CineMag manufactures many transformers with tertiary feedback windings.  Negative feedback is used in solid state and vacuum tube amplifiers to reduce distortion and flatten out frequency response.  A good feedback winding (also called a tertiary winding) can even sense the loading conditions as it may affect the linearity of the system.  

Here, we will see what can happen when using a simple multi-filar transformer which was not even designed for this purpose.  Nothing has been optimized in the circuit design.  

This circuit has an offset current which may add a minor amount of  THD (total harmonic distortion).  The HA-5002 was originally used to drive the transformer.  As of this writing, the HA-5002 may no longer be made.  There are several newer integrated circuit chips that will work as well.  In fact, it does not have to be an integrated circuit and could be discrete transistors.  This insensitivity to how it is done is because everything sits inside a global feedback network.

The CMOQ-1L transformer laminations are 50% grain oriented steel interleaved with the remainder being nickel.  The purpose of this exercise is to demonstrate the effect of feedback when using a tertiary winding.  Nothing has been optimized for better performance.