We demonstrate phase-stable generation and ultrabroadband electro-optic detection of few-cycle multi-terahertz transients tunable from 55 to 107 THz based on a versatile telecom compatible three-branch Er:fiber laser system. Difference frequency mixing of two laser pulse trains at the full repetition rate of 49 MHz provides passively phase-locked multi-terahertz transients with peak fields of 69 kV/cm and average powers of up to 1.4 mW. 8 fs gating pulses from the third laser port allow for field-sensitive sampling throughout the midinfrared with a signal to noise ratio of up to 10⁴ Hz¹/². A leadoff application in trace gas detection is discussed.