Singxer SU-6 has a design BUG
After exchanging the oscillator of my 004 and two days of burn in, I won'ted to do some listening tests.
So I sat down in my listeningchair everything was silent. I started the player, but no sound
. Ohhh, I forgot to plug in the HDMI cable comming from the singxer. I plugged it into the 004, ....... and there it was, a soft hum.
Now you have to know, that I know that very soft hum. It started when I connected chassis GND to PCB GND. But I never could understand where the ground loop was.
Well, until now. I unplugged the HDMI again, and the hum went away. Pluged it back in, an the hum was back.
Pulled out the USB from the singxer, hum was gon. There had to be some kined of connection between the USB input of the singxer and the HDMI output. So I measured it. 0 Ohm between USB socket housing and HDMI socket housing.
I opened up my sand filled singxer, and took a closer look. Indeed, as everybody can see on pictures, the USB section is clearly isolated from the rest of the PCB. BUT ONLY THE MIDDLE 4 PINS OF THE USB PLUG!!!! THE USB PLUG HOUSING IS TIED TO CHASSIS GND!!!!!!!
Almost every USB cable has shielding connected on both sides of the plug, and that's why I had a ground loop.
The solution is to eather isolat the metal part of the USB plug on the singxer side with kapton tape, or open up the connection on the PCB as I decided to do.
That solved a lot of sonic problems. I can bite me in my ..... that I didn't question the SU-6 construction earlier.