Quick thanks to Steve for these diagrams. From these and a great design from Duncs, I have made a (working!) SpeakerLink to SPDIF + two-way (C-lead) comms adapter (pic to follow).

Note, you can use a Lundahl impedance matching transformer to do the AES to SPDIF conversion. Not cheap (£18 + shipping from Canford), but no other options at the moment.

Wire as follows: connect the AES+ (pin 7) and AES- (pin 8) to the 110 Ohm side of the transformer ground is not connected) and the +ve (centre wire) and ground (screen) of the SPDIF coax cable to the 75 Ohm side of the transformer.

Roon Developer and ex-moderator of this Forum
I am #25 in the HH1 photo of fame.