I'm not sure what you mean by "grid hum," but it sounds like you have DC on the line. That could come from the utility itself or from within your house. Is this hum you can hear coming only from a component transformer, or do you also hear it through the speakers?
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the hum i am referring to is my transformer on my tube amp. PG&E has assigned an EMF specialist to help determine where the noise is generated. I do not believe at this time that it is generated from my house due to its irregular intervals, i.e., time of day, inability to duplicate it when it is not there, everything turned off in my house. Yesterday it was a constant hum for at least and hour this morning it was an oscillating hum for about an hour. I am on the same line as my neighbor. We are the only two houses connected to the transformer across the street. They have solar panels that run off of AC. I took my amp and plugged it into different circuits in my house and got the same hum. Maybe I should go to my neighbors house and plug it in to find out if it would hum over there.
It sounds like the age-old issue of having DC on the line. This will cause mechanical vibration in the transformer.
I would not just turn off appliances, I'd turn off all the breakers to see if it's something in your home. Usual suspects are digital supplies, as well as LED or high efficiency light bulb supplies. Hair dryers, and dimmer switches.
The permanent fix for this is to use a balanced power conditioner. Furman, Equitech, Torus, etc. will eliminate this completely.
There are also smarter, less expensive devices like this from Emotiva, which I have not tried:
interesting comments. There isn't anything connected to my pre-amp except CD and amp. No other external sources. The circuit is an isolated circuit. Stereo only.
Erik, you say that the emotiva reduces your transformer hum. It doesn't eliminate it? My next step in evaluation is to shut off all breakers to see if it is emanating from some DC source in the home, which at this point i doubt.
Thank you for the comments ! I will keep you posted as to the results!
Maybe I should go to my neighbors house and plug it in to find out if it would hum over there.
Yes you should!
There is a very good chance your neighbor’s solar system is causing DC offset on the AC power mains in your home. It may also be causing the utility power company’s power transformer to run hotter than it normally would, therein shortening it’s life.
Found this doing a quick Google search.
PG&E has assigned an EMF specialist to help determine where the noise is generated.They will have testing equipment that will identify possible harmonic distortion that your neighbor's solar system may be back feeding onto the power company’s AC power system.