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Old 25th October 2004, 09:15 PM   #1
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Unhappy Transformer - EM hum - Help!

Hello all,

Recently I built a Kevin Gilmore Dynahi headphone amplifier. It is an excellent design, and I have been thoroughly impressed with its performance. Anyway, to my issue; prior to placing it within it enclosure (with about 1' of space between boards and psus) I encountered absolutely no hum at all. When the psus were placed in the same enclosure as the boards, and wired up the amplifier worked, but I noticed a light hum (from the headphones). These are regular (non-toroid) transformers btw. Following the advice of a friend I disconnected the input leads from the boards. The hum was reduced, but not eliminated. I also tried moving the psus around within the case, this also made a bit of a difference, but did not eliminate the problem.

I would like to avoid locating the psus outside of the case. There is a bit of a mechanical hum, but I am concerned with the EM hum. It is my understanding that Mu metal or a copper shield around the psus could help. However, I am unable to source it, and with the border delays between Canada and the rest of the world...

The person that made the enclosure for me offered to make a 1/4" thick wall with a small 3/4" grometted hole to divide the case from the PSUs would this help?

Are there any people located in around Vancouver that can give me a quick hand with this? Does anyone in my area have this Mu Metal stuff?


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Old 26th October 2004, 12:42 AM   #2
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Old 26th October 2004, 02:03 AM   #3
Mr Evil is offline Mr Evil  United Kingdom
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Are you sure it's caused by radiation from the transformer, not a ground loop caused by something contacting the case?

If it is indeed caused by the radiated magnetic field, then surrounding the transformer with soft steel would be your best bet. It may not be as effective a shield as mumetal, but it's an awful lot cheaper, making it practical to use thicker sheets, making up for it somewhat. Copper is of no use shielding against magnetic fields (it's good for electric fields though, of course).
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