how can I get DSD signal from my Sony SCD5400 ES

Ah OK.

I can think of a few options:
a. get it modded by Modwright. It modifies the analog output stage and the power supply and is quite highly rated

b. sell the 5400ES for a better SACD player. I am quite pleased with my DSD DACs from Playback Designs and EMM Labs. Well worth an audition.

c. wait for an HDMI DSD DAC. The only HDMI DAC that is 2 channel and "audiophile" grade (ie not AV/home theatre) is the NAD M51. But that does not accept DSD over HDMI, only PCM.

Note the Hydra X you mentioned is a USB -> I2S converter. What it claims is that it is compatible with DOP (DSD over PCM) encapsulation over USB. This is a way that computers can take existing DSD encoded files and send it over USB to a DAC that can decode the DSD directly. It is not a DAC and it does not work with the Sony 5400ES.

thanks for the info.

so getting another Sony TA DA 5400ES will not work to improve sound quality

I think I will wait for HDMI DSD. I think there is
rapid development in this area so I may not have to wait that long.

Thanks also for the comment on the HydraX

Are you sure there's an HDMI DSD DAC on the way? :)

Are you sure there's an HDMI DSD DAC on the way
I once asked Audiobyte and they said that in the future there may be, the problem now is the decode license from Sony. I don't know whether it can also work with those of oppo or not. I need to confirm this info later . I may be wrong but I think I'd better wait until the DSD 512 have settled and the price has come down more.

Frankly I feel HDMI is not a great transport for audio signals. The M51 for instance often sounds better using the SPDIF than HDMI.