We demonstrate a robust linearly polarized 365 W, very low amplitude noise, single frequency master oscillator power amplifier at 1064 nm. Power scaling was done through a custom large mode area fiber with a mode field diameter of 30 µm. No evidence of stimulated Brillouin scattering or modal instabilities are observed. The relative intensity noise is reduced down to -160 dBc/Hz between 2 kHz and 10 kHz via a wide band servo loop (1 MHz bandwidth). We achieve 350 W of isolated power, with a power stability < 0.7% RMS over 1100 hours of continuous operation and a near diffraction limited beam (M2 < 1.1).