We are developing a programmable Josephson voltage standard (PJVS) system based on a compact cryocooler. The system is capable of generating two arbitrary waveforms by use of a dual-output PJVS (DO-PJVS) device. Owing to this dual-output feature of the PJVS system, the low-frequency characteristic in the ac-dc transfer difference of a thermal converter has been investigated using the 90° addition method. The evaluation of the low-frequency characteristic of a thermal converter is of great importance for connecting conventional ac-dc transfer standards and new Josephson-based ac voltage standards. To fulfill this purpose, a dual-heater thermal voltage converter (DH-TVC) with improved low-frequency performance has been developed. The low-frequency ac-dc transfer difference of the DH-TVC was evaluated to be smaller than 1 μV/V at frequencies of as low as 1 Hz.