We report on a single-frequency large mode area Er-doped photonic crystal fiber amplifier system. The active fiber had a core with 40 µm diameter and a numerical aperture of <0.04. This resulted in a mode field diameter of about 31 µm around 1550 nm wavelength. In a master-oscillator power-amplifier scheme, 70.8 W of output power at 1556 nm have been obtained. The maximum output power was limited by the amount of available pump power. Very high fractional TEM 00 content - and thus near diffraction limited output beam quality - has been verified by measurements with a non-confocal scanning ring cavity.