Link transformers need deep and thorough understanding to give their best. This applies to source and load impedance, stray capacitance, floating/grounded sources and balancing of source and load impedance. Please see the trivial example where the link transformer is put into a soundcard loopback. The card has balanced input with 18k + 18k input impedance and SE output with 1k output impedance.
1) direct loopback, i.e. the transformer is sourced from 1k impedance
Please note the 50Hz multiples with 100Hz the highest.
2) soundcard output first goes to the audio buffer with gain = 1, output impedance = 50 ohm
Please note that the 50Hz multiples have disappeared. This is because the 1k source resistance in (1) caused gross impedance unbalance of the transformer input. Its impedance is complex, with capacitive component that brings the errors if heavily unbalanced. With the 50 ohm source impedance the situation is much better and the result is completely satisfactory. Everything can be explained.
Edit: This is even more pronounced when the high sensitivity microphone input is used. Again, a loop with transformer and 1k card output impedance and then the buffer with 50 ohm output impedance is added. The suppression of mains components is considerable and their origin is only because of the transformer parasitic impedances mismatch.
1) direct loopback, i.e. the transformer is sourced from 1k impedance
Please note the 50Hz multiples with 100Hz the highest.
2) soundcard output first goes to the audio buffer with gain = 1, output impedance = 50 ohm
Please note that the 50Hz multiples have disappeared. This is because the 1k source resistance in (1) caused gross impedance unbalance of the transformer input. Its impedance is complex, with capacitive component that brings the errors if heavily unbalanced. With the 50 ohm source impedance the situation is much better and the result is completely satisfactory. Everything can be explained.
Edit: This is even more pronounced when the high sensitivity microphone input is used. Again, a loop with transformer and 1k card output impedance and then the buffer with 50 ohm output impedance is added. The suppression of mains components is considerable and their origin is only because of the transformer parasitic impedances mismatch.
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