why does headphone amplification work?

[Nate805]

So probably a dumb question, but I see lots of headphone amplifiers being advertised to greatly improve the sound quality of your iPod or other similar device. I don't understand, because wouldn't the limitations of the iPod's DAC and internal amplifier be fed right into a "better" headphone amplifier, and be, well, amplified?

[jymz4941]

Originally Posted by Nate805

So probably a dumb question, but I see lots of headphone amplifiers being advertised to greatly improve the sound quality of your iPod or other similar device. I don't understand, because wouldn't the limitations of the iPod's DAC and internal amplifier be fed right into a "better" headphone amplifier, and be, well, amplified?

Yes

[Gordy]

Yes, but... the ipod internal amplifier will be relieved of driving 16 - 30 Ohm headphones and have a much easier time driving the 10k - 47k input of an amp.

Also, I don't know for sure because I don't have one, but there may be a line-level output on the ipod connector that can be used to drive the external amp. If so it will welcome the easy 10k - 47k amp input loading.

[kevinkr]

Originally Posted by Gordy

Yes, but... the ipod internal amplifier will be relieved of driving 16 - 30 Ohm headphones and have a much easier time driving the 10k - 47k input of an amp.

Also, I don't know for sure because I don't have one, but there may be a line-level output on the ipod connector that can be used to drive the external amp. If so it will welcome the easy 10k - 47k amp input loading.

All true from my direct iPod experience.. The extra headroom a lot of these designs have means lower levels of distortion at the headphones. External headphone amps also can drive headphone impedances that an iPod will not - at least to any reasonable volume. Impedances typically below 16 ohms, and 600 or above, depending on the sensitivity of the headphones.

[qusp]

yes, iphone/ipod (all idevices) have a lineout at the dock connector and despite popular opinion, its actually harder to do a good job of driving low impedance loads. they do indeed make a nice difference to a portable headphone experience

[Mooly]

Originally Posted by Nate805

So probably a dumb question, but I see lots of headphone amplifiers being advertised to greatly improve the sound quality of your iPod or other similar device. I don't understand, because wouldn't the limitations of the iPod's DAC and internal amplifier be fed right into a "better" headphone amplifier, and be, well, amplified?

Correct

So what you need is a headphone amp that adds it's own bit of magic to the mix, not so much "a piece of wire with gain" but something that works sympathetically with the h/phones.

http://www.diyaudio.com/forums/headp...phone-amp.html

It's just like with normal amplifiers, there are a select few that can pull the trick off.

[qusp]

but no it isnt correct, dac yes, internal headamp, no. there is analogue line output at the dock. and now there is also emerging portable transports that are able to pull digital out from ipod/iphone in a portable mode. with my camera connection kit I can pull USB audio full speed from ipad. I have just finished building an adum based USB isolator and integrated it with a deconstructed apple camera connection kit. I have started working on a regulated battery supply for the downstream side of the isolator, so I can drive any standard USB audio device, regardless of whether it has its own power or normally is bus powered and it is powered by this clean regulated power instead, I will be adding a usb to spdif/i2s output and either keeping this little box externally, or integrating it with the portable buffalo II headphone amp i'm working on

some of us are pretty serious about portable/transportable audio. I use balanced JH13

[Rodeodave]

I too would have said that it's basically a matter of impedances. A headphone with it's resistance/inductance/capacitance is a harder load to drive than the input of a designated amplifier. Plus the headphone's impedance will vary with frequency, a well designed amp-input may not.

[dangus]

The IEC spec says headphones should be specified with a 120 ohm source impedance. However, somebody did some testing and found that with real world 'phones, the lower the source impedance, the better.

This has me thinking that an LM386 may be the right choice for driving 24 ohm headphones.
HeadBanger Headphone Amp

[dangus]

Or maybe try what Rane did for the MP2016 mixer headphone output. (Schematics can be found at Rane's website.) One opamp section for gain plus another configured as a buffer, paralleled with 100 ohm series resistors. But, the resistors are inside the feedback loop, so output Z should be very small.

If one extra buffer is good, maybe a quad opamp per channel, using 3 buffers instead of 1 would be better, particularly with very low Z phones. I'd just as soon use a bipolar supply and avoid the need for big coupling caps (LM386).