Most recordings are produced for loudspeakers. In stereophonic loudspeaker listening both ears hear sound from both loudspeakers. The sound is crossfeeded acoustically and things work smootly for the listener.
In headphone listening this acoustical crossfeed doesn't happen (open headphones leak some sound to the other side of head but not enough really). The result is "superstereo" effect, discomfort and listening fatique. Some people get even headache from that.
Fortunately this problem is not that difficult to solve. Some headphone amplifiers have integrated crossfeeder circuits to simulate acoustic crossfeed in loudpeaker listening.
Instead of buying a headphone amplifier I constructed myself an "passive headphone adapter". The parts for it cost about 30 euros. The adapter gets amplified signal from the B speakers terminals of my home theatre amplifier and attenuates it suitable for my headphones. It also has crossfeeder section. The circuit is based on crossfeed simulators of Siegfried Linkwitz and Chu Moy but I modified the impedance levels suitable for this kind of use where headphones are fed directly. The output impedance of my adapter is 1 ohm ensuring a very high damping factor with my Sennheiser HD 598 phones. The sound is very clean and precise.
The crossfeeder of my adapter can be switched off if needed and has 3 levels of crossfeeding; -8.5 dB, -6 dB and -1 dB. These are the relative levels of the crossfeeded signal at low frequencies ( < 800 Hz ). Above the mentioned frequency the crossfeeded signal is lowpass-filtered so that crossfeeding gets weaker when frequency rises.
Crossfeeding at level -6 dB is considered the best in general. However, recordings with exaggerated stereo separation call for stronger crossfeeder. Tangerine Dream has often this kind of hyperstereo philopsophy in their music. I must often use the strongest crossfeed level -1 dB in order to make TD's tracks sound spatially natural with headphones and to avoid listening fatique. It seems that TD's newer productions suffer from this. The older stuff is better in this sense.
I even constructed a small simple crossfeeder unit for my iPod.
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