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KatieandDad 4th October 2012 06:26 PM

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 ?

Mooly 4th October 2012 06:32 PM

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.

KatieandDad 4th October 2012 06:36 PM

The humming doesn't seem to change regardless of what else I have running in the house.

KatieandDad 4th October 2012 06:37 PM

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.

Mooly 4th October 2012 06:46 PM

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 ?

wayne 4th October 2012 06:57 PM

Quote:

Originally Posted by KatieandDad (Post 3190062)
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.

KatieandDad 4th October 2012 07:02 PM

Quote:

Originally Posted by Mooly (Post 3190093)
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.

speakerfritz 4th October 2012 07:22 PM

4 Attachment(s)
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.

:cop:
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

KatieandDad 4th October 2012 07:28 PM

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.

DF96 4th October 2012 08:16 PM

Are they mounted on rubber washers? This can help with mechanical hum, but make sure you ground the frame.


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