PC Sound Card Humming

[paul_g_787]

Hi. I have a strange issue that is puzzling me.

I have my desktop PC hooked up to my stereo HiFi amp. When I am playing music or any audio, the quality is superb and it works perfectly with no audible hiss or hum or any kind of noise.

However approximately 4 seconds after the sound stops playing, such as the end of an album or when I press mute on the PC I get a very loud 50Hz hum.

The hum doesn't happen straight away, only after a few seconds, as if something is being switched???

I have this issue with the onboard sound card, a cheap USB soudn card, 2x creative soundblaster cards, an audigy card I borrowed. All exactly the same symptom. Work fine when playing, start humming when it stops.

I have tried using a stereo ground loop isolator. Which I as far as I understand is comprised of 2x 1:1 transformers so as to AC couple the signal like a DI box. However with the ground loop isolator the problem still exists so I do not expect it is a ground loop of any kind.

Does anyone know what might be causing this?

Thanks

[magic]

 
What if you plug headphones instead?

It looks a bit like the output may be going hi-Z and the amp has extremely hi-Z inputs too and the cable picks up mains hum.
But most discrete soundcards have opamp buffers on the outputs, so they never go truly hi-Z.

[paul_g_787]

Interestingly it doesn't do it with headphones! When I stop playing sound I do get a tiny little 'pop' when the humming would usually start and when I play audio again the little pop happens just before the sound plays.. So I think the sound card is 'turning off' or something?? I am guessing here. Any clues anyone?

Also, my amp has a ground lift switch. I tried lifting the ground and this made no difference either, the humming still happens.

I connected a little pair of Tesco desk speakers I had knocking about to the PC with a 1m 3.5mm cable and it also does the humming on on here too!!

So I think this rules out ground loop problems as well as anything to do with the signal cable or my amp.

It seems to be a bad design of the sound card perhaps? But interesting it is with all of the different cards I have tried so perhaps not?

[wraper]

It might be internal issue of amplifier. Many of them detect incoming signal and switch between standby and normal mode. Probably the issue appears in standby mode.

[NiHaoMike]

Try using optical S/PDIF if your amplifier has that.

[wraper]

It seems to be a bad design of the sound card perhaps? But interesting it is with all of the different cards I have tried so perhaps not?

How it's an issue with sound card when you have exactly the same problem with different soundcards? Not to say other people don't have this problem. Also there is nearly zero chance mains hum coming from PC since it uses SMPS. Did you try different audio cable connecting PC and amp BTW?

[magic]

It's quite weird that it happens with multiple soundcards and multiple amps.

One obvious solution seems to be using a Y splitter and connecting the amp and some headphones simultaneously

You could try some other cable indeed. Those with two RCA connectors on each end and figure 8 cross section tend to use coaxial construction and may provide better shielding of the signal wire.

If the cable passes near some mains wiring, move it further away.

Off the top of my head I have no idea if that could be the reason, but are you sure your mains installation is properly earthed?

[ConKbot]

I've noticed something similar with USB headphones. When videos or whatever is playing goes silent I hear the background amplifier hiss, but a second or so after it finished it goes silent. I.e. DAC and amp shut off, so no more hiss like when its playing silence. 
Sounds like the in your situation, the sound output devices and amps are shutting down, and mains interference isnt being sunk by the source opamp.  Since headphones are quiet, maybe try a load resistor in the 400-30 ohm range to limit the coupled noise amplitude.

[Labrat101]

This can also be caused by a failing Cap in the PC power supply . As sound cards are very sensitive to ground loop interference.
 I would first check that your Amp and PC are both connected to the same mains outlet and that the Live and neutrals are the same
way round  . Not saying that this is the cause but sometimes the obvious is over looked also make sure both the PC & Amp
 have the Earth connected to a good Earth . A loose earth connection on the socket its self. or connection leads.
 Saying this as I can not see your setup or house wiring .
Mains Hum can only come from the supply . 
Just out of interest have you tried testing your Stereo Amp with No inputs . on half volume .
There should only be Gain Hiss and nothing else .
More Info required  . 

