You can get some very strange effects from class D and from overloading if you are not careful which the OP may be interpreting as 'echo'.
I do not know what Nigel's Beringer amplifier or any other amplifier, class D or not, has to do with this post. AG you seem to have a private mission about class D. I am not implying for a moment that there is anything wrong with class D, or that class D has any sort of echo per se.
But class D requires more care, especially with the filter-less chips which can have a load of HF on their output that may cause EMI.
I should have said in my post #12 that the output from portable devices: mobile phones, laptops etc, is very often by a full bridge class D driver running off 5V typically, and is used to drive the internal speakers/transducers.
Many of the semiconductor manufacturers have chips specifically for this purpose.
https://www.analog.com/media/en/technical-documentation/data-sheets/SSM2518.pdf. I guess this is the type of output from the modem as indicated in the schematic of the referenced document in post #12
(The output (earphones) for portable devices is often a half bridge for each of the left and right channels, but using the same chip.)
When coupling the modem to the sound card it is important to configure the ground return correctly to eliminate problems. This and a suitable low pass filter was the approach I was considering.
I had missed your post #9 AK. Theses were my thoughts too.
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