Positive center is way more popular: gearspace.com/board/electronic-music-instruments-and-electronic-music-production/1222518-center-negative-vs-center-positive-power-supply.html
Articles by others on the same topic
A direct current is an electric current that flows in one direction; its magnitude may vary with time. A constant or steady electric current additionally has a value independent of time. Thus a direct current need not be constant. The steady direct current model of a voltage divider assumes a constant input electric potential difference and fixed electrical resistances.