A circuit that supplies d.c. powering to condenser microphones, using the same conductors as the audio signal.
"I have just listened to the CD that you made from the at least 44 year old 33 1/3rd rpm album I sent you. The record was of lousy quality to begin with. It had been a private label recording -- a vanity record -- recorded by a group of local Dixieland musicians in the late '60s. I was never able to play it or enjoy it because it had such a ..."