Output From Differential Phase Shifter

Using Multisim, I try to simulate the Differential Audio Phase Shifter circuit from:

http://electropub.files.wordpress.c...-4-coils-wrapped-around-a-ferrite-toroid1.pdf

I simulate just a single channel with 10k resistors.

When running the simulation, Multisim complains about a convergence issue, and wants to automatically solve it. It is able to solve that issue. I have no idea what this convergence issue is about.

The problem is that the output sine waveform is distorted as shown on one of the attached screenshots.

If I change all resistors to 1uohm, I get a nice sine waveform at the output (unshifted).

How can I change this circuit to get a 90 degree phase shift, and proper sine waveforms (with two channels)?

 

Changing the signal source from AC_POWER to a function generator, and replacing the operational amplifiers from 741 to LM324N fixed both issues.