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Old 2nd April 2001, 05:05 AM   #1
grataku is offline grataku  United States
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My newly built bi-amp system is all wired up and ready to go and there is a hum from the transformer. Damn!
Now the choices are:
1)remove the transformer from the box (which removes the hum) and put it in a separate box.
2) built a sub-enclosure to be put inside the current box to shield the transformer.
Even though physical distance between the boards and the offending transformer is probably the best solution, I would like to try and keep everything in one box so here are the questions:
-Is this really going to work, has anyone done this successfully before?
If so then:
What would be the best material to build such shielding enclosure (copper,aluminum)?
-What thickness material should get the job done?
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Old 2nd April 2001, 06:41 AM   #2
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grataku,
Congrats on the system.
1) Try turning the transformer this way and that. Even to the extent of turning it on its side. Yes, this could lead to a nuisance of a mounting problem depending on the orientation. Cross your fingers that this works.
2) If you shield it, remember to ventillate well.
3) If all else fails, go for the external power supply route. Yeah, you've got that inconvenient umbilical, but it works.
Good luck.

Grey
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Old 2nd April 2001, 01:21 PM   #3
grataku is offline grataku  United States
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Grey,
it's a toroidal transformer and I can't turn it on the side so it's pretty much shield it or get it out of there.
The big question is would it really work or would I be wasting my time?
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Old 2nd April 2001, 03:57 PM   #4
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grataku,
Sorry to see you in dire straights, man. Actually, when I wrote the above, I had toroids partly in mind. The isolation transformers I've mentioned elsewhere that I use for power line conditioning are toroids. They came in (got them surplus) with an aluminum U bracket that straddles the toroid. The top of the U has a hole on each side for the through-bolt for the toroid. The bottom of the U has four holes for mounting to a chassis. Can you visualize what I'm describing? If not, say so, and I'll try again.
Anyway, such brackets are sure to be available on the market somewhere. On the other hand, given that it's just (I'm guessing) .050" aluminum, it wouldn't be all that difficult to bend your own.
Another possibility: L brackets from your friendly Lowe's or Home Depot et. al. You'd probably need fairly large ones, but two of them back to back should do the trick.
Try manipulating the transformer first. If you can't find a quiet position, then it's a moot point and we'll fall back and try something else. Also try rotating the toroid around its axis. Costs nothing, and might do the trick.

Grey
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Old 2nd April 2001, 09:00 PM   #5
grataku is offline grataku  United States
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Grey,
it's a pretty big transformer, 500VA about 5' 1/2 diameter 3' high there is NO room in the box to put it transversally.
I would have no problem building a little sub-enclosure if I knew it was going to be effective. Otherwise I might as well just built an outer box a little bigger with the same style as the main enclosure and put both the power amp the auxiliary transformers in there.
I am looking for someone that has succeded in taking the "magic" out of shielding transformers. I have seen a bunch of regular core transformer with a copper foil shield around the coil. I tried that in the past and that didn't really do much. I was wondering if anyone had better luck with other materials and(or) methods.

grataku
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Old 2nd April 2001, 10:11 PM   #6
blmn is offline blmn  Brazil
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Grataku,

Did you measure the noise? I mean, are you sure it comes from the transformer? Hum from the transformer has frequency of 60Hz or 50Hz, and from the rest of the power supply has usually frequency of 100/120Hz.

If you have the second kind of hum the new positioning or
shielding will not help.

Regards






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Old 2nd April 2001, 11:32 PM   #7
grataku is offline grataku  United States
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blmn,
I could not measure the frequency of the noise. I borrowed this Tectronix LCD scope and with a x10 probe on the maximum gain 5mV setting I couldn't see it, but I could hear it! Anyways, by getting the transformer out of the box the noise stops, so I am thinking I am on the right track.
This is my first project in 10 years and I have been having only minor problems considering I started from unknown schematics, designed double face boards and everything worked the first time around!
However, the devil is DEFINITELY in the details! I had to get rid of ground loops, I am working on the transformer noise, next in line will be dealing with relays to negotiate smooth turn on-off operation.
Chances are the final product will be different from what I have anticipated!

thank you
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Old 3rd April 2001, 02:08 AM   #8
blmn is offline blmn  Brazil
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Grataku,

I don't know if it's possible, but can you remove the speaker from the box maintaining the transformer inside to observe the hum behaviour in this case? I had a similar case some time ago with a disk drive and a 60Hz transformer. I don't know if it is possible to happen with speakers, but, you said you could not measure the hum.

Regards,

blmn
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Old 3rd April 2001, 02:39 AM   #9
grataku is offline grataku  United States
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blmn,
funny you should mention that, I am using resistors and an old tannoy dual concentric speaker for my tests and it's in open air. How easy it is to hear a 60Hz noise especially in open air? I don't know. All I know is that I heard that kind of noise many times before including in many commercial products.

thank you
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Old 3rd April 2001, 03:33 AM   #10
blmn is offline blmn  Brazil
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Grataku,

I think it's not easy to hear this frequency in open air, but 120Hz is not too hard to hear. I was thinking in some kind of magnetic interaction between the speaker and the transformer. Another possibility is any inductor or wire wound resistor near the transformer, but, in theory this kind of problem does not occur with toroidal transformers. long wires are possible troublemakers too.

Regards and I wish you luck, because this kind of problem is not really easy to deal with.
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