3D enhancement is needed to create a fully 3-dimensional sound for most stereo multimedia products. Usually, simple phase-delay circuits is used to produce a widening effect on the perceived sound field. However, there is transaural acoustic crosstalk effect. The following figure shows transaural acoustic crosstalk effect and schematic diagram of simple phase-delay circuits :
Phase Delay Network for 3D Audio Enhancement circuit schematic
Transaural acoustic crosstalk effect is a condition where some sound from the right-hand stereo speaker reaching the left ear, and vice-versa.

This is an active circuit that uses first-order section to present a phase-shift filter with ft of 1kHz for the quadrature signal  and ft = 10kHz for the linear signal. It will produce 90° phase shift between the quadrature and linear signals over the audio bandwidth of 1kHz to 10kHz. [Source: maxim-ic.com]