Noisy transformer

[evanc]

let me start by saying I am very new at this DIY audio stuff. Maybe this is something I should know, but I don't.

some time ago I built an aikido pre amp. I got the all in one board and parts from glassware audio. I used an Antek toroidal transformer 300 VA 230 volt. So far so good.

recently I finished a salas phono pre amp and shunt regulator. Still good. I noticed the phono pre got quieter when I put it's transformer in it's own box

The Aikido is fairly quiet, but not dead quiet, so I figure I should put it's transformer in it's own enclosure. I go ahead and find a steel box that just fits over the transformer and still fits in the preamps chassis. Great I think until I turn the thing on. The transformer buzzes and the center bolt gets hot?! I unplug it and check that I didn't short something out. I didn't. Take the box out It works fine put it in and it buzzes. Tell me why.....
Thanks, Evan

 

[Bill_P]

The chassis, box and bolt form a shorted turn. Insulate the bolt and try it again.

 

[evanc]

Bill, thanks for the time. You mean the box is acting like part of the transformer? I'm not sure I get it, but I will try isolating the bolt. I thought the box should be grounded to work as a better shield.
thanks again, Evan

 

[Bill_P]

You can keep the box grounded. Your present configuration has a continuous metal path through the center of the transformer and around the outside. This path needs to be broken somewhere and insulating the bolt from the box with a shoulder washer is probably easiest.

 

[evanc]

Easy enough...Thanks again