Are you sure that you're losing quality because of the 3.5mm -> 6.3mm adaptor?
If it's the one that came with the headphones it will be made to the same high quality as the headphones - particularly if it's from a company link Sennheiser.
I don't recall ever seeing a computer sound card with anything other than 3.5mm sockets.
While there are probably sound cards with 6.3mm sockets, you might be better off finding the best quality sound card you can afford regardless of the outputs it has. The quality of the output is going to outweigh any loss due to the extra connection in your headphones.
Well... it's gold plated just like the connector on the main cord. So you do have a valid point. However the onboard one is not a solution. So if I go to an external one let's have one with a proper connector, right? – Andrei Rînea – 2010-04-22T21:33:41.113
The Xonar (PCI - so not for laptop) has a 6.3mm socket.. – Andrei Rînea – 2010-04-22T21:39:01.693
@chrisf: in analog signal paths, especially in sound applications, any extra connectors or adapters adds potential signal degradation. – quack quixote – 2010-04-23T08:24:33.540
2@quack - yes, but that degradation is often slight/not measurable, particular with decent quality equipment - which the OP's headphones are. I have some Ministry of Sound DJs headphones, with the same issue, but I can't detect any problems with a 3.5mm jack. – CJM – 2010-04-23T10:14:39.187
1If these huge multi-millimeter interconnects significantly degrade signals at audio (10's of kHz) bandwidths, it's amazing that HDMI, GbE, and eSATA work as well as they do. Unless you're buying absolute rubbish, I think any degradation occurs between the ears, not on the way to them. – coneslayer – 2010-04-23T14:07:39.370