2x LM4780 + DRV134 = hum Need grounding help!

[maxw]

I have a ground loop but I'm having trouble working out how to get rid of it when I have so many grounds!

In my diagram, the power grounds and signal grounds join on the BrianGT amp PCBs and on the DRV134 PCBs. The other channel is a complete mirror of this one. I also joined power ground to the chassis other wise I get loud hum. My amp is dual mono in one case.

As it is now I get a quiet hiss (not silent) when no input is connected. If I connect one channel input to my preamp then that channel is silent and the other continues to hiss. If I connet both channel inputs to my preamp I get hum again. So I think the loop is between my power amp and preamp.

I've read the (many!) grounding threads but they only seem to cover simple GC grounding.

How do I get rid of this??
Is my ground loop caused by having (effectivly) 3x grounds (2x pwr, 1x signal) to my amp PCBs??

Any help much appreciated

[maxw]

this is the diagram of how the DRV connects to the amp PCBs.

[richie00boy]

This should be quite easy to fix - your grounding is no good. Remove all the ground wires you have at present.

You should make the joint of the transformer secondaries your star ground. From here, take a wire off to each board ground and to the chassis earth.

Input and output sockets should be isolated from the chassis, if they are not already.

The DRV board appears to need a DC power input but you are feeding it straight off the bridge. Can you lash up a pair of caps before the board?

Should OG on the amp board be grounded, or is that simply where you connect the speaker return lead? (Look on the PCB to see if it's joined to PG).

I also believe the Brian boards should be fed with a dual rectifier bridge scheme (hence why there is PG+ and PG-) and you are using only one. So one part of the board is possibly not grounded.

No offence but have you actually read the instructions that came with the board from Brian?

[maxw]

Originally posted by richie00boy
This should be quite easy to fix - your grounding is no good. Remove all the ground wires you have at present.

Good!

Originally posted by richie00boy
You should make the joint of the transformer secondaries your star ground. From here, take a wire off to each board ground and to the chassis earth.

OK

Originally posted by richie00boy
Input and output sockets should be isolated from the chassis, if they are not already.

Yep, they are.

Originally posted by richie00boy
The DRV board appears to need a DC power input but you are feeding it straight off the bridge. Can you lash up a pair of caps before the board?

This is a simplified diagram, there are 8x caps per rail after the rectifier and the DRV board have their own caps and then regs

Originally posted by richie00boy
Should OG on the amp board be grounded, or is that simply where you connect the speaker return lead? (Look on the PCB to see if it's joined to PG).

OG and PG are joined on the PCB. All grounds join on the amp PCB.

Originally posted by richie00boy
I also believe the Brian boards should be fed with a dual rectifier bridge scheme (hence why there is PG+ and PG-) and you are using only one. So one part of the board is possibly not grounded.

I don't think it makes any difference as all grounds join on the amp PCB

Originally posted by richie00boy
No offence but have you actually read the instructions that came with the board from Brian?

Of course, I've made a few GCs before but never had problems.


Thanks for your suggestions, I'll give them a go. I made a new diagram to represent what you said.

[richie00boy]

Yes that looks better.

I think to improve performance, you could connect the ground to the boards using the OG hole as it's more central, then put speaker returns to the star point.

[maxw]

Well its less but not gone
If I move the position the input cable (which is shielded) around it changes from loud to quiet but still never completely gone.

Here is how it looks now:

[BrianDonegan]

Disconnect the signal grounds from the mains ground. It will be quiet. Keep the chassis grounded to mains ground, however.

[AndrewT]

Hi Brian,
can you rephrase that?
I am not sure I understand your US side wording.

Richie & Maxw,
I disagree on the location of the star ground (audio ground) and note the omission of the smoothing caps from the grounding scheme.
Can you clarify the PSU common and it's connection to the audio ground?
The centre tap connection direct to audio ground may be aggravating the hum.

[BrianDonegan]

can you rephrase that?

Here's a pic. No link between mains/chassis ground and audio ground.

[richie00boy]

Andrew, the smoothing caps I assumed were the big caps on the amp PCBs. That's why I suggested the centre tap to be the star ground as it can be problematic connecting simple centre-tapped multi-capacitor PSUs together.