Hi there, I hope someone can help with this problem. I have my pc outputting audio via the line-out of the motherboard (Asus Z370P) into an amp (Pioneer A-400). There is interference which I don't really understand. When I have the volume up at listening level maybe 4 there is a horrible resonant interference from both speakers. I have some Rega Ara speakers connected. They work flawlessly with the amp when used with other sources so it's not them or the amp.

When I connect the amp to the pc via the RCA to 3.5mm jack, and play a music file it works perfectly. It's when nothing is playing that the interference comes. When I am on the desktop or browsing there is the interference all the time. It sort of feels like a fan. Maybe the PSU fan or a chassis fan. It seems to be when the sound hardware is being used there is no interference, it works perfectly. And when the operating system tells the sound hardware to turn off it allows the interference to come out.

What I am doing now to deal with it now is turning the volume knob on the amp all the way down until I want to listen to something and then I turn it up when there is music or sound playing and it works but it is not ideal.

I can play a music file in VLC and play it or mute it and there is no interference whatsoever but once I press pause or stop, the interference returns in about 3 seconds.

I'm using Linux Mint 20 Ulyana, I don't think this has any bearing on the interference but maybe if there was something I could do with the realtek chipset it might help.

Up until today I was using a Dell Soundbar with USB audio which caused no problems but the sound quality from the Pioneer and Rega speakers is so much better and I would love to keep using them. - thanks

Hard to tell, description points to some electrical feedback from output to input, have you tried to relocate the cable, another cable maybe. Are there any cables involved inside of case or you plug it directly into motherboard? Have you tried muting all inputs in mixer?

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I have it plugged straight into the motherboard so at the back of the pc case. I have tried relocating the cable. It doesn't make any difference. There is only so much relocating I can do with it but I did keep it away from any power cables. I don't know how to get to a sound mixer. Is there one I can install? I just have the basic one that comes with Linux Mint and there is an input tab but it just says there are no input sources. Is there a command line tool to check microphone inputs? I don't have a microphone connected but it could be something like that.

These commands might provide some clues about input devices and their levels..
If you have pavucontrol (graphical mixer for pulseaudio) installed, you could check settings using that.

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ALSA has command line tool 'amixer' which can display all inputs and their state. Mint uses PulseAudio, I have no experience with it. Basically, PulseAudio sits on top of ALSA and commands it, so there should be a full mixer application you can install from Mint repos.

Edit: ferrari knows better and types faster.

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And it starts about 5 seconds after music stops, right? It happens because when nothing is played pulse turns off the sound hardware to conserve a miniscule amount of electricity; amplifier, not detecting any input, winds AGC to the max and starts amplifying whatever is left - interference. Disable powersave in all pulse configuration files /etc/pulse/*.pa (usually there are two of them: default.pa and system.pa) by deleting or commenting out 'load-module module-suspend-on-idle' line and restart pulse.

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PulseAudio mixer is started with pavucontrol in your GUI.
Mute everything except used audio output. Normally used audio output is something like "HD Audio Controller Analog Stereo" (PC: "line out", light green 3,5 mm jack at rear side) for connection with an amplifier.

This worked...but now I have another interference problem in that the mouse and keyboard are making funny noises, actually I don't think it is the mouse itself but when I do something it increases the cpu effort or something. Scrolling through a website produces constant noise, even with the mouse or arrow keys. It sort of sounds like the hard disk accessing but it is an SSD. Something to do with the video card maybe? I had a terminal open and there was a constant popping and when I looked at the cursor I noticed the sound was corresponding to the cursor in the terminal flashing and stopped when it stopped. Weird. Same here now I can hear popping when I type a letter and even the little cursor in this page is causing a sound. I don't know. Thanks for the help so far.

Update:
Well it's not the HDMI cable. I disconnected it and used the keyboard to scroll this page and the noise was still there even though there was no connection. I removed all other usb devices so it is literally just the keyboard and mouse plugged in. I actually disconnected the monitor and it made no difference. Tried different inputs on the amp, Tape 1, Tape 2 and Line and all the same. Headphones plugged into the back of the PC work perfectly and walkman plugged into amp works perfectly too.

Here is the output from 'amixer' maybe someone can see something funny.

Well I decided to check another amp and it works perfectly. A similarly aged one and same Pioneer brand, the A-441.

I didn't think it was a Linux issue at all but maybe some other people had a similar problem here. I did check it in the BIOS too and it was making the noise so I guess the A-400 I have is just not compatible or too sensitive or whatever. Pity but it works perfectly fine with other sources. Thanks for all the help. Glad I had another amp on hand to check before spending money on filters and cables and what not