Hum is a nuisance in Amplifier circuits which is mainly due to improper filtering of AC fraction from the power supply and improper grounding. Hum will reduce the quality of sound from the speaker. You can easily eliminate the hum from the amplifier to get clear voice.
Usually we use a 12 volt 1-5 Ampere Step down transformer as power supply in High power amplifiers. The output DC must be well filtered to remove the AC fraction from the rectified DC. So do these techniques to remove the hum.
1. Transformer body should be grounded to the Metal chasis of the Amplifier.
2. Power supply section should be away from the Preamplifier section. If needed, isolate it from the amplifier circuit.
3. Amplifier Negative should be grounded to the Metal chasis of the amplifier.
4. Use a high value filtering capacitor like 2200 uF 50 V or 4700 uF 50 volt. As the value of the filtering capacitor increases, filtering efficiency also increases.
5. If hum continues, try one of the circuits given below. Figure 1 uses a diode and a capacitor to give an additional rectification and filtering. It should be between the Power supply and the Pre Amp.
6. Second circuit uses a medium power transistor to give regulated power supply. It should be between the Power supply and the Pre Amp.