Diffraction Phase Microscopy (DPM) is a common-path, single shot QPI technique that has found applications in studies of red blood cell morphology and dynamics, cell growth measurement, as well as in Fourier Transform Light Scattering. In DPM, the phase is retrieved by interfering two orders of diffraction from a grating placed at the image plane. The reference field has been, in the past, generated by low pass filtering the zero order via a pinhole placed in the Fourier plane. For achieving the desired spatial coherence, the pinhole is often only 5-10 µm in diameter, making the system difficult to align every time an imaging study is performed. In this work, we eliminated the pinhole from the DPM optical path and generated instead the reference field by magnifying strongly the zero order. We show that a gradient-index (GRIN) lens (effective focal length of 300 µm) can be used to magnify the Fourier transform of the zero order to the point where the DC component fills the camera sensor. We show that the resulting Magnified Object Spectrum Interference Microscopy (MOSIM) system can successfully reconstruct quantitative phase images, without the need for tedious alignment. Because it conserves the common path geometry, MOSIM is characterized by 1.1 nm spatiotemporal pathlength noise. Since it is single shot, we demonstrated 400 frames/s acquisition. We anticipate that this new method can potentially lead to a more robust and less vibration sensitive QPI instrument for carrying out biological studies at various spatiotemporal scales.