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Old 21st May 2003, 03:09 AM   #1
JoeBob is offline JoeBob  Canada
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Default Shielding transformers

I've got a hybrid MOSFET/tube amp, I use heatsinks as sides, with the FETs mounted on PCBs to the sinks, on the top there is all my tube circuitry, this is a pain, since the top has components that are attached to the sides, front (power switch) and back (input connectors), but it looks good so it's worth that little extra pain when I open and close it for changes.

Now that you've got an idea of how it's built this is my problem: I get hum, loud hum, and I was working on other aspects and now I've got to work on the hum. After toying with grounding, and the placement of the psu choke and transformer for my tube section, I arrived with no improvements. Until I noticed that if while on, I lifted the top (the metal plate with all the tubes and tube circuitry mounted on it) up above the rest of the chasis, the hum gets less and less obtrusive, until it all but dissapears.

My guess is that the toroidal power transformer is the cause of the hum. It's a 750VA, not shielded torroid that is mounted almost right in the center of the chasis, the sides being only slightly apart from it. So I guess it's interfering with the caps or cables, or maybe the tube sockets themselves, I don't know. I just know that the greater the distance, the less the hum, and since the hum is proportional to the distance and the magnetic field is inversely proportional to the distance, I'm guessing that the hum is inversely proportional to the magnetic filed. So I come to the question of shielding.

How should I go about shielding it? It's already mounted inside, and it's not affecting the circuitry next to it, only above it. Would an Al sheet, grounded work well enough? Or should I go with copper? I could use steel if I had to. I just don't know how much I would need for a transformer of that size. For all I know, maybe Al or Cu foil would be enough. I'm looking for guiding so that I don't waste alot of time persuing somethat that is doomed to failuer only to start over with a thicker and different shielding metal.

That is of course, if you all agree that it's a possible source of the problem, maybe you have other explanations and I'd be happy to hear them...
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Old 21st May 2003, 03:20 AM   #2
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Is your top plate grounded?

se
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Old 21st May 2003, 03:31 AM   #3
JoeBob is offline JoeBob  Canada
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The whole chasis is grounded, to the mains earth.

Although, when I say I lift the top, I'm just lifting one side of it, letting the other rest, afterall it is connected with wires to the circuitry in the chasis.
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Old 21st May 2003, 03:40 AM   #4
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Quote:
Originally posted by JoeBob
The whole chasis is grounded, to the mains earth.
What about your power supply and signal reference grounds? Are they tied to the mains earth as well? Or are they lifted?

Quote:
Although, when I say I lift the top, I'm just lifting one side of it, letting the other rest, afterall it is connected with wires to the circuitry in the chasis.
Understood. Thanks for the clarification.

se
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Old 21st May 2003, 03:49 AM   #5
JoeBob is offline JoeBob  Canada
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They too are tied to the mains earth, although I tried lifting them, yet that didn't cure the problem.
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Old 21st May 2003, 04:59 AM   #6
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Quote:
Originally posted by JoeBob
They too are tied to the mains earth, although I tried lifting them, yet that didn't cure the problem.
Ok. Just making sure it was tied to your power supply and signal reference grounds to rule out it acting like a big fact capacitor plate.

I went back and re-read your original post. I take it that your tube and FET circuitry is basically sitting on top of the transformer? Over the center of the toroid? If so, that's perhaps the worst possible place you could locate it.

Any chance of moving the transformer into a separate chassis?

If you're wanting to shield it, aluminum or copper foil aren't going to do much with regard to the magnetic fields. Other than MuMetal (which would just saturate if used by itself) your other option is steel. The thicker the better.

Ideally would be a combination of steel and MuMetal, with the steel closest to the transformer so it reduces the magnetic fields enough that the remaining field won't saturate the MuMetal.

And make sure that whatever you use, you ground it as well so it will keep electrostatic coupling down.

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Old 21st May 2003, 01:52 PM   #7
JoeBob is offline JoeBob  Canada
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I didn't think when I put them right above the torroid. I'll try shielding, but if that doesn't work I guess I'll just ship it to another chasis. Thanks for the input.
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Old 21st May 2003, 02:30 PM   #8
JoeBob is offline JoeBob  Canada
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Thinking about it, I guess I shouldn't even bother trying to shield it. But if I place the toroid out of the chasis (kinda a waste of space in the chasis then... oh well) can I leave the primaries always plugged in and switch the secondaries on and off? I ask because I already have a power switch on my amp and would like to keep it there and not on a seperate chasis for the transformer (that I'd make small as possible and throw behind the amp).
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