3,Replaced alternator on car 2 times each time shows it’s charging 14v let car run for a little bit then test drive and car voltage

Replaced alternator on car 2 times each time shows it's charging 14v let car run for a little bit then test drive and car voltage Drops to 11.7 and keeps going down till car stalls alternator and battery have been replaced both times as wel and keeps doing the same I've went through checked all the grounds on this car and everything any help would be greatly appreciated thanks.

My car has 103000 miles.
My car has a manual transmission.