The Teltone 487 solved the whole problem. It is a very complex circuit (physically it fills all the available volume that a 400-type ktu takes up in a shelf.) It weighs a lot. It has many dip pswitches for setting up all the variables. And best of all, it requires NO extra pairs between DPA's.
I have a couple of them, and I plan to hook them up in my museum which will feature a step-by-step PBX and a lot of 1A, 1A1, and 1A2.
