  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$^{-1}$ ($1758.45 \mu$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
$3.6747\pm0.0005$ 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.
