Binaural Synthesizer/Mono to Stereo Converter for Small Speaker/Phones
When we connect a mono audio output into a stereo headphone (simply by paralleling it), we would hear the sound with all of its components comes from one point, and the worst, that point is located inside your head. This perception is created by the fact that the signals from the right and left earphones have exactly the same amplitude and phase. In real condition, such as in a live concert, our left and right ears receive different signal because the sound sources are distributed in different spacial location. By using simple high-pass and low-pass filter, we can produce an artificial stereo images from a mono audio source because the right and left filter produce different amplitude and phase response for different frequency components.
This simple circuit can be used to drive two high-impedance loudspeakers or stereo phones, taking monophonic input (one channel), to produce a stereo signal which spread along the midway between loudspeakers when the input signal contain many frequency components. This circuit has two channels, left and right, right channel has high-pass active filter and left channel has low-pass active filter with cut off 750Hz. This circuit is designed for low to high-impedance speaker or phones. In the schematic diagram below, you can see the output is passed through a series resistor. Without this resistor, uncontrolled oscillation might occur on the output channel when low impedance 8-ohm loudspeakers or phones are connected to the outputs directly. Op amps will drive 8-ohm loads with low volume levels, but give ample volume with 2000-ohm phones, so the higher the impedance the higher the power.