I've got kittens at the moment and their litter tray can be smelly in the morning,but I clean it out regularly. As long as your cat isn't actually smelling, then it must be that. I've used different litters and the only one I like is the Costcutter lightweight one-some of the 'clumping types'are so difficult to clean out.I still completely change it evey couple of days,and I don't think the house smells.
You could always try one of those plug -in odour things in the same room as the litter tray.