Dear Mr Thiagu,
Brahmayagyam is mandatory after all tarpanams irrespective of wheather its Amavasai , masa sankramam , Mahalaya Paksha Tarpanam or the Tarpanam which is perfomed after the shrardham of Father , mother etc.
The only exclusion for Brahmayagyam is the tarpanam peformed after Grahanam if it falls at odd hours ( like midnight , late eveining etc)
Otherwise It is mandatory as per our Veda shastra's that Brahmayagyam has to be performed after Tarpanam.
I hope that other esteemed and respected members would also agree with my view.
Thanks & Regards
N.Narayan