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Transformers humimg........again

Yes I'm starting a new transformer hum thread. I read through most of the others, but did not find the info I was after, sorry.
Let's start. I have a dedicated 20A line for my audio gear. Gear consists of a BPT 3.5 Signature ac isolator, Jolida JD9 II phono pre, Naim 5i-2 int. amp., two REL T-5 subs, and a Clearaudio Concept TT. Now the important part, ALL components that utilize a transformer (BPT, Naim, REL's) all have transformer hum. And yes, it is transformer hum, NOT sound from the speakers. I had a buddy, who's an electrician, over last night, and we tore everything apart from the main box to the outlet, no results. Tried many other outlets in the house, with and without the BPT unit. No difference, all transformers still humming, whether alone or not. Unfortunately, my power is fed to my house from overhead lines, but so be it. I can't change that unless I move.
Question to answer, "what can I do to stop the transformers from humming?" The hum is not overwhelming, fairly subtle actually, but nonetheless, it is something I would like to eliminate.
Can anyone offer experienced or educated replies?

Thanks :)
128x128shawnlh
sbrownnw
710 posts
 
And what are the next steps if the following checks are not in spec?

  1. Just a few things your electrician friend should check. Have him check the mains voltage at the main service electrical panel. Make sure the voltage is the normal voltage for your area.. Have him check hot L1 to hot L2.
  2. Then check hot L1 to neutral, and then hot L2 to neutral. The two readings should be within a couple of volts of one another.
  3. The combined voltage of L1 to neutral + L2 to neutral should equal the L1 to L2 voltage reading.
  4. Next thing he needs to check is the Hz. It should be 60Hz possibly 59Hz.
  5. Next have him check the voltage at the receptacle the audio equipment plugs into unload, nothing powered up, then check the voltage again with everything powered up. The voltage should not vary more than a couple volts.

Might PM some of the people on this thread directly if I don't hear anything.  Thanks everyone...
erik_squires
16,412 posts
 
The most likely cause of a transformer mechanically humming, besides poor lamination, is DC on the AC line. That is, the + and - portions of the waveform are not equal and opposite.

DC can be injected from certain dimmer switches and switching power supplies (or fluorescents sometimes), among other things. 

If so, a balanced transformer will minimize this issue.

If not, it's a problem with the transformer's itself.

Best,

E


atmasphere's avatar
atmasphere
11,973 posts
 
A balanced transformer does not seem to sort the problem of DC on the line although it does have advantages.

There is a simple circuit consisting of a bridge rectifier and four electrolytic capacitors (bypassing the rectifiers in the bridge in reverse bias mode) that can block DC. They are effective enough and cheap enough that they are often included in equipment to help prevent the noisy transformer issue.

http://www.diyaudio.com/forums/solid-state/2080-dc-filter-16.html
Scroll down to post 159.

They can also be inserted in the AC line although this should be done by a qualified technician. The circuit is compact enough that it can be installed in an electrical junction box.
puiu
31 posts
 
Hi Shawn,
First off, you should know that transformers in NAIM gear tend to hum.
My Superline top line phono stage powered by HiCap DR power supply hums. Not all the time. When I have and can measure residual DC on the line. Also, when any dimmers are used in the house. My original Nait5's transformer also hums. I am not sure as to what extend Naim audio specs their transformers for core saturation but this is another element that triggers it at times. They are very high tech audio grade custom transformers and sometimes that is a drawback in  real world conditions. NAIM has always been an idiosyncratic company producing product as so. Much better after the passing of Julian Vereker the founder, CEO, and past designer. 
I hope this helps. Good luck with everything.

PS: You can also contact Chris West (ex-service manager at NAIM UK) at AV Options in IL (the only factory trained and authorized service center in US).

Best,

Puiu
puiu
31 posts
 
High core saturation  margin I meant.
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