i did not mean the motherboard in itself but the onboard audio of the motherboard
i wanted to buy headphones and i read that one should buy headphones(load impedance) with impedance 8 times the output impedance of the source
i wanted to buy a sound card but did not cause again i read that sound cards have higher output impedance than onboard audio in motherboards and hence it is better to stick with the audio on your mobo
hence the question