Carrots are fussy, and they do indeed prefer sandy soil. However is it really worth the trouble of altering your soil just for carrots? Potatoes, leeks, runner beans,courgettes and many others will all grow in soil that will not produce a single carrot. Raised beds filled with free-draining compost will work, but if you are going to that much trouble it would be better to use a raised bed for an expensive crop like asparagus, since carrots are cheap and readily available.