We report on the generation of 2.06 W of tunable cw light at 780 nm by a single-pass frequency doubling in a PPMgO:LN crystal with a seeded high-power fiber amplifier. A 1560 nm distributed feedback (DFB) diode laser, a Littman-type grating external-cavity diode laser (ECDL) and a DFB-type erbium-doped fiber laser (DFB-EDFL) were separately used as a seeding laser source of an erbium-doped fiber amplifier (EDFA). Here we use the EDFA which has a narrow linewidth option, the fundamental frequency light will not be obviously broadened. The influence of laser linewidth on the conversion efficiency of frequency doubling is investigated. The injection power and output power of EDFA must be consistent. So when temperature of the PPMgO:LN crystal is fixed, the conversion efficiency for different seeding resources can be obtained as follows. When the fundamental power is 12.42 W, using the DFB as seeding resource yields 1.36 W of 780 nm doubling output, and the corresponding conversion efficiency is 11.0%. Using the ECDL as seeding source yields 1.78 W of 780 nm doubling output, and the corresponding conversion efficiency is 14.3%. While using the DFB-EDFL as seeding source yields 2.06 W of 780 nm doubling output, and the conversion efficiency is 16.6%. The measured laser linewidths of the three seeding sources are 1.2 MHz, 200 kHz, and 600 Hz for the DFB, ECDL, and DFB-EDFL, respectively. The experimental results show that the narrower laser linewidth, the higher doubling efficiency, and the experimental results agree with our theoretical analysis.