Since Sanyo discontinued these modules, many "new" units on the market are clones. Genuine units usually have sharper laser-etched text and better weight due to the metal substrate. What is the supply voltage of your transformer?
Protection features:
: Due to its ZIP/DIP package and internal power dissipation, engineers must use adequate heatsinking . Prolonged operation near full load in enclosed spaces requires careful PCB layout and forced airflow. Stk411-210e Datasheet
When working with the STK411-210E, always respect the absolute maximum ratings, use adequate heatsinking, and avoid 4Ω loads. With proper care, these modules can deliver warm, detailed sound that rivals many modern chip amplifiers. Since Sanyo discontinued these modules, many "new" units