Although  more complicated and expensive, another interesting amplifier project is the open source O2 amp:<p><a href="http:&#x2F;&#x2F;nwavguy.blogspot.com&#x2F;2011&#x2F;07&#x2F;o2-headphone-amp.html" rel="nofollow">http:&#x2F;&#x2F;nwavguy.blogspot.com&#x2F;2011&#x2F;07&#x2F;o2-headphone-amp.html</a>