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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
erik_squires
15,807 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
11,615 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.
georgehifi's avatar
georgehifi
10,312 posts
 
atmasphere
What is your line voltage? It should not exceed 125V for more than a second. If its high that could explain the problem.
+1 for checking this.

Canada is supposed to be 120v.
In Australia we’re supposed to be 230v mine measured quite constantly around 250v, incandescent light globes would last a couple of months, yes my transformers were a little noisy with a screwdriver to the ear test, but the worst is your paying more for your power, when using unregulated appliances, fridges, aircon, heaters,lighting, poweramps, microwaves ect ect.
I complained very hard, and eventually they put the whole street on a different tap on the pole transformer down the road. Now I have correct 230- 235v no more globes blowing, and my usage is down. And I swear my system sounds better, even after I re-biased my amps, back to what they were with the 250v.

Cheers George
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