Precision M4400 & M4500: Noise throught External Speakers

Hi everyone

So here's my sad story of woe:
Machine:
M4400 w/QX9300, 8GB, FX1700
AND
M4500 i7-820QM, 8 GB, 880XM, etc.

When plugging in fairly decent (keyboard monitor) speakers, loud noise levels come through the external audio port.

Focused on (new) M4500. Dell sent repair guy - replaced MB, audio daughter board. Noise still there. While repair guy was on-site, called it in, and had machine sent to depot.

Came back, same problem, new scratches, BIOS reverted to A01.
Threw hot**** dip fit, got NEW replacement machine sent out.
Same problem.

How do I know it's not the speakers, KVM or cables? M6400 has much lower (acceptable) levels of noise. So does Lenovo T61. Dell Optiplex 755 and HP XW Desktop/Workstation make NO Noise - all plugged into the same environment.

Troubleshooting revealed: Noise generated when A/C power supply connected to laptop. Tried 4 M4400/M4500 power suplies - all resulted in same problem. Tried M6400 Power supply - same problem.

Conclusion: Lousy design of M4400/M4500 chassis means you simply get bleading of (switch mode?) a/c supply noise coming through the chassis to the external speaker port. Cannot repurpose mini-jack for SP/DIF, either, so that's not an option.

Any ideas? I'm wondering if I can put some filter inline on the power supply cord (before it connects to the laptop).

Why is this such a PIA problem is? I'm constantly using WebEx/Webinar tools, Skype for conferencing, demos, etc. While sitting at my desk, I really don't want to use headphones... after several hours a day, it feels like a microwave has fried my ears (and brains).

Any ideas? Anyone?

russ

 

That sounds like a good old-fashioned ground loop. The fix is to remove the earth connector for the power supply of either the computer or the speakers. The PSU for my Logitech external speakers has a plastic earth pin on the power plug.

If my prognosis is correct then I'm disappointed that Dell couldn't reach the conclusion.

John

 

Hi again.

I'm pretty sure anyone who has plugged one of these machines into their home theater or A/V system has come accross this...

I have found numerous postings on AVS Forum, and recording studio sites.. no one here?

Anyone?

 

Hi John

Thanks for the reply.
While there is a lot of chatter about removing the connection to earth, the power supply is undoubtedly designed to dump surplus energy (spike, etc) to earth. I don't feel this would be the safest route, but I am considering it.

BTW - the noise is not a steady grond-type hum, but rather a series of squeaks, pops, etc. I'll be trying a signal-level ground loop remedy on the audio output (ala Radio Shack) later today, and if that does not work, a "Hum destroyer" like "HUM-X" on the input AC power.

I am really disappointed at Dell, and frankly surprised that no one has come up against issue in a more professional setting (Recording studio-types, DJ's etc).

russ