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Old 10th January 2006, 07:37 PM   #1
amisdad is offline amisdad  United States
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Default Shielding a Power Tranny

I have built a 12AX7/EL84 SET, but need to eliminate some noise coming from the Power Tranny being too close to the output trannys. I had quite a bit of hum which was only noticable through headphones, but I moved the Output Trannys away which reduced the hum to a tollerable level though Headphones. I would like to eliminate it completely and I'm sure a proper shield would help even more. Is there a ready made, easilly available shield? What do you guys use? I could fabricate one, but what should I use?
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Old 10th January 2006, 08:58 PM   #2
anatech is offline anatech  Canada
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Hi amisdad,
Try rotating the power transformer, or the output. Nail them down when you have the lowest level of hum.

-Chris
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Old 10th January 2006, 09:03 PM   #3
amisdad is offline amisdad  United States
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Thanks for the reply. I have already found the positions with the least amount of hum which is very low. I'm sure if I had more room on my chassis, Id be able to get rid of it all, but I don't. So I'm thinking I need to make a shield. I've heard MU metal mentioned, but haven't found it yet. What exactly is it? Remember, I'm using this for headphones as well as Fostex/MLTL. The headphones are really sensitive and I want I silent noise floor.
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Old 10th January 2006, 09:33 PM   #4
anatech is offline anatech  Canada
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Hi amisdad,
Try a steel shield first. I believe that copper sheet around the power transformer may help too, solder the ends and make it a close fit.

-Chris
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Old 10th January 2006, 10:28 PM   #5
AJT is offline AJT  Philippines
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Default copper banding

this is it:
Click the image to open in full size.
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Old 10th January 2006, 10:41 PM   #6
anatech is offline anatech  Canada
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Okay, so remove it and have a listen (kidding!).

Do you have a piece of steel laying around to play with? Also, if you isolate the power transformer from the chassis, does the hum drop?

-Chris
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Old 11th January 2006, 01:15 AM   #7
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Several years ago I was working on a problem where the noise from the display (400Hz) was corrupting the VCO in a two way radio. Our mechanical engineering team had made some shields from several materials and I tested them all. The Mu-Metal shield worked the best, followed by cold rolled steel. I don't remember the details (20 years ago), but these two were the only materials that worked good enough. There were about 10 different ones including copper and aluminum.

Mu_Metal is a material designed for magnetic shielding. The metal case that a phono cartridge is made from is likely Mu-Metal.
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Old 11th January 2006, 08:09 AM   #8
beamnet is offline beamnet  Netherlands
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did they have a lead shield too?

lead whas the first thing thet came to mind for me..


Bas
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Old 11th January 2006, 09:57 AM   #9
EC8010 is offline EC8010  United Kingdom
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Lead is for nuclear stuff - a very different kind of radiation...
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Old 11th January 2006, 10:12 AM   #10
beamnet is offline beamnet  Netherlands
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really?


so that means that i DO have to buy that nuclear fusion lifetime power supply on e-Bay!! Neat!!





So it works for subatomic particles....does that mean it is useless for magnetic shielding?

my experiments only covered earthed sheets of pertinax unetched pcb's

they did not make an audible difference. I gave up on magnetic shielding because someone said it was useless, but now i think of it, they shield speakers with steel too!


I WAS goint to add a steel shield between my trannies and the electronics, now my hopes are up for effectiveness!

Bas
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