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why low impedance phones are hard to drive?

post #1 of 47
Thread Starter 
Hi all. like the title says.. i was always wondering about this. because is seems logic that low impedance cans should be relatively easy to drive.
and from what i heard it exactly the oppiste..why?
well ,my guess is that maybe because low impedance cans need to be handled more gently. because of their sensitivity (iems as example)...?
post #2 of 47
huh? i always thought they are pretty easy to drive, to the point that using an amp is not really needed.
post #3 of 47
Low impedance/high sensitivity headphones, like IEMs, are easy to drive. It's low impedance/low sensitivity headphones that are hard to drive.
post #4 of 47
Thread Starter 
Quote:
Originally Posted by scompton View Post
Low impedance/high sensitivity headphones, like IEMs, are easy to drive. It's low impedance/low sensitivity headphones that are hard to drive.
ahh! ok. so it's the low sensitivity then! sorry for the confusion and thanks for the replies.

and by the way..if we are talking about this..what factor more counts when driving headohones.. high impedance or low sensitivity?
post #5 of 47
I've had more problems driving low sensitivity headphones than high impedance. I have a few 600 ohm headphones that aren't that hard to drive. Some of my orthos are 75-150 ohms but with low sensitivity and they distort horribly when under driven. Neither do well from battery driven amps. They're driven fine while the battery is fully charged, but they drain the battery in a hurry.
post #6 of 47
Thread Starter 
thanks! a few days ago i posted in the headphones thread about why the denon ah-d5000(and the AHD series in general) needs amplification,and i didnt get any answer.
its a mystery to me..because the specifications say: 25 ohm impedance and 106 db sensitivity. well they dont sound good right from a portable, i mean..it's ok but very lifeless and grey. when amplified they alive and kicking!

but my grado rs-1 are: 32 ohm (higher impedance) and 98 db (lower sensitivity) and they are just fine out of my d2. ofcourse better when paird with an amp ...but just fine unamped.

so..can you explain this to me..? maybe it's because of the sound signature diferences between the two cans. maybe the grados piercing highs make them sound more light, and as a result more easy to drive..? or maybe it's the d2..?
although i heard in other places (like headroom)that the denons do need amplification.

thx again for the replie.
post #7 of 47
It has more to do with the output impedance of the amp you're using. For the record, iPods, soundcards, etc. have internal amplifiers - those have an output impedance, as well.

There are two things to consider with impedance. First, how closely output impedance matches the impedance of the transducer (i.e. headphones or speakers) the more efficient the power transfer is. If there is an impedance mismatch, power transferred from the amp to the transducer will be affected. There are formulas where you can determine this.

The second part is the damping factor. The damping factor is the ratio of the output impedance of the amp to the impedance of the transducer. What you're looking for is a low output impedance of the amp compared to a relatively high impedance of the transducer. The higher the damping factor, the greater control the amp has over the transducer. This is like improving the brakes and handling of your car. It's about control.

This, incidentally, is why tube amps don't always do well with low impedance cans. Tubes tend to have a high output impedance, especially OTL (output transformerless) amps. You can use an output transformer to lower the output impedance of an amp, but transformers introduce other issues and add a few hundred (if not more) to the cost of the amp.

Solid state, on the other hand, tends to have a very low output impedance. This is why you almost never see output transformers on a solid state amp. McIntosh made a few solid state amps with output transformers, though.

Once you know how much power makes it through the impedance match, you can then use that with the sensitivity of the transducer (measured in Decibels, or dB) to show how loud it will get given the power that comes through.

There is a lot more to this but I hope that makes some sense.
post #8 of 47
Low impedance headphones/speakers make the amp work harder because low impedance means less resistance to electrical current so they draw more power from the amp, which is not a good thing. That's one of the reasons low impedance headphones sound louder than high impedance headphones at the same volume setting. The other is of course sensitivity of the headphone.
post #9 of 47
Thread Starter 
thanks everybody for your kind replies! its good to be here.
post #10 of 47
Lower impedance means higher current means higher distortion (from the amp). It may not be of importance, though, as long as it's below 0.1% and the like.

