I suggest that the expression "source singer" can only properly be used in relation to "folk song" - and at least in this context the latter has a specific meaning - the 1954 World Council of Folk Music meaning.

In that case, the "source singer" will be the singer who sang the folk song (as a folk song) to the collector who collected the song from the oral tradition. There may of course be more than one such, and the versions of the song may differ but that is the nature of the beast.

There are a number of singers who were the source singers of a number of songs. Walter Pardon springs to mind. Lloyd of course will have been a collector, and therefore not a source singer. MacColl was largely a writer, but he claimed that a number of songs he sang wre traditional. In some cases he will have been the collector, and assuming that his story about Proud Maisrie (or the Gairdner Child), namely that he learned it from the singing of his mother, when he was a child and she puttering about the house, then he learned it as a folk song and he was the source singer.

Curiously, applying that train of logic, the Copper family will largely have been collectors, rather than source singers.

But what I don't understand is how the term is demeaning.