As an old circuit designer my idea is that the noise we see at higher frequencies on the RCA-output is noise from cpu and other logical circuits reaching the Op-amps inputs capacitively via the PCB-layout or via the Op-amps supply or grounding pins. It is hard to get rid of this problem completely at the low levels we are measuring here, and it might be so that impedance levels are changed in different modes (DAC och Headphone or Pre-Amp) and therefore the amount of capacitive coupling to inputs is changed between the modes. It may also be that same noise is reaching the head phone Op-amp in somewhat antiphase and therefore the sum of the amplitude of the noise levels at the head phone output will be less. It can be quite a tricky work for the designer to improve such things further, but should not be impossible.Does anyone have an explanation for the paradox that headphone output is significantly better for SNR and THD?
Indeed, in the block diagram Topping headphone amplifier is after RCA. Therefore, THD and SNR can only be the same or worse.
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amirm , can your jack3.5 cable be better than RCA?
The noise can also sometimes originate from out of band high frequency noise which is translated to the audio band via intermodulation/mirroring on the testtone and nonlinearities in the following amplifiers, so not so easy sometimes to find where noise is coming from. But under these circumstances it could be improved by a following bandwidth limiting stage.
Just some speculative thoughts. Cheers!
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