An audio line is a cable transferring an audio signal from the output of one device to the input of another. Normally the input impedance is fairly high, >1k for mic preamps and >10k for line inputs.
If the impedances in both ends of an audio line are high, any noise current picked up by the cable will be easily audible, as the current will generate a significant voltage across the input resistor. But if one side is low impedance, the noise current will be short circuited and no voltage will be generated.
Even if the impedance of your output amplifier is fairly low, the use of an LL1540 will add almost 3k to the output impedance, which in most situations is far too much. In addition to the noise problem above, you will also get a certain signal loss across the LL1540, in particular if the input impedance of the next device is less than 10k.
A better choice is one of our general purpose transformers (LL1527, LL1588) or a dedicated line output transformer such as LL1517 or LL1585.
