I am puzzled as to why this should be affecting your equipment at all if it's this:
with, presumably, XLR cables of somewhat decent quality. All these components should be IEC Class I (earthed) devices with balanced connections. Unless your mains wiring is entirely lacking a proctective earth connection (where are you located and how old is it?), I can only imagine an RF ingress problem in one of the components - does moving the router around make any difference?
Hello and thanks for your reply!
I had been using RCA cables (stock cables / Mogami 2497) between the DAC and A90, then XLR cables from A90 to the Neumann KH 80.
Buzzing noises could be softly heard on the Neumann, and it was very loud when I used IEMs plugged into the Topping A90.
1. I added an ifi iDefender to the USB cable, 90% of the noise is gone. This showed that there was a ground loop. Still some noise remained.
2. The remaining noise waxed and waned with GPU usage (eg. playing a video without sound), so there must be some EMI noise as well. Moving the A90 away from the computer and router lessened this noise. Moving the Gustard X26 Pro had no effect, perhaps due to shielding by the thick aluminum casing.
3. Occassionally, there was an extra 1kHz whine and intermittent buzz that the iDefender couldn't remove. I unplugged the router, it disappeared So the router (which was ungrounded), must be one of the culprits of noise too.
Today, I received my much needed second pair of XLR cables and now both my DAC-Amp and Amp-Neumann connections are Mogami 2549 XLR. All noises are gone, even without the iDefender.
There are a few interesting observations.
1. If I connect the DAC and the A90 with BOTH RCA and XLR cables in place, even if I choose XLR input on the A90, all the buzzing comes back.
2. The Wifi router related noise could not be replicated now, after I unplugged the router and all the LAN cables and replugged them in. I hope they don't come back.
3. The if iPurifier doesn't always reduce noise. In my previous configuration where the iDefender reduced 90% of the ground loop noise, adding an iPurifier to the USB gave me double the noise.
Finally, all my equipment are plugged by UK plugs (except the router) into two power strips (Belkin/ Schneider) going into ONE wall socket via a multi-plug adapter. My apartment block is only a few years old and the UK plugged wall socket is grounded by law.
Happy to share my experience and to learn from you all.