yet another hum

[lshtram]

Sorry to bore everyone, but I have a hum problem in my LM3886 amp. It's a 6 channel amp, two separate power supplies (powering 3 amps each).

The hum appears when input is connected (or shorted to ground) without input it is completely quiet.
I turned off the power supply and working on the caps, the hum is not there.

The gounding I thought was decect, connected to a star for each set of 3 amps (and its power supply) and then the two are connected together to the chassis and supply ground.

Let me know if you have any idea, or at least can narrow down the search by telling me what it's not...
thanks!

[Ted205]

what is the input ?

[pacificblue]

Most probably you are using a source with an internal connection from signal ground to PE. That could be a tuner, TV or similar thing that is connected to PE through the antenna cable. A sheath current filter can help there.
Another possible cause can be equipment, where the signal ground has an earth connection through the metal case, e. g. a PC. Then it is difficult to avoid hum other than disconnecting the earth from the amplifier's signal ground. But this article might help you then.

[lshtram]

The problem is evident also without any external source. It's enough for me to short the input signal to the input ground (using a wire, resistor or large cap) and I hear the hum. Without the short, there is no hum.

[AndrewT]

Hi,
it must be a ground loop inside the amplifier.

Build a successful monoblock and learn what needs to be done to silence it.
Then build a stereo amp and relearn how difficult it is to silence it.
Now, you're ready to tackle the 2channel amp.