All the best
 

[Twoflower]

Any chance you're using Linux with Pulseaudio subsystem? In that case you can try to modify the file /etc/pulse/default.pa: Disable the line load-module module-suspend-on-idle (add a # in front). That prevents the humming by not disable the power down of the sound-chip if in idle (Source: Boxen brummen unter Linux, sorry German only).

[greenpossum]

Any chance you're using Linux with Pulseaudio subsystem? In that case you can try to modify the file /etc/pulse/default.pa: Disable the line load-module module-suspend-on-idle (add a # in front). That prevents the humming by not disable the power down of the sound-chip if in idle (Source: Boxen brummen unter Linux, sorry German only).

I think you may have hit upon it. Something similar happened with my Linux desktop hooked up to play to an external amp when the playback ended, but it was just a faint hum so it didn't bother me. I always suspected the sound system of doing something to the sound device. Powering it down sounds plausible.

OP, can you recognise the song it's trying to hum? Sorry, little joke. 

[ogden]

To me it seems like AC mains ground loop. Disconnect amp cables from any other device except PC and speakers, plug both - PC and amplifier in same wall plug using splitter/extender and see if it helps. Try another audio cables. If no change - then your amp is faulty.

[paul_g_787]

This can also be caused by a failing Cap in the PC power supply . As sound cards are very sensitive to ground loop interference.
 I would first check that your Amp and PC are both connected to the same mains outlet and that the Live and neutrals are the same
way round  . Not saying that this is the cause but sometimes the obvious is over looked also make sure both the PC & Amp
 have the Earth connected to a good Earth . A loose earth connection on the socket its self. or connection leads.
 Saying this as I can not see your setup or house wiring .
Mains Hum can only come from the supply . 
Just out of interest have you tried testing your Stereo Amp with No inputs . on half volume .
There should only be Gain Hiss and nothing else .
More Info required  . 

All the best
 

Yes tried the amp with no inputs, perfect silence and can barely hear a hiss when it is up loud.
No other input devices cause this noise just the PC so the problem must be the PC.

Tried different audio cables and routing, nochange,  so must be an issue with the source (PC)

[paul_g_787]

Any chance you're using Linux with Pulseaudio subsystem? In that case you can try to modify the file /etc/pulse/default.pa: Disable the line load-module module-suspend-on-idle (add a # in front). That prevents the humming by not disable the power down of the sound-chip if in idle (Source: Boxen brummen unter Linux, sorry German only).

Yes indeed I am! I am using Ubuntu 20.04

I will try that when I get a few minutes. Thanks

[paul_g_787]

Try using optical S/PDIF if your amplifier has that.

It was built in 1977. A little before they had thought of that hehe

[paul_g_787]

I've noticed something similar with USB headphones. When videos or whatever is playing goes silent I hear the background amplifier hiss, but a second or so after it finished it goes silent. I.e. DAC and amp shut off, so no more hiss like when its playing silence. 
Sounds like the in your situation, the sound output devices and amps are shutting down, and mains interference isnt being sunk by the source opamp.  Since headphones are quiet, maybe try a load resistor in the 400-30 ohm range to limit the coupled noise amplitude.

This could make sense! However PC the power supply is a decent Antec 80+ Gold supply so should not have significant noise on the DC rails (I would hope). I would have thought that there would be decoupling capacitor by the op amp IC to prevent this sort of noise. But maybe not.

I will try the resistors to see if there is any change in the noise.

[paul_g_787]

I think you may have hit upon it. Something similar happened with my Linux desktop hooked up to play to an external amp when the playback ended, but it was just a faint hum so it didn't bother me. I always suspected the sound system of doing something to the sound device. Powering it down sounds plausible.

OP, can you recognise the song it's trying to hum? Sorry, little joke. 

I think it is called "50Hz" by "The National Grid"