I have four of these amplifiers and they all exhibit hum on output regardless of output level even with nothing connected. The hum is actually quite loud and annoying. I believe this could be a design fault with these amplifiers?
Thanks to Bonsai I have completely eliminated the hum even with headphones on the output.
First I cut the track on the 0v line between the smoothing caps and experimented adding a 1ohm HBR between the 0v smoothing caps and the ground 'amplifier module' this reduced the hum slightly but hiss and DC offset increased. Reducing the resistance to 0.8ohms 'struck a balance' Higher values made it much worse.
Then I noticed that the speaker return wires are connected to chassis ground via a approx 8ohm resistor and cap. Also connected to this is earth mains. (resistor on top right of 1st pic)
I removed the black speaker return wires from the board and connected them to the 0v smoothing caps. Hum completely gone! Maybe I should restore the cut track with the 0.8ohn resistors?
Thanks to Bonsai I have completely eliminated the hum even with headphones on the output.
First I cut the track on the 0v line between the smoothing caps and experimented adding a 1ohm HBR between the 0v smoothing caps and the ground 'amplifier module' this reduced the hum slightly but hiss and DC offset increased. Reducing the resistance to 0.8ohms 'struck a balance' Higher values made it much worse.
Then I noticed that the speaker return wires are connected to chassis ground via a approx 8ohm resistor and cap. Also connected to this is earth mains. (resistor on top right of 1st pic)
I removed the black speaker return wires from the board and connected them to the 0v smoothing caps. Hum completely gone! Maybe I should restore the cut track with the 0.8ohn resistors?
