretty much at the top of my price range, and they are being offered in 80 and 250 ohm versions. Most of the time Ill be using them with my iPod for music or theyll be hooked up to an Astro mixamp when gaming, but even then I dont know if they have sufficient power to drive these cans.

 

[TheOtus]

80 ohm version is more suitable for weaker sources. 250 ohms is quite much...

 

[joelpearce]

If you're looking for the Beyer sound but don't want too much bass, I'd recommend the DT-250 instead of the DT-770 pro.  You'll get a much better sound quality for the same price. (at least, here they are the same price).  They are also easier to drive.
 
I know that doesn't fully answer your impedance question, but in reality I think sensitivity has more impact.  I know my DT-990 pro sounds awful fed by a portable player, but the DT-150 sounds just fine, and they are both 250 ohms.

 

[d.g]

I would try not to over think about it - have a read of the following article which has a specific section on headphones which might help you!
 
http://www.soundonsound.com/sos/Jan03/articles/impedanceworkshop.asp
 

 

[Ypoknons]

Yes, ohms are measure of electrical resistance. However, that doesn't mean its necessary harder to drive. 
 
Now, I will first say that it's embarrassing how much high school physics I've forgotten. I'm trying my best.
 
First, what does it mean to 'drive' a headphone? It's the provision of power. When electricity flows around a magnetic field, amazing things happen! Now power is divided into voltage (V, 'energy' of the electrons) and current (I, # of electrons). When they flow across a resistor we turn to ohm's law, V=IR, rearranged into:
 
R = V / I 
 
Now see why higher impedance headphones generally need more voltage now? However, it's not quite simple as that. Different driver designs have different current and voltage requirements - if impedance was the only that that mattered, then the 600ohm Beyers would require beefier amps than virtually anything else, right? And 8ohm speakers would be driven by iPod headphone out. Now, naturally, that's not the case. A good amp design also needs the ability to cope with spikes in the voltage, low noise floor, avoid circuit crosstalk, spikes in the power supply, output impedance, and of course, circuit layout! 
 
Here we enter into the territory where even the experts and builders have a lot of fights, they all have different ideas on how to do things!

 Very confusing for the rest of us. Though as an inference, why do portable headphones usually have such low impedance? Well, most portable devices don't have that voltage out there. What's an iPod battery like, 3.7V? That's not a lot - mains are like, 120V or 230V.
 
Also, remember that these ohm ratings are rated at 1K Hz. See how they differ with frequency? 
 

 
Hopefully I didn't get anything wrong here. (edit) Now concerning the issue of bass, a good amp (in the sense of a Kevin Gilmore no caps, no coloration kind of thing) shouldn't add or take bass (and even colored amp won't add lot of it), it just moves the headphone drivers to their max. In that sense, if you're worried about bass, then you should look at EQing and replacing the DT770's. Headphone amps can 'tweak' your sound but not totally change it. 
 
Quote:

Its beginning to confuse me alot, I think I am putting in too much thought into it. So if I am right, ohms just refer to electrical resistance? So a 250 ohm headphone would be more difficult to drive than an 80ohm?

 

 

[SillyHoney]

Ypoknons, I hate to say this but I love you.
 
That's the easiest reading on headphones' ohm I've read thus far and my dumb brain likes it

 

[thelastpolarebear]

Thanks for the help guys, Ypoknons that was very informative post!
 
I'll be going off to try out the Beyer DT770's 80 ohm version today, I havent heard them yet but I do hope itll suit my needs. Do you guys think an ipod/iphone would have sufficient power to drive these cans?

 

[Maljunulo]

If one uses the term "resistance" one is usually referring to DC resistance. When discussing AC it is better to use the term "impedance" DC resistance is constant but impedance varies with frequency and is a more complex parameter as it depends on more than the physical properties of the conductor.
 
Don't over think, but don't oversimplify either or you'll never understand what is going on. Just read everything you can find and eventually someone will explain it in a way that makes sense to you. Find a source more reliable than a forum, for there is a lot of very bad information floating around. Stick to good sources and you will be fine.
 
Good luck!

 

[Lifted]

Quote:

Amazing post
 

Dude... where were you a month ago when I took a test on this stuff in my physics class? MAN, I so would have done better if I read this back then...

 

[Ypoknons]

Thanks. As it is my specialty is in law and accounting, and I am limited in my accuracy when it comes down to electronics... Say, do you have a good resource to read up on the basics? Wikipedia okay?
 
Quote:

If one uses the term "resistance" one is usually referring to DC resistance. When discussing AC it is better to use the term "impedance" DC resistance is constant but impedance varies with frequency and is a more complex parameter as it depends on more than the physical properties of the conductor.

