Ground loop causes hisses and fizzes in captured audio

R

ricecrispies

Member
When I record through my desktop computer (via a Scarlett 2i2 which seems to work OK when connected to my laptop), I get a lot of unwanted hisses and fizzes consistent with a ground loop. If I am just recording with my microphone or my bass plugged directly into the audio interface, the sounds are below the level of hearing, but clearly visible in the level meters in my DAW, which of course becomes clearly audible with certain effects. However if the audio interface receives a signal connected to another device with its own connection to the electric mains, such as my bass amp (via a DI box) or my digital piano, then the problem is bad enough to make any kind of recording impossible. I've managed to reduce the volume of the noises as far as the bass amp goes by (slightly dangerously) isolating it from ground, but I've not got rid of the problem altogether.

I assume that there is a faulty or sub-standard component in my computer, but I need to know which one it is so that I can replace it. Is it likely to be the motherboard or the power supply or something else?
 
If there's a grounding problem, typically you have hum or buzz, not 'hisses and fizzes'. Have you optimized your desktop for recording - making sure nothings running in the background, for example?
 
I agree - but it's not a ground loop in the usual sense. Ground loops occur because of a difference in ground potential between two ground sources - and manifests by a hum at the mains power frequency, or a harmonic - 20 60/120Hz or 50/100Hz typically. Hiss, noises and crackles that can be cured by lifting a main ground. The noise is being induced so must be coming from somewhere. ferrite rings in the USB link might help, but a big toroidal magnet from an old loudspeaker with lots of turns of the usb cable wound round it might were better? My guess is there is bus noise getting down the usb to the Scarlett, on the ground of the USB connection, and this is upsetting the Scarlett. However plugging in the DI means that something else is guilty. If I read your response, you 'accidentally' lifted the ground on the bass amp to make it float, and this cures the hum? When you reconnected it, for sensible reasons, the hum comes back? So grounding the Scarlett via the input audio cable makes it take off? Might be worth trying the ferrite winding on the input. Does the DI ground lift do nothing? Are you sure the computer is grounded properly? I'd be prodding around with a voltmeter to see if bass amp or keys ground is the same as the computers case - as in the meter reads 0 volts on the AC range. Grounds causing increased noise suggests a problem in the D/A. Maybe something as simple as the 5v from the USB is not 5v for some reason - perhaps connection to the real ground is pulling it down a little, and the increased noise is a byproduct. Guesses really - but weird!
 
Thanks very much for your replies.

Here is a sample of the noise. One thing I should have mentioned before is that many of the random sounds are pitched at a single frequency, several octaves higher than mains hum.

I said in my OP that lifting the amp from ground seemed to reduce (but not eliminate) the noise, but now I wonder if I was hearing things. If there is a difference between the sound from the earthed and the unearthed amp, it's not a big one. In any case I also get the problem when connected to my piano, which has a two-pin power supply and is therefore not earthed.

The most powerful electrical devices close to my computer or the audio interface are my TV/monitor, TV decoder, wifi router and audio monitors, but powering off all of these makes no difference.

I'm not clear what you mean about trying the ferrite winding.

I've been trying to prod around with my multimeter, but I can't find anywhere on the outside or the inside of the computer case that conducts electricity that I feel confident to prod without doing some damage. For that matter, I can't find a conductive place on the amp casing either.
 
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