K701 impedance/amping question
Nov 2, 2008 at 2:40 AM Thread Starter

#### tintin47

Being as they are only 62 ohms, why are the k701s so amp-dependent? I heard a pair unamped and they sounded terrible. Amped, I absolutely love them. Usually, though, amping is needed to drive phones because they have a large impedance, such as the dt880s or hd6X0. This being the case, do the AKGs have better synergy with different types of amps than higher impedance phones?

Nov 2, 2008 at 2:51 AM

#### Uncle Erik

##### Uncle Exotic
Impedance is only part of the puzzle of amplification.

There are four factors at work:

2. Output impedance of the amp
3. The amp's output power
4. The sensitivity of the headphones

The closer the headphones' impedance matches the output impedance of the amp, the better the power transfer from the amp to the headphones. There is a formula that will show you the power loss from a impedance mismatch.

The second part is just how much power transfers from the amp to the headphones. If the amp puts out 250mW, consider how much of that actually goes to the headphones based on the impedance match. If there's a 50mW loss from an impedance mismatch (for example) then a maximum of 200mW is going to the headphones.

Next, you have to consider the power reaching the headphones in light of the headphones' sensitivity. You'll see this listed in terms of Decibels (dB) on the package. Given a certain amount of power, the headphones will get so loud. Take the amount of power coming from the amp (in light of loss from impedance mismatch) and you can see how loud the headphones will get given the amount of power that gets through.

There are a few complicating factors. First, the power output of the amp is almost never the listed amount. A 500mW amp only puts out 500mW cranked all the way up with the source cranked all the way up. If you have the volume at 9 o'clock and the source is playing a quiet passage, you might be putting only 80mW (or whatever) through the amp into your headphones. Second, amps and headphones have impedance curves - impedance doesn't stay the same all the time. If impedance changes, it affects how much power transfers.

This is where it starts getting complicated. But I hope you see how the relationships interplay here.

Nov 2, 2008 at 2:55 AM

#### ph0rk

Voltage swing.

Nov 2, 2008 at 3:14 AM

#### tintin47

uncle Erik; Thanks so much for the quick and detailed reply. I am a moderately advanced college physics student, so I understand the math and physics involved. I just had no idea about the relationships between the phones and amp that I needed to worry about. This helped a lot. I am off to my text books/roomates/internet to find the equation for impedance mismatch equation. If you know it, could you post it or PM it to me?

Nov 2, 2008 at 7:25 AM

#### QQQ

Those are of very low sensivity, around 93db\mw.

Nov 2, 2008 at 12:21 PM

#### JMCIII

As Uncle Erik pointed out in his very thorough answer, this is a case where the numbers don't tell the whole story. That's why listening is still the best way to make up your mind. You've heard it yourself - with a good amp the K701s sing.

Nov 2, 2008 at 1:17 PM

#### QQQ

Numbers always tell the story, when they honest, of course.

Nov 2, 2008 at 2:04 PM

#### JMCIII

Quote:

 Originally Posted by QQQ /img/forum/go_quote.gif Numbers always tell the story, when they honest, of course.

Numbers, no matter how honest never, tell the whole story. Never have, never will. That's a fact of life. We still do not (and never will) have the capability to measure all the fawcets that make a piece of equipment sound good. That's why we have to actually listen.

Nov 2, 2008 at 2:29 PM

#### BigEat

The formula can be found on Wiki under the title "impedance matching". Does a good, although technical explanation.

Go Harvard. LOL

Nov 2, 2008 at 5:03 PM