How to calculate db drop at balanced signal

[axl85]

Hi Guys, here is my problem:
I have a balanced signal coming out of my audio interface (Apogee Symphony) feeding into my active monitors. I want to attenuate the signal by let's say 1db with a resistor between the positive and the negative pole of the balanced signal.
How can I calculate the resistance of that resistor...? Is there any formula I could use?

Thanks a lot for your help!!

 

[user 37518]

Here you go Uneeda Audio - Build your own attenuator pads

 

[axl85]

Here you go Uneeda Audio - Build your own attenuator pads

Thank you so much!!!