[hpk]

I am in the process of shopping for a set of headphones. I will be driving the units with a Rotel RC-995 preamp that has a headphone jack with 150 ohm impedance. My understanding of impedance matching is that the driver and load must be as closely matched as possible to acheive maximum power transfer.

I was in a high end shop lately that told me that I should be considereing as a rule of thumb headphones that are up to half the impedance of the driver, in my case 75 ohms maximum.

Anyone have any comments or suggestions on a specific headphone with impedance charateristics that would be suitable in my case? I am not considering a separate headphone amp at this point.

 

[Budgie]

It really becomes an issue of theory. Yes, maximum power transfer will be with the impedances matching each other. However, best control of the headphone drivers would happen with the amp impedance being lower then the phones (ie-dampening factor). But there are also people using headphones that are lower impedance then the output of the amp they use and reporting good results from the combination. It seems like impedance matching is not a big issue. The general opinion around here seems to be that some amp/phone combinations seem to sound better then other combinations. The sound of the phones seem to dominate the combination, in my exsperience. It becomes akin to picking speakers, in alot of ways.

(I have no idea were that guy came up with the 1/2 the source impedance idea. That makes little sense to me.)

 

[AndreYew]

hpk,

Impedance matching is not important for headphones, because headphones are driven by voltage. As a consequence of that, you want the output impedance of the amp to be as low as possible. Generally, 1/10th of the impedance of the phones, and 1/100th to completely eliminate audible voltage divider effects. This is for impedance from 20 Hz to 20kHz.

Edit: I noticed I didn't answer all your questions. Since you can't select which headphone amp you use, you want the impedance of your phones to be as flat and high as possible. Generally, most companies don't give out this information, so you'll probably have to audition by ear to hear which phones pleases you most.

--Andre

 

[Tyll Hertsens]

Impedance matching for maximum power transfer is important . . . in long distance power lines! But not really important with headphones. Typically you want the source to have a maximum of 1/10th the impedance of the driven device. With speakers you would want a maximum output impedance of 0.8ohms for a nominal 8 ohm speaker. Less is more in this case, and the lower the output impedance the better. This is because the output of the amp looks like a short circuit to energy reflecting back from the headphones. Reflecting is really the wrong word here, back EMF (electro-motive force) would be a more accurate term. At any rate, this Juice comming back from the headphone, if not shorted out by the output impedance of the amp would develope a voltage at the output of the amp that would become part of the feedback signal, which will cause distortions in hte amp. HeadRoom amps have less than 0.1 ohm output impedance.

And that would be the end of the story but for the fact that headphone manufacturers build into the headphone acoustic damping. Part of the acoustic damping formula is the electrical network around it. And part of the electrical network is the output impedance of the amp. So when headphone manufacturers develop the acoustic damping for headphones, they usually assume a crappy headphone amp; with a crappy, relatively high, output impedance. Then they design the damping. THe European "standard" is 120 ohms, and this is why Jan puts a 120 Ohm output impedance plug on his amps. Unfortunately, it's not really a "standard"; even in the standard it's not really a standard but more an observation. Headphone manufacturers know that they are going to be plugged into things with widely varying output impedance; no manufacturer (that I'm aware of) has ever setr the output impedance at 120 Ohms, in fact, I'm sure they usually do the ncommon sense thing and make the output impedance low as they can and still meet the power and parts price targets they have for the headphone jack----which is usually next to nothing so they just slap in an op amp and call it good. This state of affair mainly makes headphone manufacturers to have to design for as wide a range of output impedance as they can. Lovely, let's optimize by making the headphones insensitive to the quality of electronics.

To sum up, two things:

1) Output impedance---lower is still better on average, but some headphones may benefit by tweeking output impedance.

2) Headphone could sound MUCH better if headphone makers could rely on good amps. As it is now they mainly assume the amp is crap and if they have to do damping they have to do it acousticly rather than the RIGHT way wich would be to rely on a good amp for it's low output impedance and damping factor.

