Advice for Newbies re: toroidal mains transformers

[soundguy]

damn! I love this place. I always wondered how that worked. It DOES work though. Also works wonders with the panatape on panavision cameras to keep the motorboating off of the wireless headphone feeds if anyone works i the movies.

dave

 

[Michael Krusch]

If you put a shield around the power transformer, you can run in that same problem, if you put your box on top of another box with an unshielded transformer.
Better look for shielded audio transformers.

 

[zebra50]

A nice clear explanantion - thanks.
It's only 10.30 and I've learned something already today.
:thumb:

 

[Michael Krusch]

It exist a mu metal foil: http://home.t-online.de/home/electrosmog.com/Shielding.htm
But it's not nealty as good as a mu metall can.

 

[Rob Flinn]

I`ve used encapsulated torroidal transformers mounted on the outside of the box before.

 

[dale116dot7]

I've found a simple steel shield works well in conjunction with rotating to find the least hum. I mount the transformer, get the circuit working, then rotate it for least hum (and that's usually pretty low). Then I mark it, remove it, add a C-shape shield over the transformer, and bolt it down again. No more hum. My mic power supplies (which have an audio transformer in them) have two sections separated by a simple steel shield - for the same reason. I've also only noticed hum problems on mic preamps and power supplies. Even using steel as a shield, I find hiss comes up before any hum does after the shield, with no shield, the hum comes up about 3 to 6 dB more than the hiss.

I've also found another source of hum is coupling in the phantom power feed when I forget that RC filter (220R, 220uF) by each channel. It didn't show up on transformer coupled or dynamic mics but it did on transformerless ones.

-Dale

 

[Eric H]

Great thread.
Anyone have experience with the flat-pack style of transformer (side-by-side) when mounted inside a case? They are claimed to have a low emitted field (by manufacturers)
I'm asking because I already have several that would work well in mic-pre designs.

Thanks,
--Eric

 

[Eric H]

I've read that the shield you find surrounding the end of the CRT in many old monitors is mu-metal. Is there a way to test for this stuff, or easily recognize it?

-Eric

 

[smilinfu]

I did not realize ferrite beads came with different inductances. Is there a particular value either of you tend to use?

 

[John Hardy]

Regarding "mu-metal", first some trivia: It is actually a trade name for a material with about an 80% nickel content (Google search seems to indicate 77% nickel). Another trade name is "Hy Mu 80".

I talked to Deane Jensen when I was developing the M-1 in 1987. He explained that mu-metal is better for shielding at the "destination" (an audio transformer that is near a troublesome power transformer, for example), while silicon-steel is better at the "source" of the problem (the power transformer). Silicon steel is not as efficient as mu-metal, but it can handle much higher levels of magnetic field before saturation, so it is better suited for use at the power transformer. It is also much cheaper than mu-metal.

Regarding toroidal power transformers, they generally have much lower stray magnetic fields than the more conventional "E-I core" power transformers, but there can still be a big difference from one manufacturer to another.

John Hardy
The John Hardy Co.
www.johnhardyco.com

 

[Eric H]

[quote author="John Hardy"]

I talked to Deane Jensen when I was developing the M-1 in 1987. He explained that mu-metal is better for shielding at the "destination" (an audio transformer that is near a troublesome power transformer, for example), while silicon-steel is better at the "source" of the problem (the power transformer). Silicon steel is not as efficient as mu-metal, but it can handle much higher levels of magnetic field before saturation, so it is better suited for use at the power transformer. It is also much cheaper than mu-metal.



John Hardy
The John Hardy Co.
www.johnhardyco.com[/quote]

That's very useful, thanks, John,

-Eric

 

[Our Religion]

Fabulously useful thread.......man, I love this place.

Ric

 

[Flatpicker]

[quote author="John Hardy"]...Silicon steel is not as efficient as mu-metal, but it can handle much higher levels of magnetic field before saturation, so it is better suited for use at the power transformer. It is also much cheaper than mu-metal...[/quote]Thanks a bunch for the info, John. I?ve been curious about silicon steel since I read that you used it. Can we buy that in small quantities anywhere?

 

[John Hardy]

Thanks a bunch for the info, John. I?ve been curious about silicon steel since I read that you used it. Can we buy that in small quantities anywhere?

I am not aware of anyone who sells it in small quantities. The industry has changed over the years too. I get my shields fabricated 100 at a time, and the company has changed hands a couple of times. I just keep my fingers crossed that they don't disappear. I have small quantities of silicon steel in 4" squares and 1.5" wide strip from when I used to make my own shields, but I'm not parting with it under any circumstances.

A Google search would probably turn something up though. Good luck.

John Hardy
The John Hardy Co.
www.johnhardyco.com

 

[jrmintz]

www.mcmaster.com has some shielding products - I don't know if they'll suit the purpose, but go there and search for 'EMI-RFI'.

:guinness: :sam:

 

[SonsOfThunder]

The (wise and experienced) old engineer that I have the pleasure of working with, is fond of saying that a "bean can" is the best shield for a power transformer...its already round and you just cut it to the height you need! Fasten it around the trafo with a couple of dabs of silicone glue (making sure not to short anything!), solder a wire to the can and experiment with the grounding point.

Just don't ask me what qualifies as a "bean can" these days. Try it...it won't cost you anything!

:thumb:
Charlie

 

[clintrubber]

Hello,

I'll be fitting a 30VA toroid power-TX in a 1 HE box and will be taking care
not to create a shorted winding by letting the mounting-bolt touching the top-cover.

But how much of a gap is needed to be safe ?
Is some simple plastic isolation (say 0.5 mm) between top of the bolt & the bottom of the top-cover already enough to stay out of trouble ?

BTW, as an alternative I've been toying with the idea to use a few tiewraps for securing the power-TX, but the bolt is of course preferred.

Thanks,

Peter

 

[gyraf]

even small insulation is enough IF it is not going to be breached by the bolt over time. The voltage is very low (and the current very high)

 

[clintrubber]

Thanks Jakob, good news !
I'll be adding a thin but solid plastic plate then and all will be fine.

Regards,

Peter