An assembly for passing controlled impedance electrical signals between two different environments via feedthroughs in an electrically-conductive feedthrough block. The feedthrough has a signal coupling, a dielectric spacer surrounding the signal coupling, and a ground coupling surrounding the dielectric. The feedthrough is sealed to hermetically separate the two environments. One or both ends of the dielectric are recessed from the face of the block forming a gap. The gaps is filled with a potting material to form the hermetic seal.