60 Hz Hum from Powered Sub

Why does that work?

If the source and preamp are on one circuit (L1) in the main, and the sub is plugged into the other side of the main (L2), the difference between the two is the hum you are hearing. Lifting the ground with a cheater removes the common difference between the two. A ground is for fault protection and a drain to ground vs YOU.

A common on the other hand completes the circuit. A common and a ground CAN (not should) share the same bus with a jumper in the main. I keep them separate. That jumper can be the source of a lot of noise in house wiring..

You can get a hum using the same side of the bus if a two component have a slight difference in voltage but share a common cable like a RCA or XLR, (good place for a ground loop eliminator). Cable boxes are notorious for it, so are laptop computer supplies.

Regards

oldhvymec

... A common and a ground CAN (not should) share the same bus with a jumper in the main. I keep them separate. That jumper can be the source of a lot of noise in house wiring ...

It isn't clear what you're saying here. All neutrals and grounds must be bonded together at the main panel - and only at the main panel - for safety. That is very a basic NEC requirement. No exceptions.

Thanks cleeds I got a phone call from a city official I been waiting on.. I got excited, only took a month for the call back. :-)

Excuse me, I meant to say sub panel, and must be bonded at the main.

At the a sub a jumper can be between the TWO. It will work but I separate and have found it to be a source of noise in my house wiring.

I check ground and commons for amperage to ground. It always amazes me what I find there and the difference between the TWO.

A 220/240 sub panel, with a blower motor using L1 and L2 and in the same sub panel, stereo receptacle (s) being uses on one or the other hot rails. That is a no no in my system for noise..

My house is quiet and I have a  mix of nob and tube on the old AND Romex on all the new construction.  SOOW 4/3/g feed both shops, and 12/2/g for all shops, basement, kitchen and attic outlets..

The house is 14/2/g and nob and tube.

Bottom line OP shared grounds can make noise..:-)

@ spacecadet65,

Which sound best describes the hum you hear from the sub?  

60Hz hum  
  https://www.youtube.com/watch?v=SVZ2P0KsLic     

60Hz buzz  
https://www.youtube.com/watch?v=FBRPHojSGAs

120Hz hum  
  https://www.youtube.com/watch?v=pC4Uzt0qm2E


Did you try a 3 wire to 2 wire cheater plug yet?  
Make sure you insulate the ground tab on the cheater so it does not contact the 6/32 trim screw that holds on the outlet cover plate. The 6/32 screw is grounded.

Almost all subs have a "plate amp" with a switching power supply and are very prone to this type of problem. Check with Tannoy and see if they have an exchange program where they send you a rebuilt plate amp, which you can change out yourself. If not, some repair shops will work on the removed plate amp, saving a lot of shipping cost. Also if you remove it, sometimes it is easy to spot a split cap.