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Old 4th October 2012, 06:26 PM   #1
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Default Humming Transformers

I've got a pair of 500VA transformers that are loaded to 400VA.

They both hum very slightly which can be annoying.

Would I get any benefit from encapsulating them in potting compound ?
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Old 4th October 2012, 06:32 PM   #2
Mooly is offline Mooly  United Kingdom
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Are you sure its not the old "DC on mains" problem ? although if you are running them at 400va loading then you may be hearing the harmonics as the bridge comes into and out of conduction. Worth looking at though.

Wouldn't like to say on potting compounds. Over the years we had this type of problem on numerous ferrites and HF transformers (TV Trade) and there is so much energy involved that encasing the coils wasn't the success you might have expected.
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Old 4th October 2012, 06:36 PM   #3
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The humming doesn't seem to change regardless of what else I have running in the house.
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Old 4th October 2012, 06:37 PM   #4
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As it's a Class A amp, the loading is virtually constant.

It's a mechanical hum, there is no humming from the audio perspective.
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Old 4th October 2012, 06:46 PM   #5
Mooly is offline Mooly  United Kingdom
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It may just be the way they are.

Another thought, could the VA rating be being exceeded if you have a massive reservoir cap. The charging currents will be huge but the conduction angle relatively small. Could that high peak current be causing saturation ?
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Old 4th October 2012, 06:57 PM   #6
wayne is offline wayne  United States
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Quote:
Originally Posted by KatieandDad View Post
I've got a pair of 500VA transformers that are loaded to 400VA.

They both hum very slightly which can be annoying.

Would I get any benefit from encapsulating them in potting compound ?
Put them in a box that is non magnetic cover with sand and fill with melted paraffin. Potting compound still mechanically couples the noise.
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Old 4th October 2012, 07:02 PM   #7
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Quote:
Originally Posted by Mooly View Post
It may just be the way they are.

Another thought, could the VA rating be being exceeded if you have a massive reservoir cap. The charging currents will be huge but the conduction angle relatively small. Could that high peak current be causing saturation ?
I am using 3 x 33000uF per rail. (12 caps in total).

I might try some larger transformers if I can get them in the chassis.
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Old 4th October 2012, 07:22 PM   #8
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try a DC block. They are relatively inexpensive. Only mod I would make is to add a parallel film cap of the highest value you can find.......point of marginal return would be around 250uf on the film bi-pass caps.


The ESP images have been removed as Rod Elliot specifically asks that any reference to his work be linked directly to his site.
http://sound.westhost.com/articles/xfmr-dc.htm
Attached Images
File Type: gif DC Blocker.GIF (8.2 KB, 144 views)
File Type: gif DCblock.gif (19.8 KB, 143 views)
File Type: jpg DCBLOCK.jpg (10.8 KB, 142 views)
File Type: gif dcblocker.gif (2.8 KB, 139 views)

Last edited by Mooly; 5th October 2012 at 06:47 AM. Reason: Removal of copyright material.
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Old 4th October 2012, 07:28 PM   #9
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I'll try a DC blocker as suggested, it's easy enough to iron out that possibility.

Can anyone tell me why circuit 4 above works. D1 and R2 surely conduct into destruction on the positive half cycle.

Last edited by KatieandDad; 4th October 2012 at 07:30 PM.
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Old 4th October 2012, 08:16 PM   #10
DF96 is offline DF96  England
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Are they mounted on rubber washers? This can help with mechanical hum, but make sure you ground the frame.
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