Torroid Transformer Hum

I think "DC" is a simple term for a complex phenomenon. However the simple series capacitor with bypass diodes "DC Trap" does appear to work for some.

Chris, we were just giving Peter the benefit of the doubt. Perhaps Peter does have a buzzing transformer?

So what makes transformer laminations buzz?

Could it be that the laminations aren't stuck together with enough varnish?

Varnish does help, but it is magnetisation current which causes one transformer to buzz more than another.

A high magnetisation current results in a lot of buzz, and conversely, low magnetisation current results in a quieter transformer.

Magnetisation current depends on the quality of the lamination materal used. When I'm talking to my manufacturer we just call it steel because we're past that "one upmanship age".

Poor quality transformer "steel" takes a lot more magnetisation current, and most off-the-shelf transformers from the "shed" type suppliers (RS etc) are made using inferior stuff from you know where.

Our transformer manufacturer is also an aerospace/military supplier, and is also quite close to a source of specialist "steels" (the benefits from being near Sheffield ... ).

Our transformers therefore buzz less. But another benefit is that more power is available at the secondary, because less power is wasted making the transfomer transform.

And for all who did ONC, you know, where the electrical guy goes on about transformers being a "perfect machine" - well, they're not!

It does seem that the hum is above that that should be acceptable, RS will most likely replace it as it should be classified as faulty.
I have previously used Antrim transformers, now part of Trans-Tronic I understand. Those I have (25, 50 and 300VA) don't hum. The smaller ones have potted centres which makes mounting easier and perhaps helps reduce hum. I don't know if they sell direct, ut I'm sure they would advise where to buy.