gustabo
Using balanced connections would probably have taken care of that.

I needed my hum destroyer for a test with something else and I realized that I was not using the hum destroyer with my reference speakers, like I said above. I was indeed using balanced cables for that as you indicated. I was using the hum destroyer in between my audio interface and external mixer/PA to take care of a hum problem.
 
The test I was running was to connect two guitar amps in stereo but I only have 1 4x12 cab that can be run in stereo. I had to put the hum destroyer inline with the signal going to the second amp to get rid of oscillations. It turns out that my cabinet used a common terminal when in stereo.

Anderton

• I put a USB port PCI card into my computer that handles only audio.

PCI? PCIe? Model number?

tenfoot

Paul P

tenfoot
If you are using a 3 pin AU plug it can be caused by an earth loop. I had the same problem using a Toshiba laptop. Lifting the ground on the power supply fixed it for me.

I wouldn't go lifting ground on any mains cable.  It's there for a reason, mostly to save your life.

Paul P.....As stated in the post above yours, you are not unearthing your entire system, simply removing a second ground that is causing the loop. Your circuit is still finding earth.

 
You should never lift the chassis/mains ground on anything.  The ground is there to prevent you electrocuting yourself and in Australia, with your 230vac, that could very well kill you.  If there's a loop between systems, it's the fault of an audio cable connecting the two grounds together, through the shielding.  Go ahead and disconnect the shield at one end of a cable.  There are connectors to do this.  But that will only mask your real problem.  Better is to make sure your chassis grounds are all at the same potential by plugging mains cables into the same building circuit close together.  And not have your cabling routed to create large physical loops that'll act like UHF antennas.