We built a transformer-based voltage summing box for low-frequency transport measurements. It combines a DC bias with a small AC excitation from a lock-in amplifier, allowing the sample to be driven with a well-defined voltage of the form (V_{\mathrm{out}} = V_{\mathrm{DC}} + V_{\mathrm{AC}}). The device was tested for transfer ratio, load dependence, linearity, phase shift, and electrical pickup over the relevant low-frequency range.