Buzzing sound (sample linked) with USB and 3.5mm only when connected to same PC

[new23d]

Mystery noise. Audio sample here. Piano is turned on at the 2 sec mark. Noise is from the 7 sec mark. Clip length is 23 sec.

I have a brand new Kawai ES920 digital piano. It has USB MIDI and unbalanced 3.5mm Line In. If I connect USB (for MIDI in) and 3.5mm (line out from PC) to the same PC, a constant buzzing is heard on the piano speakers.

What I have tried:

1. Different USB cables, with and without ferrite cores.
2. Different 3.5mm cables - all of mediocre quality.
3. A laptop with no other connections - no power, nothing connected at all but these two connections.
   1. When laptop is turned off, the buzz disappears.
   2. When laptop turns on, the buzz returns instantly. No need to wait for boot.
   3. With the laptop turned on, the buzz returns as soon as the outer part of the USB-A plug on the cable gets in contact with the port on the laptop without inserting.
4. A totally unplugged Raspberry Pi with just the USB and 3.5mm from the piano plugged in. Buzz returns with the Pi turned off and having nothing else plugged in.
5. Running the piano on AC power not from a wall socket but from an AC-mains disconnected UPS. Piano power is 2-prong into a DC 15v supply.
6. The buzz does not occur if I plug the USB into a turned on laptop and the 3.5mm into a turned on Raspberry Pi.

How can I troubleshoot this further and potentially fix it?

 

[Chazz6]

"The ES920 on the other hand has only USB-MIDI and on my piano there is some strange USB power leak that makes my MacBook lose its ground. You can feel it when you touch it and when you send the sound from the MacBook back to the ES920 via a 3,5 mm cable there is a hissing sound."
https://forums.musicplayer.com/topic/184091-yamaha-p515-vs-kawai-es920/

Your cases 1 to 5, problem. Case 6, no problem. Maybe try cases 1 to 5 with a ground wire from some part of the piano to a true ground?

 

[AnalogSteph]

I'd suggest picking up a USB isolator, presumably a basic ADuM3160/4160 job should do the trick (MIDI is pretty low data rate, I can't imagine USB 1.1 wouldn't do). You'd think in a digital piano selling for about 2 grand they'd find the budget to include one of those, but perhaps they didn't envision both connections being used at the same time.

You can observe the exact same problem with basic USB-powered PC speakers sporting 3.5 mm input. The noises then go away when upgrading to a dedicated USB charger.

 

[new23d]

Thanks everyone. Ultimately went with a third-party MIDI to USB interface: the CME U2MIDI Pro. The piano had MIDI ports. Full discussion on this topic is on the Piano World forum.