I'm a hobbyist and I have 2 doubts regarding a power supply described into [this schematic](http://www.redcircuits.com/Page181.htm):

1) How do I differentiate a center-tapped power transformer from a non center-tapped one? My primary language is portuguese and I cant manage to translate it... Can I assume that "[24V 2A](http://www.soldafria.com.br/transformador-24v-2a-entrada-110220vac-p-4139.html)" is a center-tapped transformer and "[24+24 2A](http://www.soldafria.com.br/transformador-2424-2a-entrada-110220vac-p-4136.html)" it's not?

I need a +-20V power supply. Into the diagram, it recommends a 30V center-tapped or a 15+15V secondary in order to build one. But:

2) If I split a 30V center-tapped transformer, it will give me +-15V, not 20... Neither if I use a 15+15V, it will not amplify to 20V...