One decisive downside of low-impedance headphones is the operation on portable players -- a target group which they're actually designed for, since batteries don't provide high voltages (which would be ideal for higher impedances). To prevent direct current, their headphone outputs incorporate serial capacitors, acting as high-pass filters: the lower the load impedance, the earlier the bass drop-off. For minimizing cost and size, these capacitors are notoriously undersized, so 99% of all PDPs suffer from an audible bass drop-off even with 32-ohm headphones (let alone 16 ohm and the like).
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post #11 of 47
It's a matter of power. Certain phones need different amounts of power to be driven correctly, and then also certain amounts of **voltage swing (potential difference) to be driven to a certain volume (sensitivity)...
P=IV
where P = Power, I = Current and V = Voltage
we see that P/V = I, meaning that power and current are directly related whereas voltage and current are inversely related (i.e. higher power = higher current, lower voltage = higher current).
and then we have ohms law which is (swapping letters around to match the above)
V = IR
where R= Resistance.
Substitution...
P = I(IR) = I^2R thus
P/I^2 = R
We see that Power is directly related to the resistance, and current is also directly related to power, but they (power and resistance) are inversely related. I.e. Low Resistance = High Current required to get desired power.
A lower impedence version of the same phone will require a larger amount of current to be powered and we also see that (Via V=IR) that a larger current is required to achieve the desired voltage swing.
But when you compare different models theres things like sensitivity and efficiency.
That's the basics of it though.. You'll also have a higher noise floor on low impedence versions of the same phones because they are running "hot" (i.e. more current). Volume knobs work differently though (they are a variable resistor) which will squeeze more power from the same current thus driving the phones louder
Disclaimer: its 5:34 am and my maths might be wrong because of this (and so might the concept). Feel free to correct and i will edit/remove
I've tried to clear up my english as much as possible
post #12 of 47
On the subject of impedance 3 quick questions ....

Am I right or wrong in thinking that most modern headphones are designed for
a 0 ohm output impedance from an amp ?...

...And that some headphones such as a few beyerdynamic models were actually designed to the 120 ohm standard .. ?

-- And that if you add an impedance adapter between an amp and a headphone it will ADD to the output resistance ie .. if the amp already has a 100 ohm output resistance... to make 120 ohm I only need to add a 20 ohm adapter ?


Thanks.
post #13 of 47
At the same voltage, a low impedance phone will draw more current than a high impedance phone. On units such as Grados and IEMs, it's not such a big problem because those headphones are very sensitive. This means that they can achieve loud volumes at a relatively low voltage. Lower voltage = lower current.

However, low impedance doesn't automatically mean that a headphone will be sensitive. The 60 ohm AKG K701 has a pretty low impedance, but it's not a very sensitive headphone. With the K701, I find that I have the volume knob at about the same position I use for the HD650. With the volume knob in the same position, the 60 ohm K701 demands 6x the current used by the 300 ohm HD650.
post #14 of 47
Quote:
Originally Posted by plonter View Post
thanks! a few days ago i posted in the headphones thread about why the denon ah-d5000(and the AHD series in general) needs amplification,and i didnt get any answer.
its a mystery to me..because the specifications say: 25 ohm impedance and 106 db sensitivity. well they dont sound good right from a portable, i mean..it's ok but very lifeless and grey. when amplified they alive and kicking!
I am not sure if this was answered directly, but I think the issue is more about the stability of the load than anything else. A lot of your run of the mill amplifiers tend to claim optimum performance from the 30-600 ohm impedance range. I can't help but wonder if the low impedance of the Denon tests the stability of the amp in such a way that only properly powered designs can drive them.
post #15 of 47
Quote:
Originally Posted by deaconblues View Post
...

However, low impedance doesn't automatically mean that a headphone will be sensitive. The 60 ohm AKG K701 has a pretty low impedance, but it's not a very sensitive headphone. With the K701, I find that I have the volume knob at about the same position I use for the HD650. With the volume knob in the same position, the 60 ohm K701 demands 6x the current used by the 300 ohm HD650.
According to their respective specifications the K701 is actually more sensitive (105dB/mW) than the HD650 (103dB/mW). Please correct me, if i'm wrong here.
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