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Hi
OK I'll concede that 50uF is adequate if driving 10 K Ohms but what may be of greater interest is the HF behaviour with different cables.
This is actually transformer related rather than the coupling caps.
With 1 metre of twin screen cable on the output there is a 11 dB 'peak' at about 160KHz, with a small rise on the nominal 20K area.
If you increase the cable to 14 Metres then you get a 7dB peak at about 85 KHz with a smaller rise at 20KHz.
These peaks are much better controlled if the transformer is loaded with about 2 - 3 K Ohms resistive.
If the output is loaded with 600 Ohms the response is flatter '20 - 20K' but then 50uF is not sufficient at the LF end.
Since the sending impedance is approximately 150 Ohms (2 X 75) the capacitor values have a reduced effect.
Thus the general excitement that are transformers, juggling the send impedance, load impedance and load capacitance.
Matt S
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