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power grid issues?

Hi  Agoners. By chance, are there any bright EE around who may have an answer to this? I get "grid hum" on my Balanced Audio Technology tube amp. It is irregular, happens for different intervals and different lengths of time. I have check my house system by shutting evert appliance of during these "hum intervals" and I do know it is not my house systems that are creating this noise, or fluctuation. I had PG&E check their system. They used a neutral detector and said that their neutral to the source is good. The power is stable. This morning the hum, which is audible in the transformer, was a wave. on off on off on off, for a good hour. then it stopped. Yesterday it was just a straight hum for a period of time. no fluctuations at all. What is going on? 
Thanks for any input!
Greg
greg22lz
erik_squires
15,657 posts
 
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:
https://emotiva.com/products/cmx-2

Best,
E
erik_squires
15,657 posts
 
I found this article, and it seems to be pretty comprehensive:

https://www.pooraudiophile.com/2015/03/how-to-fix-dc-offset-and-transformer.html
theaudiotweak
2,769 posts
 
Can be DC on an RG6 co axial on a device which in turn is hooked to an input on your preamp . Tom
three_easy_payments's avatar
three_easy_payments
2,097 posts
 
@erik_squires 

I use the Emotiva CMX-2.  It's pretty effective in reducing my transformer hum w/o degrading the sonics.  In fact I think it may help reduce some high frequency edge but it's very subtle.
greg22lz OP
182 posts
 
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!
Greg
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