 

[Jan Meier]

"
1) Output impedance---lower is still better on average, but some headphones may benefit by tweeking output impedance.

2) Headphone could sound MUCH better if headphone makers could rely on good amps.
"

Tyll just said it all

J.

 

[hpk]

Thanks you so much, in summary I should be looking for a high impedance headphone much greater than 150 ohms.

However, in reality it's best to experiment reardless to see what senergies actually exist between scource and load.

So given the fact I do not have local access to product. WHich do i place my bets on the Sen's hd-590 (120 ohm) or the Beyer's 931 (250 ohms) given my scource Rotel RC-995 amp is operating at 150 ohms?

 

[AndreYew]

hpk,

The Beyers and Senns sound different for so many other reasons other than their impedance. I think you'll really have to listen to them to decide.

--Andre

 

[SuperCompromiseMan]

Hello everyone

I need some advice: I'm in the process of buying closed circumaural cans for office work (loud place, coworker sitting next to me, music helps me concentrate). I'll be wearing them about 8hrs/day with breaks.

They will be connected to a Creative Xmod:

Creative Xmod External Sound Amplification System - Gadget specifications and review - Softpedia

The phones that come with it in the box:

Quote:

So far I have these two candidates:
Beyerdynamic DT 660 (32 ohm)
Beyerdynamic DT 770 (80 ohm)

I have chosen them based on reviews at headphonereviews.org saying they are very good quality for the money, also very comfortable. I was trying to find them in a shop to try them out, but I could not find them anywhere... I'll buy them in an online shop.

My questions:

Does anyone know the quality/specs of the amp in the Xmod?

Should I go safe and get the 660s because they have the same impedance as the bundled earphones (32 ohm)?

Or should I take a risk and try the supposedly better 770s (80 ohms)?

Or should I go crazy and get some 250 ohm cans?

Or do you know of any cans that could fit my needs better than Beyerdynamic cans?

Thanks a lot for your answer(s), cheers!


"Come Middle Course Boy! To the BargainMobile!"

 

[Pirmasens]

Hi, I been in comm business decades. Impedeance is a complex subject. It has many faces, radio, audio, input, output, you get the jist. You get best power transfer when they are equal. However, the two devices should be equal quality for best reproduction of sound. Many headphone transducers, thats a speaker to everyone, have paper cones, good ones have mylar cones. Mylar is extremely light. So less drive power produces same sound level. This also allows more faithful reproduction of sound too hi's and lo's.

Fot the tekkie, if you have the gear, you can measure your own impedeance. You need an accurate AC millivoltmeter (any model Fluke), a variable resistor, and something to create a 1000 cycle steady tone. Connect the resistor wiper to the audio jack high, one of the other sides to ground. Set the generator to say.... a 50 millivolt output, connect sound across resistor. Adjust the resistor until the AC voltage drop is half the generators voltage. You will have to measure each a few times to get it set correctly. Disconnect the generator and measure the resistor setting, in ohms. This will be your impedeance. Its 99% accurate without having Sony Engineering at your disposal. Hope this is a bit of help. Out

 

[cyberkeeper1]

i have to disagree based on fact. from an electrical standpoint, the impedance (regardless of what device) is the measure of electrical resistance given to a specific AC voltage. if the impedance on the headphones is lower, the amp needs to drive a higher AC voltage than it was designed to to provide the same output. the lower the Ohms, the closer to a total short. 1 ohm of resistance provides almost no output and requires a HUGE amount of power to be driven. so, matching is definitely recommended by me, an audio technician who runs sound at my church. impedance is key, or you could face distortion clipping, and much more problems. going higher is also not recommended. i would estimate the voltage coming out of most devices is between 5 and 12 volts AC, depending on the source. ( i have not yet taken a voltage meter to the source to check... lol)
 
Case and point, as far as ac voltage with or without a load, the higher the load, the lower the voltage. the lower, and the opposite happens.