Good thinking Batman:)  My rgb was 17,17,17 rather than 0,0,0. That solved that problem - both using the alpha node(boost =0%)  or the transp. node((transp=100%) the background square disappeared. It's not much of a scene - a cylinder with my name on it. The left cylinder is with alpha and the right with transp. (just moved the connection arrow in the function editor.) The transp one seems a bit brighter. I dont need it for anything in particular just trying to help out the OP and learning some things myself in the process.

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
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