Well, it's the DC offset measured on the 3.5mm jack of my mobile phone.
I was asking because I want to connect it (the mobile phone) to a Grado RA-1 clone, which will be connected to my headphones.
The audio chip is a 2134 which I have used in the past .Quite a good chip but it is a dual amp chip no means of adjusting the offset that leaves a resistance network at the output to bring it down to zero. There have been some posted on Head-Fi in the past. I am sure somebody quicker than me can tell you where although I could dig one up at home.
This exactly how mine is built (as soon as I finish it).: http://www.williamneo.blogspot.fi/2008/03/grado-ra1-headphone-amplifier.html
I might add a sijosae discrete rail splitter, though...
It is not a big job to provide several resistors connected to the output and supply connections to zero down the offset.
Just remember to do it while your mobile phone is connected to make sure it is zeroed in working condition.
I also have to say UNLESS you have an expensive version of the chip[military grade] you will find the chip itself has some offset as it is a dual version.
This is covered by the chip manufactures under the heading---The----chip/ic/ etc is within the MANUFACTURERS SPECIFICATION it is a "coverall" for legal statement to cover the use of their components as used by the -GENERAL PUBLIC.
still going through the thread to get my head around the science behind this (and the dangers!) but keep going guys... really ground break stuff.
just one quick question - would you think that it would be possible to create a "magic box" that gives users control over some of the key variables (resistance?) to allow quick matchups between a variety of headphones and amps?
This thread has nothing to do with what you are asking.
Very easy. Take an amp with 0.0ohm output impedance and add resistors to suit.
If you have an amp with high output impedance things get harder, but are still very easy. Install a transformer at the output of the amp to reduce the output impedance for headphones that benefit from that.
As you said, it requires magic (actual magic, Give Gandalf or Dumbledor a call) to design a one-size-fits-all solution.
On second thought:
Do not meddle in the affairs of wizards, for they are subtle and quick to anger.