Yirushalayim.....Amen.

Within the context of the Bircas Hamazon (blessing after meals) there seems to be confusion as to the proper appropriation of the word "Amen" after the Bracha of "Uvinei Yirushalayim."

In O.C. Siman 188 S' 1 the S'A says that after finishing the bracha of Uvinei Yerushalayim one should answer Amen (after his/her own bracha) because it is the end of the sequence of brachos which are Deoraisa (Commanded from the Torah). This Amen should be said quietly, in order that passer byers should give the same amount of respect to the next bracha (hatov vihmeitiv) as they did to the uvinei yirushalayim.

The Sefer Piskei Teshuvos ascertains that the aforementioned law only applies to Sfardim and that Ashkenazim are encouraged to say Amen with a normal tone [1][2].

The Mishna Brura adds on this topic that one should not say "Bonei Birachamav Yirushalayim, Amen" in one breath. Rather he or she should wait a little bit after saying the word Yerushalayim and then follow it with the word Amen. The reason for this is to separate the Amen from the Bracha. However one should be sure[3] not so pause for more than the shiur of Toch Kidei Dibbur (the amount of time that it takes a person say, "Shalom Alecha, Rebbe U'morei").


________________________________________________________________
1. There are Ashkenazic Poskim who support the psak of the S'A as seen in the Shulchan HaTahor S' 2, in the Orach HaShulchan S'3.
2. Ba"Ch [Sham]
3. PM"G 48 S"K 1 - others who are listening along may answer Amen immediately.