We present the results of a multisite photometric observing campaign on the rapidly oscillating Ap ( roAp ) star 2MASS 16400299-0737293 ( J1640 ; V = 12.7 ) . We analyse photometric B data to show the star pulsates at a frequency of 151.93 d ^ { -1 } ( 1758.45 \muup Hz ; P = 9.5 min ) with a peak-to-peak amplitude of 20.68 mmag , making it one of the highest amplitude roAp stars . No further pulsation modes are detected . The stellar rotation period is measured at 3.6747 \pm 0.0005 d , and we show that rotational modulation due to spots is in anti-phase between broadband and B observations . Analysis and modelling of the pulsation reveals this star to be pulsating in a distorted quadrupole mode , but with a strong spherically symmetric component . The pulsational phase variation in this star is suppressed , leading to the conclusion that the contribution of \ell > 2 components dictate the shape of phase variations in roAp stars that pulsate in quadrupole modes . This is only the fourth time such a strong pulsation phase suppression has been observed , leading us to question the mechanisms at work in these stars . We classify J1640 as an A7 Vp SrEu ( Cr ) star through analysis of classification resolution spectra .