If you are looking for that ultimate headphone replay amplifier, you couldn't do any better than with this discrete circuit. Richard Marsh develops his circuits from the well-known diamond topology but with a twist. His headphone amp is a very clean and linear discrete design that's equally at home driving low-impedance or high impedance phones. But Richard doesn't stop there; he realizes that many headphones do not produce flat frequency response at your ears. Therefore, he borrows from room equalization' technology and develops a simple way to use that to equalize the headphone response at your ears.
A Headphone Buffer/Amp and Auto-EQ for Headphones