Weird electrical interference

[eflamm]

Hello.

I have a weird issue with my setup.

Here's the tested configurations :
Motherboard===usb==>Dac Fiio KA5 (balanced output)==balanced cable (4,4mm to XLR)==>SMSL SP200==>Hifiman Ananda : clean sound
Motherboard===usb==>Dac Fiio KA5 (unbalanced output)==unbalanced cable (3.5mm to RCA)==>SMSL SP200==>Hifiman Ananda : electrical interferences
Motherboard===usb==>Dac Fiio KA5 (unbalanced output)====>Hifiman Ananda : clean sound
DAP==balanced cable (4,4mm to XLR)==>SMSL SP200==>Hifiman Ananda : clean sound
DAP==unbalanced cable (3.5mm to RCA)==>SMSL SP200==>Hifiman Ananda : clean sound

So, as you can see, I have these interferences only when using the amp with the DAC through unbalanced in/out.

Does someone have any idea of where the problem can come ?

Since I have a clean sound with DAP through unbalanced, I suppose it comes from the motherboard (moreover, the noise is affected when moving the mouse), but then, why it it is clean when using the headphone directly on unbalnced output of the Fiio KA5 ?

 

[Vincent Kars]

My guesswork:

Dac Fiio KA5 (balanced output)==balanced cable (4,4mm to XLR)==>SMSL SP200

Balanced output in this case means 2 amps per channel so there is no ground connection between the 2

Dac Fiio KA5 (unbalanced output)==unbalanced cable (3.5mm to RCA)==>SMSL SP200

Here it is 1 amp per channel and possible a grounding issue between KA5 and the SP200 (tried KA5 and SP200 on the same powerstrip?)

the noise is affected when moving the mouse

This is commen when you have a fanatical GPU, it simply modulates the ground plane creating a ground loop. https://www.audiosciencereview.com/...ground-loop-best-solution.51732/#post-1863472
However, if this is the case I expect it to affect the Fiio all of the time.

 

[eflamm]

Hello Vincent.

Thank you for these explanations.

tried KA5 and SP200 on the same powerstrip?

The KA5 is powered by the PC's USB connection, so this isn't possible.

I know there are USB devices that can eliminate this interference.
Do you think this could solve my problem?

 

[Vincent Kars]

The KA5 is powered by the PC's USB connection, so this isn't possible.

PC and SMSL on the same strip?

 

[eflamm]

PC and SMSL on the same strip?

Yes.

 

[wwenze]

This... is definitely computer ground noise. More technically, common-mode noise.

Cheaper USB products especially tiny USB-powered dongles should be considered as having the same power supply as the computer.

Why is it called common-mode noise? Because the ground is also called the common, and is used as the common reference voltage for single-ended signals. Common-mode noise occurs when the ground voltage at your computer is different from the ground voltage at your load, in this case the SMSL SP200. Consider this scenario - The voltage at the RCA signal is the same on the Fiio KA5 and SMSL SP200, but the ground voltage is different. The Fiio KA5 intends to send 1V, which it generates by adding 1V to its ground voltage. The SMSL SP200 sees 1V, which it references against its ground voltage, which is, for example, 0.1V. This difference of ground voltage is not constant, especially in a computer with huge ground currents, hence shows up as noise.

When you use a differential or balanced connection, the signal is interpreted as hot minus cold. Since the ground noise appears equally on both hot and cold, you don't get common-mode noise.

You don't see this noise when plugging headphones directly to Fiio KA5, because headphones are 2-pin devices (per channel); transducers are 2-pin devices and floated except for where both pins connect to the source, and are thus differential by technicality. (You can, however, turn it into single-ended by connecting its ground to another reference, say, earth. If just to prove a point.)

Solution? Break the need for ground reference with optical connection. And if you want analog connection, balanced output is exactly the solution to this problem.

There are other solutions, like USB isolater, or getting a DAC with a proper built-in power supply to begin with so it ignores the computer noise and has a more consistent ground voltage. For cars we would use ground loop isolater (just a transformer) but those have poor performance.

 

[staticV3]

Does someone have any idea of where the problem can come ?

Ground loop between PC and SP200.

Since I have a clean sound with DAP through unbalanced, I suppose it comes from the motherboard

Not the motherboard alone. It's the PC<->SP200 connection.

but then, why it it is clean when using the headphone directly on unbalnced output of the Fiio KA5 ?

Using the KA5 directly breaks the loop, as does a differential interconnect between KA5 and SP200.

 

[eflamm]

Thank you @wwenze, for these clear and insteresting explanations.
I now fully understand the issue.