We demonstrate a unique terahertz (THz) source that is compact, utilizes recently developed all-fiber Q-switched lasers, and is based on difference-frequency generation in a GaSe crystal. A single piezo simultaneously Q switched the two fiber lasers by using stress-induced birefringence, to achieve the temporal overlap of pulses from the two fiber lasers. These correlated pulses then combine in the GaSe crystal to produce coherent and highly monochromatic THz pulses. The peak power for this THz source can reach $0.53\mathrm{mW}$ , corresponding to an average power of $0.43\mu \mathrm{W}$ and a conversion efficiency of $4.75\times {10}^{-7}$ . The estimated linewidth of this THz source can be as narrow as $\sim 35\mathrm{MHz}$ or $1.17\times {10}^{-3}{\mathrm{cm}}^{-1}$ .