Advice for Newbies re: toroidal mains transformers

Experienced engineers may choose to skip the following information, I?m posting it here for newbie diy-ers that it may be useful to.

On the old message board I made a couple of posts relating to positioning mains toroids for least hum; true to nature I managed to disregard my own advice in the last few days...

A friend asked me to evaluate and comment on a prototype stereo preamp that had been sitting on the shelf for a while. As with many prototypes, the casework was quite rough and ready. Upon listening to the unit I found that it sounded superb, but had a frustratingly loud hum. With it being a prototype I suspected the grounding-scheme. Spent days re-jigging the ground scheme - bleeping all connector bodies, minimising cable lengths to the star-ground, moving the star point around the ground-plane, experimenting with local grounds as well as the star-ground etc etc, but to no avail.

After much head scratching I decided upon the obvious tactic of loosening the bolt in the mains toroid and rotating the body, thinking this would just be too easy if it solved the problem... Turns out that the whole problem was inductive hum introduced by the field of the toroid! Weirdly, I found that the best position for the toroid was with the cable outlet facing the preamp board, and moving it in one direction introduced the hum on one channel but not other, and vice-versa if rotated the other way. Eventually I found the quietest way was to turn the toroid upside down with the wires facing the pcb (I used a 10mm thick rubber washer I had lying around to prevent the leads being squashed against the chassis).

The moral of the story? Whenever you build anything using a toroidal mains transformer, ALWAYS loosen off the bolt and rotate / invert until you get minimum hum.

This hum really was pretty bad, I cannot believe a procedure as obvious as this would cause such a profound improvement.

I?ve read a couple of posts complaining of hum in solid-state diy projects recently, if you haven?t already, try moving the toroid around / inverted etc, it could be just the cure you?re after.

Justin

thermionic,

I guess I can be considered one of the newbies around here and your comments are much appceciated.

One question though. If I do as you suggest and find the point of rotation/optimal position is it still beneficial to add some sheilding around the xfmr and if so what metal, copper, aluminum, or Mu metal.

thanks

James
Big Ugly Van Audio

In many cases where an inboard psu is used it may be beneficial to add shielding to, or partition a mains transformer if it is anywhere near audio circuitry. To my knowledge mu is the best shield (if you've got the corn to pay for it). Steel will be good, and alloy acceptable.

J

thanks for the advice. It has been proofen again simple things can achieve alot!

You can get a mu metal shield from many transformer companies. Cost ca. 100 ?. Or you can put the transformer outside in an extra housing.

do you use audio transformers inside your preamp? The shielding of that transformers seams to be the weak link.

Where to get MU metal? How much do you get for 100euro? What's the dos & don't with MU? Something about not bending it..?

Michael Percy has TI Shield, which is a copper alloy designed to shield RFI and EMI. It's expensive, but not as much as mu metal.

http://www.percyaudio.com/

You have to download the pdf catalog.

edited 6/30/04 (fixed web address) with mea culpas to all.

-neil

Dead link :?

Thanks Justin,

I'll link to this in the Newbies META.

:thumb:

Mark

http://www.ektypus.de/mu_metall.html
http://www.ektypus.de/guss_trafos.html

So if you bust open an old iMac, there is massive shielding in the case that looks like, kinda resembling, I could be wrong, but might it possibly be Mu Metal? Maybe?

:?:

Looks like John Hardy already knew this. Here?s a quote from his pdf document about his ?M? pres:

?Toroidal Power Transformer with additional silicon-iron shielding eliminates hum problems caused by stray magnetic fields? Each transformer is carefully tested for stray magnetic fields under worst-case full-load conditions. The optimum rotational position is determined ( the italics are mine), then the silicon-iron shielding is added to assure hum-free performance??

this is also true with normal power transformers.

This was the problem in the EQ I made. I did the same things, re grounding changing star ground points...finnally I just unbolted the power transformer from the back of the chassis and rotated it around a bit. The hum lessened at certain positions. so I just built a seperate box for it with a heavy duty multi-wire cord and a big military looking connector.
problem solved.

even a steel chassis wil not sheild the magnetic field transformers create.

> heavy duty multi-wire cord and a big military looking connector.

Do you have the part number for the connector? How many terminals does it have?

[quote author="bluebird"]even a steel chassis wil not sheild the magnetic field transformers create.[/quote]
FWIW, I've actually heard several times that steel isn't a good metal for "shielding" and that compared to aluminum it will pick up more interference.

[quote author="bluebird"]even a steel chassis wil not sheild the magnetic field transformers create.[/quote]
FWIW, I've actually heard several times that steel isn't a good metal for "shielding" and that compared to aluminum it will pick up more interference.

In discussing this, it is important to differentiate between electrostatic and electromagnetic hum induction.

The electrostatic part of the noise can be reduced with a thin layer of any conductive material - a barrier of aluminum foil will do. The electrostatic noise of a toroid transformer is highest in the direction of the wire breakouts.

The electromagnetic part of the noise is much harder to control. Here you will need to use a ferromagnetic material like mild steel, iron, nickel, or mu-metal. Mu metal is prohibitively expensive these days - it seems like extremely few companies manufactures it still. My transformer manufacturer was quoted 900DKK(120?)/Kg for simple sheets! But mu metal surely is effective - it has the shielding properties of iron about ten times it's thickness, so it's very space saving.

The hard part of shielding power transformer hum is that it is a relatively low frequency, and needed shielding material thickness for same attenuation grows exponentially with reduced frequency.

Maybe it should be pointed out here, that toriods have vastly lower stray noise fields than any other construction tecnique due to the closed magnetic field. It is rare that you have to shield a toroid - even when you have audio transformers in the same box.

And btw, if you need to shield a toroid, remember that you under no conditions can allow both of the ends of the mounting bolt to connect to chassis. This would be a shorted winding on the transformer..

Jakob E.

I was just going to mention tin foil.

Ive solved lots of wierd buzzes in my boxes by simply taking some aluminum foil, folding it over so it stands sort of like a pyramid. I plug in the output of the box (with nothing in the input) crank the gain and put on some headphones and move the tin foil around until the bozz goes away. Works suprisingly well. A little hot glue holds it in place just fine. Looks really silly, but for DIY, its a great band aid for poor planning. Ferrite beads on the mic inputs are also a great fixit for RF noise. Between the two Ive solved a lot of annoying garbage that would have eventually inspired me to redo the entire wiring.

dave

Can anyone explain to me exactly how ferrite beads work?

..throw a wire through a ferrite bead, and you have a half-turn coil, effectively forming an inductor. In series with the signal, it forms a low-pass somewhere up where RF starts to get annoying..