Here is a 1957 aircheck of a station that touted itself as “high fidelity.” The station was Minneapolis-St. Paul KSTP. I am not sure whether this recording was made off the air or off the modulation monitor.
http://www.twincitiesradioairchecks.com/images/Sirg_06_KSTP-AM_New_xmitter_1957.mp3