[Tenson]

Hi Guys,

My Mum just got herself a pair of AKG K701. She also tried a pair of Audio Technica AD700, but although they sounded nice they didn't fit her (or my) head!

So anyway, the K701 seem like really good headphones but I have heard they need a beefy amp to drive them. Now, rather than buy a headphone amp it makes sense to me to use a resistor voltage divider to power the headphones form her Cyrus 3 power amp. Headphone Adaptor for Power Amplifiers

Now the questions are...

Is the sound quality going to be as good or better than a dedicated headphone amp? I'm just thinking there must be a reason why more people don't do this. I had some AKG K1000 a while ago and they sounded brilliant powered from my power amp (though without need for voltage divider) so I wonder why so many people bother with headphone amps? Do you think they sound better?

The second question is that it says on the Rod Elliot page most headphone amps have 120 Ohm output impedance. I'm just curious, but why? A normal power amp driving speakers aims to have as low an output impedance as possible (providing good damping factor), as do line level sources. Low impedance source, high impedance load. Why do headphone amps not work this way?

Thanks for your helps!

Simon

 

[brainsalad]

One reason for NOT using a power amp is that headphones work in the milliwatt range. You are throwing away a 1000 times the power needed. All the power in power amps require completely different component types than a several hundred milliwatt amp design. The circuit boards can be smaller, better built, closer tolerances at a fraction of the cost not to mention some headphone amps run in full time Class A biasing.

 

[Gradofan2]

Quote:

Originally Posted by Tenson /img/forum/go_quote.gif Now the questions are... Is the sound quality going to be as good or better than a dedicated headphone amp? I'm just thinking there must be a reason why more people don't do this. I had some AKG K1000 a while ago and they sounded brilliant powered from my power amp (though without need for voltage divider) so I wonder why so many people bother with headphone amps? Do you think they sound better?

As noted elsewhere... AKG recommends a speaker power amp of at least 30 watts per channel to drive the K1000s properly - much more than most headphones require.

Others have amped their Senns with speaker amps successfully. I would guess you could AKGs also - except allowing for the difference in their impedence and sensitivity. I would think that most power amps, with a lot more power, designed to power speakers, might have a bit more noise, which sensitive headphones might pick up. Whereas headphone amps are designed with that potential in mind - and may be quieter.

 

[Tenson]

Hi,

Thanks for the reply. That seems intuitively correct, but I think things may not be so clear. When you look at the specs for headphone amps they are no better than power amps. Most of the headphone amps I have seen state THD+N (so includes noise getting through from power supply) of 0.01% or so. Many good solid state power amps can match that or exceed it. The latest designs from Doug Self routinely achieve THD+N as low as 0.001%.

On top of that, the power amp has the advantage of almost infinite headroom for short duration peaks (which can use a lot of power) and can be as much as +20dB in classical music, or more than 65x the average power level requirement to put it another way.

Now I'm not trying to argue one way or the other really, despite what I've just written. I'm really just giving food for thought and playing devils advocate! The more I think about it though, the more using a power-amp with voltage divider seems like the ideal solution, and I just wondered if anyone could point out some reason I missed why a dedicated amp would sound better?

Anyway what I really wanna know is, why would you not want a headphone amp to have as low an output impedance as possible, and apart from Rod Elliot's page, are there other tutorials on how to get the right resistor values for making a power amp drive headphones?

Happy listening!

 

[JadeEast]

I'f you look down the page linked here you can find a bit more info.

HeadWize - Headphone FAQs

I've gone 10ohms across outputs and 50-330 in series with the driver, this way the amp sees nearer to its expected 8ohm load.