Sub-enclosure for power transformer

grataku

Member
2000-12-31 9:31 am
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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?
 
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
 
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
 

grataku

Member
2000-12-31 9:31 am
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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
 

blmn

Member
2001-02-01 2:43 am
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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
 

grataku

Member
2000-12-31 9:31 am
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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
 

blmn

Member
2001-02-01 2:43 am
.
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
 

grataku

Member
2000-12-31 9:31 am
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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
 

blmn

Member
2001-02-01 2:43 am
.
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.
 
Actually, toroids do have magnetic fields--they're just less prominent than EI transformers. Somehow, they got this magic reputation that's a bit better than reality.
grataku, I hate to say it, but it may be time to go the external route, even though it's a PIA. (There's another mystery for you--why does the "T" get left out of the acronym?) There might be other solutions, but it's going to be a bear trying to sort through the possibilities without seeing the situation first hand. That's the downside to this spread-all-over-the-world thing: Most of us will never meet. It could be something as simple as a cold solder joint on a ground connection (of the high resistance sort, not an open connection or surely you would have noticed by now).
Steel or iron are your best bets for shielding, but some feel (not without reason) that it's a Bad Thing to have ferromagnetic material near a circuit that thrives on fluctuating high currents. It's a tradeoff for you to consider.
If you go the external route, you might want to use a little heavier wire for the umbilical in order to keep resistive losses down. Twist the wires (rather than using just straight zip cord, for instance) to reduce AC fields.

Grey
 

grataku

Member
2000-12-31 9:31 am
-
Thank you all for your input.
This is got to be the most useful websites with the best user community on the net!
I am going to built the transformer enclosure tomorrow, I already have the design in my head. Grey, the point you made about the voltage drop is very well taken. I'll have to find some surplus cannon connectors.
 

blmn

Member
2001-02-01 2:43 am
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GRollins,

In many catalogs and books you can find comparative charts on core shapes that states poor shielding performance for EI or E types and good shielding characteristics for toroids. In theory it really not happens and in the pratice it is much less proeminent than in EI transformers without belly bands or other kind of shielding.

I have already done many designs using toroids in switched mode power supplies for audio amps and I had very good shielding characteristics with excellent noise results even in compact mounts.

In fact, the grataku's problem is unnusual.

Regards
 
blmn,
We're saying the same thing. Toroids are better than stacked laminates. (Just more expensive.) My Thresholds have large toroids up front (away from the front end boards, which may be the problem here, dunno without seeing grataku's setup) and zip for hum. I'm all for toroids, I only wish they were more widely available in the voltages I need for tubes. (Yes, there are a few, but not like the selection at lower voltages.) My only point is that they aren't perfect. In fact, I think I recall either Self or Slone (don't remember which...you've got those books, help me out here...I turned 'em back in to the library) saying that it might be necessary to fiddle with the transformer placement.
And you know what happens when we get around to saying the same thing...time for you and me to go have a beer.

Grey
 

blmn

Member
2001-02-01 2:43 am
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GRollins,

Sorry about the last notes (the last in the other thread about ZEN Vs. Opti-mos included, right?).Your land has good beers and I think it was a very good invitation. I'm a little bit 'hard' in my statements and for sure it is an attitude used by me on the debates only. Nothing personnal here (seriously).

Regards