This digital to analog converter circuit was found in the March 1980 Radio Electronics magazine. The power supply should be symmetrical, and the digital controls can be TTL.
The oscillator shown in the figure generates a rectangular signal with a 50% active cycle (square)...
This power supply can be implemented with a TIP32 providing an adjustable output at the 2k2...
This switched circuit was found in an old documentation and no information about the transformer...