Newbie Mains Transformer problem

[simon111]

Excuse this post in case it's all ready been answered or silly ( I have checked using the search function). I am currently putting three effects in one box, and as such I am building in a PSU. Like the geo spyder one. Problem. Mains Hum. At the moment it all isn't in a shielded box, (does this effect the mains hum bit?). But once the transformers are further away than around about 1 foot away from the effect circuits they don't seem to be effected by the transformers. But i need to fit it in a fairly tight box. Is there anything I can do to reduce or better still eliminate any mains hum?
Any help to solve this would certainly make my already drilled metal enclosure useable for it's intended function!
Simon111

[brett]

Hi.
There's 2 very simple things you can do.
First, build a Faraday shield around your transformer(s).  The shield is just a sheet of metal or metallic mesh surrounding the transformer(s).  Ideally, these shields are grounded to the same point as the PS and signal (some kind of star ground).  In practice, it isn't going to matter too much. 

Second, all signal cabling should be shielded cable.  Earth the shielding at one end only.
good luck.

[Paul Perry (Frostwave)]

If it is magnetic fields getting into a grounding loop, then mesh shielding won't help. Changing the angle between the transformer and the board might (but, likely to be difficult or impossible).

[brett]

Hi.
Paul makes a good point.  Try to use one, and only one, grounding point.  However, use separate lines to ground PS and effects to that point.
Effects can share earth lines, but never with a power supply.  Heavy earth wire is always a good idea (I use speaker cable inside my boxes).

Also, try placing your rectifier and smoothing caps as close as possible to the transformers.  Twisting the low voltage AC lines together can also help (use 1 to 2 turns per cm).  Use heavy wire for these lines.

cheers

[R.G.]

Both Paul and Brett have good ideas.

There is one other thing to be done. If reorienting the transformer, reworking the wiring, and wire dresing don't do it, you can provide some magnetic field shielding with iron or steel sheet between the transformer and the circuits. This is similar to the "faraday cage" in that what works best is a completely covered steel box around the transformer, but even a single sheet of ferrous material will help. This works by being a low-impedance shunt to the magnetic field, so the field preferentially stays in the iron.

It's better if the ferrous material is soft iron - which you can't get, so use the softest steel sheet you can get. Use the thickest steel you can get and put into place.

You can also try a shorting band of the thickest copper sheet you can get soldered into a loop all the way around the transformer's outside. This shorts the external hum-field flux and forces the M-field to mostly lie within the copper loop.

If by "Spyder one" you mean you rewound a flat pack transformer, I'm a little surprised at the problems with hum. Those things are semi-toroidal, and are usually better for hum than ordinary shell-type EI stuff.

[simon111]

Thankyou all for your assistance here. I will be trying putting it in an enclosed metal box tomorrow. In response to RG's comments, the transformers I am using are not the type specified on the Spyder circuit. I am using PCB mounted ones. I take it these are not ideal then. I will try the above before changing them to the specified type. I only mentioned the spyder circuit in terms of the design of the isolated outputs. Sorry for any confusion caused.
Simon111