First off, what is an "acoustic simulator"?
It can be anything to simulate the effect of being in a room, creating an effect of open space, or even give the illusion of hearing sounds from different directions. This is usually achieved through software. Results vary, as there is coloration to the sound and the simulations can be very invasive to listening.
Unlike software implementations, what I will show here is a simple hardware solution to get rid of the "in-head" effect that headphones have. It is not designed to give the illusion that you're in an auditorium with reverb. Rather, it is designed to simulate the feeling of hearing audio in front of you. Its purpose is to improve the soundstage.
The main goal is to reduce fatigue and make headphone listening natural and pleasant.
In this instructable, I will show my build of a Linkwitz crossfeed. This particular design was modified by Chu Moy, the person responsible for the famous "cmoy" headphone amp design. I take no credit for this design, only my simple modifications.
Step 1: What is a crossfeed? (Theory)
In reality, you're hearing with both. Your brain recieves signals from both ears and interprets the direction from which the sound is coming from. You may be hearing to the right of you, but your brain also interprets if the sound is close, far, in front of, or behind you. When you listen to audio with headphones, this directional information is lost.
To remedy this, we welcome the crossfeed. A crossfeed circuit bleeds the left and right channels just a bit to give off that lost sense of direction. While it does not give the illusion of surround sound, it does give you the feeling of listening to audio from speakers in front of you rather than through headphones on your ears.
Music from live performances and orchestras sound particularly good with a crossfeed. Mono recordings and music that generally sits "in the middle" will not benefit from a crossfeed much, or at all.