Strange transformer hum

[Atilla]

Nothing related to the transformers themselves was causing this, I tried quite a few things to rule them out.

It is possible that the lid might touch those cables, so I'll take a more careful look at them when I come back from holidays. I need to secure them better and pair them up in heat shrink anyway, so I'll position them in a different way. I'll report back my results if that helps.

[robmil]

The transformer is a toroid. The secondaries are quite a distance from the lid (7-8cms).
The secondary cables are quite long - 13cms and run along the length of the chassis bottom. The secondaries are twisted together which has reduced the hum A LOT. The transformer is 50va and only about 5cms high.

It's only when I bring the lid (aluminium) close to the chassis when the hum (50hz) starts to be audible through the loudspeakers.

Would it help if I shielded the secondaries using a steel woven screen and attached the screen to the chassis.

Happy new year

rob

[godfrey]

Originally Posted by robmil

It's only when I bring the lid (aluminium) close to the chassis when the hum (50hz) starts to be audible through the loudspeakers.

That's different. The OP had mechanical hum from the amp itself. If you're getting hum through the speakers the pre-amp circuitry must be picking up a hum field. Maybe some steel or mu-metal shielding would help?