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Help I think my AC is polluted

After months of trouble shooting, I've come to the conclusion that my AC is the culprit of a very annoying hum. The thing that's odd is it only occurs every couple weeks, and only for two or three days at a time. The other thing that's strange is it happens on the weekend and ends usually by Monday night.

Let me give a few details about my system.
- I recently installed two dedicated 20 amp lines. This did not change the intermittent hum.
- I've tried cheater plugs during the hum and they had no effect.
- I'm in the process of auditioning two power conditioners. The Equi=tech Son of Q Jr. and the Furutech e-PT609. Niether of them has had any effect on the hum when it's happening.

I've put my ear right next to each component during the hum and found the Equi=tech transformer humming. I turned off all other components and the Equi=tech continued to hum, even in standby. When the hum is gone, the Equi=tech is dead quite. The hum doesn't change whether the powercords are plugged into the wall or either power conditioner.

I'm going to give the power company a call but I don't expect much help from them. I may try putting in an isolated ground and see if that helps. According to Equi=tech, balanced power should take care of most all ground loop issues, so I don't think an isolated ground will help much with this problem.

Has anyone else ever experienced this? Any ideas on a solution?

mootsdude
mootsdude OP
51 posts
 
It appears that an iso transformer may be the answer to this problem. From what I've read, the iso transformer will take care of external AC noise, voltage fluctuations, and DC finding its way into the house. I'm first going to call the power company and see if they can do anything. Does anyone know if there is a maximum allowable voltage swing that the power company must maintain within established limits for residential applications? If the problem is DC finding its way in, can I hold the power company responsible for rectifying the problem on the assummption that its a safety hazard? I'm guessing this is a somewhat unique problem given that it occurs intermittently.

Thanks for all the helpful responses.
tbg
5,263 posts
 
Jgreene, wow your electrical service is third world. Is it an old house or an old apartment?
jea48
4,441 posts
 
Jmho excessive mechanical noise of a toroidal transformer is not due to low voltage. It is due to DC offset voltage on the AC power mains.

Here is a link for the industry standards for power companies. The nominal AC power voltage they should maintain to the customer shoud be within plus or minus 5%. Usually a + or - 10% range for equipment.
http://powerstandards.com/tutorials%5CVoltageRegulation.htm
stilljd
160 posts
 
All,

Thanks for the discussion and information. It points out a misunderstanding I had about causes of transformers humming - not low voltage - but DC offset voltage (see stilljd above, I started the low voltage stuff). Now I have to understand what DC offset voltage is.

Also explains why my Alpha 9's CDP transformer hummed occasionally at home but not at work.

Appreciate the information and I didn't mean to post erroneous info, just hoping to learn more myself. My apologies.

Best Regards,
Jim S.
oldears
386 posts
 
I seem to have a similar problem. Is there an easy way to measure DC offset on the AC line? I have access to a scope, but no skill in using one.
Also I discovered when I added dimmers to the dining room light that the electrician had not run a ground to the switches (single pole). I get hum even when the dimmer is off.
Any advice?
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