 

 

[Maljunulo]

Ypoknons;
 
I just took a quick look at Wikipedia under "Impedance" and I wouldn't recommend it. If you think you are confused now, don't even think about reading Wikipedia.
 
Offhand I can't find anything to point to that gives a good, simple and basic explanation. It seems to be something which sort of builds up over the years as a little bit sinks in every so often.
 
Apiece of straight solid conductor (wire) will have a constant DC resistance (at a constant temperature) which depends on the cross section and the physical properties of the material. i.e. Big wire, less resistance than small wire, short wire, less resistance than long wire, silver wire, less resistance than copper, iron and so on. When you squirt an AC signal through that same wire, the impedance will be (pretty much) constant over the audio frequency range. If you form it into a coil (as in a headphone driver or a speaker coil) you introduce "inductive reactance" (among other things) so the coil will behave differently than the straight wire. These effects are much greater at radio frequencies, but are present at audio frequencies to an extent.  That is why headphone impedance graphs display the whole audio range and why the curve has one or more bumps.
 
When you use stranded wire, that muddies the water even further, but I am just not too sure how significant those effects are at audio frequencies.
 
I'm not going to go any farther lest I run out of understanding or ability to explain. I refuse to wander off into equations, since you are not (I hope) looking for an explanation which would satisfy a physicist. If you are, I am not the right guy.

 

[xnor]

Quote:

Its beginning to confuse me alot, I think I am putting in too much thought into it. So if I am right, ohms just refer to electrical resistance? So a 250 ohm headphone would be more difficult to drive than an 80ohm?

Quote:

Though as an inference, why do portable headphones usually have such low impedance? Well, most portable devices don't have that voltage out there. What's an iPod battery like, 3.7V? That's not a lot - mains are like, 120V or 230V.

@Ypoknons: What matters is what is coming out of the headphone jack and in case of the ipod that is about 0.8V and I've read that the EU version has a limit at 0.3V.
 
You can combine that max. output voltage with the sensitivity of the headphones to get the max. achievable volume.
With a 250 ohm DT770 you get about 100 dB. (and about 92 dB with the EU limit)
Assuming that the 80 ohm version has the same sensitivity (96 dB/mW) the max volume would be 105 dB.
Not much of a difference, eh?
 
However, what is different with higher impedance headphones is that you'll get less frequency response variations (less is good, imo) that vary from output device to output device.
Best "worst case" example would be to take the HD595 (a low impedance headphone) and switch between a portable player and stereo receiver as output device - it will sound like a different headphone, FR variations will be huge. (Simple explanation for this is that amps tend to more or less follow the impedance curve, as seen in the graph above. Or read it up on wikipedia: "damping factor".)
Additionally, some portable players struggle to provide enough current for low impedance head/earphones, but with 80 ohm this is nothing to worry about.
 
 
And remember that sensitivity depends on the design and construction of the drivers/headphones and not on the impedance.

 

[eucariote]

With ohm's law, lower impedance headphones need *more* current to maintain a constant voltage.  V=I/R
So amplifiers also help low-ohm headphones by providing plenty of current to support big movements in the drivers (bass).   For that reason I found that DT770-80 sounded bass-thin (and painfully bright) without an amplifier.  And they improved the *most* of six decent headphones I own with a high-current tube amp.

 

[xnor]

Quote:

With ohm's law, lower impedance headphones need *more* current to maintain a constant voltage.  V=I/R
So amplifiers also help low-ohm headphones by providing plenty of current to support big movements in the drivers (bass).   For that reason I found that DT770-80 sounded bass-thin (and painfully bright) without an amplifier.  And they improved the *most* of six decent headphones I own with a high-current tube amp.

 
Yes, more current which especially portable devices cannot always provide:
 

With a higher impedance headphone the result will be an almost flat line down to 30 Hz or lower.
(But I mention it again, 80 ohms aren't that low so nothing to really worry about.)
 
I wonder what device you used when you talk about your experience "without an amplifier".
 
And about the big improvements you heard: I don't know how the DT770-80's impedance curve looks like but what you've experienced might very well be the effect I've described above.

 

[eucariote]

Quote:

 
I wonder what device you used when you talk about your experience "without an amplifier".
 
And about the big improvements you heard: I don't know how the DT770-80's impedance curve looks like but what you've experienced might very well be the effect I've described above.

 
That was with a computer out, ipod nano 3g, ibasso d10 and a nuforce udac.  All produced very bass-thin sound on the DT770-80 compared to a stereo or tube amp.
 
Thanks for the graphs, they illustrate the point very well.