This circuit has been published in the magazine "Electronics Australia", Vol. 46, April, 1984, p. 54, and it has been reprinted in the USSR magazine "Radio" for June of 1985.
This simple device (see figure 1) converts mono signal into stereo signal. The base of this device is two double T-shaped bridge circuits connected in parallel to each other, that is the two-poles notch filter. This notch filter contributes two poles (200 Hz and 2 kHz) to the frequency response of the right channel. The difference signal between the input signal and the signal from the right channel output goes to the left channel, so the total gain in both channels is constant.
Figure 1. Circuit diagram of the device that creates stereo signals from mono signals. Op-amp DA1 - TL084, almost any general purpose op-amp can be used in this circuit.
The frequency response of the circuit is shown in figure 2.
Figure 2. The frequency response of the mono to pseudo-stereo converter circuit.
Figure 3. LTSpice model of the mono to stereo converter circuit. Download it: how_to_create_stereo_from_mono_signal.asc
Figure 4. The frequency response of the mono to pseudo-stereo converter circuit. As we can see, the first pole is about 300 Hz, the second is just above 5 KHz. It is different from the parameters mentioned in the magazine "Radio".
This circuit was simulated in the LTSpice (see Fig. 3.). The result of simulation (Fig. 4) is slightly different from the characteristic of the original circuit. In simulation, the notch filter has two poles at about 300 Hz and 5 kHz.
How this circuit works? The notch filter changes the phase of the input signal for almost 180° between its poles. Along with the phase changes, the amplitude of the signal in the right channel changes in opposition to the amplitude of the left channel. When the signal is louder in the one channel, then it is weaker in the other channel. As a result, we can hear the pseudo-stereo sound at the output.