The grounding line of the reference voltage regulator ICTL431 is wrong. The grounding of the same secondary reference voltage regulator IC has similar requirements as that of the primary IC, that is, it cannot be directly connected to the cold geothermal ground of the transformer. If this is combined, the result is a decrease in the carrying capacity and the howling is proportional to the power output. When the output load is large and close to the power limit, the switching transformer may enter an unstable state: the duty ratio of the switching tube in the previous cycle is too large, the conduction time is too long, and too much energy is transmitted through the high-frequency transformer; The energy of the dc rectified energy storage inductor is not released fully in this period, and it is judged by PWM that there is no drive signal or too small duty ratio to make the switch tube conduction in the next period; The switching tube is in the cut-off state in the whole cycle after, or the conduction time is too short; After more than one full cycle of energy release, the output voltage drops, and the duty cycle of the switch tube in the next cycle will be large...... This cycle causes the transformer to vibrate at a lower frequency (the frequency of regular intermittent full cut-off cycles or drastic changes in duty ratio), producing a lower frequency sound that can be heard by the human ear.