GND loop help Please

[TPM]

I have searched and read many threads on hum hoping to find a solution, which I haven't so am now requesting your input.

I have built a 7 channel HT power amp from Brians LM4780 boards, which makes sound tracks sound as good as I had hoped except for the Hum/Buzz, which is significant.

I have figured out that with zero or only one input connected, I'm hum/buzz free, except for the physical buzzing of the two (one???) 500 VA dual secondary trafo's.

As soon as a second input is connected the hum/buz starts. It is very similar if not identical to the physical buzzing of the trafo. This leads me to suspect a ground loop or trafo induced interference.

I have 18 ga wire from the CHG of each PCB to the Star gnd (tag strip connected to the chassis) which is then connected to the 3rd prong of the power plug. I have tried to add more wire between the tag strip and the 3rd prong, I've tried a power cord with 3 wires and one with 2 wires, I've tried connecting the star ground to the chassis/heat sink in various places.

I haven't triple or quadruple checked if I wired up the inputs incorrectly (but I double checked hot is on the inside on construction).

I suspect that the slight difference in length (max. 5 inch diff) is causing this? What do you think?

How else can the physical buzz get into the signal? (Shield my inputs?)

Thanks
T

[jeff mai]

Originally posted by TPM
I have 18 ga wire from the CHG of each PCB to the Star gnd (tag strip connected to the chassis) which is then connected to the 3rd prong of the power plug.

If you are already using RCA jacks which are isolated from the chassis (if not, try isolating them first), I think this CHG star may be the problem. The way Brian's boards are arranged, there is a local star at each chip (as there should be.) The issue is the CHG connection.

It's probably very installation dependent, but I could see how current might flow between two boards when their CHG are connected at a star - especially if they are supplied by different PSU boards. A 10-15 ohm NTC thermistor (as suggested by Peter Daniel in another thread) between each board's CHG and the star should isolate them enough to stop this current flow.

It could be something else, but I'd try this first.

[TPM]

I sure wish I did a search on multi channel, before starting this project.

RCA's are isolated.

I didn't copy it, but in my new searches I did see a post where I believe Per Anders said something like, "that's why multi channel amps have multiple trafos"

Surely there is a fix to this beyond buying 5 more trafos?

T

[jeff mai]

Originally posted by TPM
Surely there is a fix to this beyond buying 5 more trafos?

Yes there is - as I said, try the 10-15ohm NTC thermistors in series with the CHG connection and the chassis star earth. You'll need one per board and they are only a few cents each.

You don't need 5 more transformers!

[moamps]

Originally posted by TPM
...Surely there is a fix to this beyond buying 5 more trafos?...

Yes.

BrianGT LM3875 NIGC Gounding Scheme

Regards,
Milan