Groudning the chassis for the chipamp?

[pyromaniac_]

Hi, I've recently built a TDA2040 stereo amp, two chips and PSU in the same box. Because the chassis is alu, it's connected to the safety ground (don't sure about the english term btw). My question is then, should the zero V point of the PSU also be connected to the chassis as well?

The thing is that the amp works except one annoying little error. Then it's connected to my iBook I get some quite loud hum (sounds like the old familiar about 60 hz sound). The strange thing is that this doesn't happens then it's connected to my iMac instead. Because the imac is grounded (safety grounded) and the ibook isn't, I thought this could have something to do with the grounding of the amp, since I also get this hum then nothing is connected to the amp.

[Rainwulf]

If you are getting hum with nothing connected, something in your circuit isnt right. Try connecting the ground of your PSU to the case and see what happens.

[sangram]

RCA input sockets should be insulated from chassis or you'll get hum from sources which are plugged into a common earth point...

[pyromaniac_]

Originally Posted by Rainwulf

If you are getting hum with nothing connected, something in your circuit isnt right. Try connecting the ground of your PSU to the case and see what happens.

Thought so too, I'll try to connect the PSU ground to the case and see what happens. Worst case; I'll black out the whole apartment, but whatever...

Originally Posted by sangram

RCA input sockets should be insulated from chassis or you'll get hum from sources which are plugged into a common earth point...

They are.

[sangram]

Do you also get the hum when the iBook is on battery power?

[pyromaniac_]

Yes, there no different when it's on battery power or not.

[remmelt]

Are you using the grounded Apple plug or the duckhead? That might be the difference between the iBook and the iMac.

[sangram]

Then it's a ground loop for sure. And it's inside the amp - if it quietens when you connect the iMac.

Are the two channels sharing any ground apart from the central star? The two channels share a single power supply?

[pyromaniac_]

Originally Posted by remmelt

Are you using the grounded Apple plug or the duckhead? That might be the difference between the iBook and the iMac.

The duckhead (ungrounded) for the iBook, which probably makes the difference between the iBook and the iMac. I thought so too.

Originally Posted by sangram

Then it's a ground loop for sure. And it's inside the amp - if it quietens when you connect the iMac.

Are the two channels sharing any ground apart from the central star? The two channels share a single power supply?

Ok, if it's from inside I should try to connect the chassis to the ground of the power supply right? Haven't tried this yet, hope I'll get the time to do it tomorrow.

Well, let me think.... to start with - they share the same power supply. But I can't think of anyway they could share the ground except the central ground star... it's like this;

signal ground ch 1 -> (single connector) -> output and psu ground ch 1 -> PSU* <- output and psu ground ch 2 <- (single connector) <- signal ground ch 2

Ok, quite hard to describe but I you'll get it anyway.

*And here's there I should connect the chassis?

[sangram]

Can you try to route the grounds back individually to the central star, instead of to the individual grounds on the boards?

So you will have a single point at the PSU where four grounds will connect - the two PSU grounds and the two signal grounds. For some additional noise benefits you can use a small resistor (~2-4 ohms) between the signal grounds and the star.