James Perrett wrote:
On Fri, 26 Mar 2010 17:45:55 -0000, Ian Bell wrote:


Here's the scenario. I am measuring the noise from a mic pre. Its
input is balanced and shorted and anyway the gain control is turned
down to zero so there's no chance of pick up at the input. Output is
unbalanced with a source impedance of about 800R feeding a battery
operated Lindos MS1 which is connected to the laptop via a serial link.

The Lindos has a built in speaker so you can actually hear any noise
present. With the laptop running on batteries the output noise is
close to -80dBu as expected and sounds like hiss in the speaker. Turn
on the laptop PSU though and the reading rises to -40dBu and you can
hear a high pitched whistle in the speaker.


That's a standard ground loop problem typical of laptops.

James.




Yes and no, because it does not go away if I disconnect the serial cable.

Also, if I pull the mains cable out of the laptop PSU so there can be no
ground loop, the whistle is still there and you can hear it change pitch
and disappear as the PSU dies. Not only that, the PSU mains cable is a
double insulated two pole type so there is no ground connection to loop.

I strongly suspect the SMPSU is putting crap straight onto the mains.

Cheers

Ian