Lundahl LL1545A and 50Hz hum

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Hi to all.

I decided to put a Lundahl LL1545A at my dac output (voltage output). It has a 230 ohm Zo with 1.9Vrms at 0dB.
The dac output is unbalanced and at the first time I tried the R2U->L2U (1:1) connection, as described at the datasheet. Because I haven't the dac eclosed in metal case, put the earth of 11,13,14,15 to the IEC earth.

The datasheet of LL1545A is here: http://www.lundahl.se/wp-content/uploads/datasheets/1545a.pdf

Unfortunately, there is a huge hum of 50Hz that appears at the 20-20KHz bandwidth.
I am very curious why this happens, because in theory this trasfo has a complete isolation btw input and output.

At the attachment is the FFT capture, with red is the Right channel of dac output, with the black is lundahl Right channel also.
The connection cable is a shielded coaxial
 

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You should 'earth' the 15 later in your groundpath.
But anyway since it is 50Hz and not 100Hz then it cannot be rectified powersupply noise you are picking up, so this would probably not change much.

It must be something that picks up the transformers magnetic field, so you must have an ground-loop somewhere. You could try and remove ground from all those mentioned pins and see if the hum is gone. If it is then re-establish them one by one until you 'hit' the one that is causing it.
 
If I am reading/understanding the lundahl datasheet correctly they are describing it as an input transformer.
i.e. it is located at the input of the Receiver .

You seem to be using at the output of the DAC.

Input transformers should not be used as output transformers. They are different products.
 
AndrewT, the LL1545A is a general-purpose audio trasformer and of course can be used like an input transformer due to its high impedance. I have seen to used this on a dac output or the pre-input as well.

Nrik, the pin15 at the R2U isn't joined with the earth, it is joined only with the pin10 that signal comes.
By the way the asterisk that is over the signal gnd what is the meaning?
In my connection the pin10 is central pin of coaxial and the pin16 goes the signal gnd. At the output the pin1 is central pin of rca the pin3 the rca_gnd.

I tried with one lundahl to find the problem, I rised up the lundahl to take away from other parts and toroid trasfo but there is no any change.
Secondly, I removed the earth joined of pin 11,13,14 but no lucky.
If a ground loop is there why there isn't at the dac output at FFT (this with the red)?
At the dac output there are two parallel cables, one goes to rca and the other goes to LL1545A input.

The FFT measuring is with the EMU-0404 as ADC. This device has a gnd-lift option, but there is only a little improvment with this.
 

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OK, I have resolved the hum problem. It has gone completely.

I followed the directions of Nrick and joined the pin15 to the earth. Additional the signal ground of input-output joined with the IEC earth.

I done the input connection as seems exactly at the following icon.

An externally hosted image should be here but it was not working when we last tested it.

Now the last question: The output of above schema has a balanced way. There is a 15K resistor btw pins 1 & 3 of output, is it necessary because at the datasheet of LL1545A there isn't at the output.

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