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Old 27th February 2005, 08:58 PM   #1
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Default transformer isolation

Hi,

I've been building a p3a (80w) for some time now, and finally got to the boint that it works. Almost. The toroidal transformer (Avel Lindberg Y23 35-0-35 250VA) is giving off some nasty RFI which the input picks up, and as a result, one channel hums when the input wire is in the position of where I want it to be. I moved the wire around and found a position where there is little noise, but I'd rather not have any at all. Does anyone know of a good way to shield the transformer so thre is no noise?

I can post pics if needed.
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Old 27th February 2005, 09:08 PM   #2
DonoMan is offline DonoMan  United States
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I'd say try caging the transformer, put some metal around it... or you could get a new transformer with less bandwidth... You can also try a small non-polarized capacitor on the mains (like .1uF or something) - just make sure the voltage rating is really high (I'd say like 1000v, just so it won't easily burn even if your amp gets a spike)
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Old 27th February 2005, 09:13 PM   #3
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I have some metal foil tape, wondering if that will be fine. Do I need to connect the tape to ground?
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Old 27th February 2005, 09:16 PM   #4
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I'm not sure if it will be enough... Can't hurt to try it, though. And go for it on the ground, can't hurt either, as long as you don't short anything with it
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Old 27th February 2005, 09:20 PM   #5
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Will try that and the .1uf cap. Hope it makes a difference; shielding the inputs certainly didn't.
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Old 27th February 2005, 09:36 PM   #6
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Is your problem RFI or hum? These are two totally different things.

Have you isolated your signal input sockets from the case? Are you running screened leads from the signal input sockets to the PCB?

Aluminium foil/tape will do nothing for magnetically induced noises.
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Old 27th February 2005, 09:40 PM   #7
DonoMan is offline DonoMan  United States
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I assume he is sure that it's the transformer, since he said moving the wires around changes it...
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Old 27th February 2005, 09:57 PM   #8
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It definitely is the transformer. The input wire has a position where it picks up almost no noise. My signal input sockets are isolated and the leads are coaxial.
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Old 27th February 2005, 10:08 PM   #9
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If the transformer is radiating noise, then there must be noise entering it. Thus I would suggest that your best bet is filtering the incoming mains.
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Old 27th February 2005, 10:59 PM   #10
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I put a .1uf cap on the mains, parallel to the trafo. No change; noise is still being radiated.
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