The warning against playing a stereo record with a mono cartridge stems from the fact that the early mono cartridges didn't have or need any vertical compliance. The lack of vertical movement effectively chews up any vertical modulation found in the 45º cut stereo groove. Some current day manufacture cartridges like the Miyajima's adhere to this while others have coils in the lateral plane with compliance in both the lateral and vertical plane. The Denon 102 was designed as a mono cartridge with vertical compliance so radio stations could use the same cartridge to play both mono and stereo cuts.
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