With the development of ocean strategy, the working bandwidth of equipment in fields such as communication, electronic countermeasures, and testing is gradually expanding towards low frequencies, and the bandwidth of power amplifiers is also extending downwards to 9kHz. Broadband power amplifiers in this frequency band need to overcome two challenges: broadband matching and magnetic core loss. A power amplifier operating at 9kHz~400MHz is developed using the technology of loading ferrite cores onto Transmission Line Transformer (TLT) for broadband matching, successfully expanding the bandwidth of power amplifier with this power level to more than 13 octaves in this paper. The testing results show that within the entire bandwidth, the output power is > 200W, the gain is > 53dB, and the additional power efficiency is around 35%. This power amplifier is suitable for systems that require both wide bandwidth and high power.