May one answer Amen to the blessing of Al Mitzvas Tefillin said according to Ashkenazi custom?

• May an Ashkenazi answer Amen to the blessing of Al Mitzvas Tefillin of another Ashkenazi?
• May one who does not say the blessing of Al Mitzvas Tefillin on the Shel Rosh [i.e. Sephardi/Chabad] answer Amen to the blessing of one who does [i.e. Ashkenazi]?

Initially, those who are accustomed to reciting the blessing of Al Mitzvas Tefillin on the Shel Rosh, are to do so quietly [even in an all Ashkenazi Minyan], in order not to enter others into a Safek Amen Levatala.[1] However, in the event that one heard the blessing, those who are of Ashkenazi descent, and themselves recite the blessing, are to answer Amen.[2] However, those of Sephardi descent, as well as those who never recite the blessing due to Safek Bracha Levatala [i.e. many Chassidim], are not to answer Amen after the blessing.[3] Rather, they are to answer Amen in their minds[4], or say the verse of Baruch Hashem Leolam Amen Veamen [making sure to conclude Amen as the person concludes his blessing[5]].

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[1] Derech Chaim; Kitzur SHU”A 10:4; Likkutei Maharich in name of Ramatz 13; Divrei Yatziv 1:83; Aveni Zikaron p. 269 that so was custom of the Chozeh Milublin; Piskeiy Teshuvos 25:11 and footnote 90 that so is custom of world

The reason: As there is a question as to whether one may answer Amen after hearing the blessing, as the person recites Baruch Shem to remove himself from the doubt. [See P”M 25 A”A 10; Teshuvas Harambam in Peir Hador 105 ]

Other opinions: Some Poskim rule the blessing may even initially be recited aloud, as we hold that there is no Bracha Levatala involved at all. [Peri Kodesh Hillulim and Zohar Chaiy Vayigash, brought in Piskeiy Teshuvos 25 footnote 90; Darkei Chaim Veshalom 47 that so was custom of Munkatcher to say it aloud]

[2] Misgeres Hashulchan 10:8; Halef Lecha Shlomo 57; Piskeiy Teshuvos ibid

The reason: As they themselves hold that the blessing may be recited, and the Baruch Shem is only recited as an extra insurance. Likewise, an Amen Levatala is less severe than a Bracha Levatala. [Misgeres Hashulchan ibid based on P”M 215]

Other opinions: The P”M 25 A”A 10 leaves this matter in question.

[3] Kaf Hachaim 25:40; Yabia Omer 1:29; Yechaveh Daas 4:31; Teshuvas Harambam in Peir Hador 105 that Safek Amen Lihakel

[4] Kaf Hachaim ibid

[5] Heard from Harav Yaakov Yosef z”l