This project is a follow-up to the article A DIY Headphone Amplifier With Natural Crossfeed. The author shows a natural crossfeed filter design with a bass emphasis to compensate for low-frequency cancellation effects.
If you have an amplifier with a mono-switch, then here is a little experiment: listen to a stereo recording (by headphone) in stereo mode, and then press the mono-button and watch the bass. If you hear the same way I do, then you will notice that the bass suddenly seems to have weakened – it has become less pronounced. The effect is similar to that what is heard with a crossfeed filter, only much stronger. Listening in mono does introduce cancellation of low frequencies, but there is also cancellation higher frequencies (which is generally is even stronger since, with normal stereo recordings, low frequencies are more in phase). With the crossfeed activated, a weak cancellation will only be present at low frequencies, but at all frequencies, the sum of the sound pressures at both eardrums always equals the sum of the pressures in stereo mode!
At first I also wondered about the apparent loss of bass, but actually, it is this unnatural, larger then life-size, uni-directional bass, that counts for most of the annoying effects of headphone listening. I know, the crossfeed sound is nothing for a bass-freak. One should not expect a punchy bass, only a relaxation of the sound. It’s like listening to loudspeakers – a balanced speaker does not jump at you at first hearing but is rather colourless/neutral/unobtrusive. The rewards come while listening for longer periods of time. A good speaker does not fatigue, and this exactly is the strength of the natural crossfeed filter.
To add bass or not to add bass….that is the question. I believe that most bass-losses are due to psychoacoustic effects, but after thinking it over more carefully, enhancing the bass response of the natural crossfeed filter could be legitimate, because headphone sound is optimized without using crossfeed. If there really is a psychoacoustic effect (a uni-directional bass is unnatural and I believe that, with headphones, this emphasizes its existence), then the effect has been (unconsciously) corrected for in the sonic design of the transducers.
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