The 1:1 +C.T. transformers (Vigortronix VTX-101-1604) have arrived and seem to work well once soldered to some strip-board. Powering this board using the existing microphone amplifier and an external sine-wave source gives the desired result. It seems as though a buffer amplifier is required on the output due to signal drop through the ring modulator. Can always use that as a volume control for the speaker output. Nearly there, just being held back by another project deadline.