Signal noise?

Hi,

I am trying to troubleshoot a problem with an electro-dynamic shaker. There is a coaxial output from a computer to an amplifier and outputs from the amplifier to the shaker. I am sending a 15-150Hz 0-1V signal to the shaker.

There was a "ground loop isolator" between the computer and the amp (coax converted to rca, then back to coax), which had been used to remove 60Hz line noise in the past. I know 60Hz is the AC line frequency but have not yet understood why this particular frequency manages to cause interference through ground loops so often; an explanation of that would be great here as well.

What I am ultimately hoping to understand is what might be going on with the signal:

The shaker is intermittently knocking and does not seem to be working properly (with THIS amp only). If the ground loop isolator is hooked up and I touch the shielding around the connectors with my hand, the knocking goes away (otherwise the ground loop isolator makes the knocking occur on a regular basis). I'm curious how my touching the connector could have anything to do with this? I would initially guess ground it has something to do with me being grounded, but to such a low voltage does my being grounded (through shoes) make a difference to the signal?

The knocking occurs intermittently when the ground loop isolator is not connected. I've hooked a Y up to the output of the computer and watched the signal with an oscilloscope (no ground loop isolator) and have not noticed anything strange there, but when the scope is hooked up and the input to the scope is disconnected (but the coax remains connected to the Y), the shaker starts knocking more often than in any other situation. I've also noticed that if I use the other (red) set of rca leads on the ground loop isolator (rather than the white), there is no knocking.

I'm thinking there may be something wrong with the ground loop isolator since only one set of its leads causes regular knocking, but seeing as the knocking occurs even without it and my touching it effects it, I've though perhaps electromagnetic waves or something else may be involved here as well.

Any input would be appreciated! Thanks,

Dave

Line powered equipment often leaks significant current into ground through either resistive paths or capacitive ones. This current alters the local potential of ground at line frequency. If you amplify a voltage compared to local ground, and it was generated with respect to a different local ground potential, the difference between the two local ground voltages gets added to the signal.

There is probably something in the vicinity that is radiating strong electric fields that the cable shield is picking up as capacitive current (probably where multiple cables run parallel). Your body adds a lumped capacitive load to that shield when you touch it. How is the cable shield on each side of the ground isolator grounded?