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Message: <blockquote data-quote="abbey road d enfer" data-source="post: 1075151" data-attributes="member: 15112">In the late 90's, I won a contract for an intercom system using the RTS party line system, which is based on a common unbalanced line where the individual stations and beltpacks inject audio current. The signal is between one conductor and the shield. The nominal impedance is 200 ohms. The lines circulated all over the building, for a total of about 3km (2 miles), resulting in about 400nF capacitive load, which limited the audio BW to about 2kHz.Since all lines were actually shielded pairs, I drove the unused conductor with a voltage follower. It instantly doubled the BW.I don't remember the details of the circuit I used, probably a TDA2050, which I used extensively at the time.</blockquote>