Transformer HUM.

Hello folks,

Need some help in trying to eliminate transformer hum. It's not a buzz, it's a hum. It's audible from the transformer itself, and also on speakers, from a meter away. Transformer is 500VA 2x25V Nuvotem Talema:

0500P1-2-025 | 2 Output Toroidal Transformer, 500VA, 2 x 25V ac | RS Components

What I tried so far:

1. Steel shielding cover placed, no change
2. DC blocking circuit from here: Mains DC and Transformers, no joy.

The only thing left that comes to mind is to purchase more expensive transformer from RS which is specifically declared as very low induced noise:

0500P1-2-025K | 2 Output Toroidal Transformer, 500VA, 2 x 25V ac | RS Components
 
IMG_6266.JPG
 
The transformer you have looks like a potted toroid type.
They are normally low noise.
Make sure there is no electrical connection to the fixing bolt in the centre of the transformer, if there is it will produce a noise as it is a shorted turn.
There is also a connector block that looks like it has been screwed to the transformer .... I hope not!
 
The transformer you have looks like a potted toroid type.
They are normally low noise.
Make sure there is no electrical connection to the fixing bolt in the centre of the transformer, if there is it will produce a noise as it is a shorted turn.
There is also a connector block that looks like it has been screwed to the transformer .... I hope not!

The transformer is the one from the first url, with added outer steel shell. PCB is attached to outer shell, not the transformer itself. So it's not potted, nor declared as "low hum/low noise", unlike the one from second url. There is no connection from center screw to outer shell. It hummed the same way before shell was added anyway.
 
Did you tightly twist all relevant pairs of cables together?
Such as:
Twisted pairfrom transformer to rectifier bridge?
From rectifier to the input legs of filter cap?
From filter cap output legs to amp?
...
Did you treat each of the filter caps as a Four legged device?

1. yes
2. they are on the same board, very close.
3. too short to twist.
4. ?
 
How are all the ground returns mapped out?
Speaker return?
Zobel return?
Input stage differential pair return?
Nfb decoupling cap return?
Rail decoupling cap returns?
How are these connected to mid point of main PSU filter caps? What sequence?
...
No offence. Just fishing for possible trouble spots.
 
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