The MK3 evolved from the thought that the SPDIF transmitter chip (DIT4192) runs off 5V power - if I replaced this with a WMM8804 which runs off 3.3V power, I could dispense with the 5V external supply & run everything off battery. I was also aware that the output transformer used in the Hiface caused some overshoot on the SPDIF signal on my oscilloscope so I decided to try a test without this transformer & corrected the output impedance to a true 75ohm for SPDIF. The SPDIF waveform had no overshoot & was clean as I had ever seen from any of the various SPDIF devices I had looked at on the oscilloscope.
I wondered if the lack of a transformer & therefore galvanic isolation on the SPDIF output would allow noise to bleed through from the PC/laptop through the MK3 & into the DAC, amp & eventually be discernible at the speakers? No such noise was evident, the background between songs & in the silent gaps in songs was an enveloping blackness. There was no difference in sound when I had the laptop charger plugged in or when I ran the laptop on it’s batteries. I later changed my laptop & heard a very slight haze & whine in the sound. This went away when I unplugged the laptop charger. The cause of this noise was a ground loop. The laptop charger had a grounded mains plug which set up a ground loop with my DAc & amp. Easy solution was to use a double adapter from which I had removed the ground connection. Result, enveloping blackness & happiness again.