I finished putting together a system for equalizing my DML speakers, using an Raspberry Pi running camilladsp and while evaluating it I noticed a problem. The speakers were constantly emitting a low-level high frequency sound. I had previously listened to my analog electronics + DMLs separately and knew they were OK so suspected some issue with the digital side.
But nothing I tried eliminated that annoying sound.....until I unplugged the computer monitor connected to the Rpi. Silence ensued. Simply turning the monitor "off" didn't do it. I'm guessing that even when the monitor is turned off via the power button, it really is NOT completely powered-off. It's possible that the only thing that's actually turned off is the backlight. I figured out it was the monitor when I moved its power cable away from the other system cables and the noise level decreased. Additional experiments gave me the final solution.
I knew that the old-style CRT monitors can emit all sorts of EMI but hadn't considered the possibility that an LCD style monitor could, too. In retrospect that's not too surprising because the thin-film transistors associated with every single pixel on the monitor screen are switching off and on. Then there's the inevitable switching power supply inside the monitor as well.....
It won't be too difficult to fix this. I have a spare switched power strip I can use to separately turn the monitor off when I'm actually listening to music. It will add to the clutter a bit but I need to clean up the wiring anyway.
But nothing I tried eliminated that annoying sound.....until I unplugged the computer monitor connected to the Rpi. Silence ensued. Simply turning the monitor "off" didn't do it. I'm guessing that even when the monitor is turned off via the power button, it really is NOT completely powered-off. It's possible that the only thing that's actually turned off is the backlight. I figured out it was the monitor when I moved its power cable away from the other system cables and the noise level decreased. Additional experiments gave me the final solution.
I knew that the old-style CRT monitors can emit all sorts of EMI but hadn't considered the possibility that an LCD style monitor could, too. In retrospect that's not too surprising because the thin-film transistors associated with every single pixel on the monitor screen are switching off and on. Then there's the inevitable switching power supply inside the monitor as well.....
It won't be too difficult to fix this. I have a spare switched power strip I can use to separately turn the monitor off when I'm actually listening to music. It will add to the clutter a bit but I need to clean up the wiring anyway.
Try a USB isolator that uses an external power supply.
All switching supplies emit some level of noise that can get into the common ground path. The "housekeeping" portions are always alive, so you're right in that many things never really turn off. Some only turn off displays and lights and don't respond to button presses except power.
All switching supplies emit some level of noise that can get into the common ground path. The "housekeeping" portions are always alive, so you're right in that many things never really turn off. Some only turn off displays and lights and don't respond to button presses except power.
Try earthing the display shield, just check if the monitor is really connected to Earth, some strips or wires may not be conducting.
And the monitor cable may lack Earth continuity, check that also.
In my house, the CCTV recorder, its PSU, and the WiFi are on the same outlet box, but the monitor stays off mains unless needed.
I made a 4 switch 4 outlet switch box, wall mounted. That uses heavier duty switches and sockets in comparison to power strips.
And the monitor cable may lack Earth continuity, check that also.
In my house, the CCTV recorder, its PSU, and the WiFi are on the same outlet box, but the monitor stays off mains unless needed.
I made a 4 switch 4 outlet switch box, wall mounted. That uses heavier duty switches and sockets in comparison to power strips.
I meant metal screen frame of the LCD should be connected to Earth.
Might be difficult to open the covers without minor damage.
Might be difficult to open the covers without minor damage.
The line filter I had lying around solved my problem. Apparently it was being injected into the AC line.
I don't have any earth grounds except through the monitor's 3-prong plug. According to Wikipedia class II isolation has no connection to earth ground so no loop there. And my RPI's wall-wart is just a 2-prong, confirming it has no earth ground connection.
The power amplifier, connected to the output of the USB sound card, is connected to earth through a ground-break resistor.
Everything else, other than the monitor, is either transformer-isolated (linear PSU or see below) or only has a 2-wire power cord.
I do have two switching power supplies for powering my headphone amplifier which have 3-prong plugs but they were not plugged in at the time I was observing the problem. When in use they are plugged into an isolation transformer setup I made so that potential loop is broken anyway.
I added the isolation transformer because the HA was behaving strangely and I DID suspect a ground loop problem, but that turned out to be an oscillation problem.
I do have a USB isolator but the RP5's USB sockets are too close together. I have all 4 positions occupied and the isolator is too wide. A short extender or hub would address that problem.....but at this point there's no indication I need it.