Is this a ground loop or equipment issue?

[JayGilb]

Have you tried to switched order from:
PC -- [USB] --> DAC -- [RCA] --> subwoofer -- [RCA] --> powered speaker
To:
PC -- [USB] --> DAC -- [RCA] --> powered speaker -- [RCA] --> subwoofer

Probably won't make a difference, but a simple enough test.

 

[sq225917]

It's dc leakage across the negative pin of the pc psu. The sub completes the circuit.

 

[valsidalv]

Have you tried to switched order from:
PC -- [USB] --> DAC -- [RCA] --> subwoofer -- [RCA] --> powered speaker
To:
PC -- [USB] --> DAC -- [RCA] --> powered speaker -- [RCA] --> subwoofer

Probably won't make a difference, but a simple enough test.

So with the reverse order I still hear the whine in the speakers even if the subwoofer is turned off. As soon as the RCA cables are hooked up the whine is audible.

 

[valsidalv]

It's dc leakage across the negative pin of the pc psu. The sub completes the circuit.

Zolalll initially mentioned transformer hum EMI. Is this the same thing you are talking about? Could this be solved with a new PSU or a power surge bar/conditioner that has separate power banks?

 

[Pdxwayne]

I used to have loud ground loop noise when going from
streamer (coax)-> dac (RCA) -> amp/preamp (RCA) -> power amp

The power amp have ground lift option, but did not help.

The noise was gone after I used a RCA to xlr cable in the chain
streamer (coax)-> dac (RCA) -> amp/preamp (short RCA to XLR cable, connect to long xlr cable) -> power amp (via xlr in).

Not sure if this will definitely help. But if you have Amazon account and can do free return, you can try something like this:
6 Foot - RCA to XLR (Male) Cable Pair - Canare L-4E6S Star-Quad Audio Interconnect Cable & Amphenol ACPL RCA & Neutrik Male XLR Gold Plugs - Custom Made by WORLDS BEST CABLES https://www.amazon.com/dp/B094RHFDY3/ref=cm_sw_r_apan_glt_i_SPFR86MQVM2KK9N3RT9R

 

[vert]

FWIW I'm going trough a similar issue, and bought this cheap USB to s/pdif converter which was well reviewed by a member here.

PHIREE U2SX USB to SPDIF Interface SA9023 OTG 24bit 96kHz Black - Audiophonics

The Phiree U2SX is a simple, efficient and compact USB to SPDIF and Optical to USB interface. It is based on a Bravo SA9023 chip, supporting sampling rates up to 24bit 96kHz.

www.audiophonics.fr

Hopefully it will break the ground loop, which I assume is caused by my laptop's power supply (loop disappears when running on laptop battery).

I'm not sure an audio interface always resolves ground loop issues BTW, because I briefly had an audio interface (Motu) before returning it ; at the time I hadn't identified yet the intermittent SQ issue I perceived as ground loop-related, but in retrospect I'm pretty sure it was there, as I swapped between interface and unbalanced DAC, with either device, to decide which I preferred (both sounded... identical).

There are many "hum destroyer" boxes on the pro audio market, RCA such boxes are less common. A member mentioned a Jensen box. If you're in Europe, I found this https://www.oehlbach.com/en/audio-equip/electronics/audio-linear-8. I didn't buy it, going to try the Toslink solution.

 

[ZolaIII]

Zolalll initially mentioned transformer hum EMI. Is this the same thing you are talking about? Could this be solved with a new PSU or a power surge bar/conditioner that has separate power banks?

Yes he is (AC/DC transformer hum EMI).
I already told you going optical is the best approach. Showd you both sides (PC and standalone DAC) propositions as the foundation where to look. You don't really need balanced (normal RCA cable is perfectly fine for home use) but if you wish it then go for it (it won't resolve anything and will cost substantially more). Before buying anything try to barrow a deacent stand alone USB DAC with it's own power supply (cut the dirt in the root) to see if that's all you really need. Even using the separate power supply (like phone charger) to power the USB DAC without it's own deticated power supply line helps a lot. Buying a new PSU also won't fundamentaly resolve the problem (it just might help it become less prominent) and same goes regarding making shielding - cage on any transformer or DAC/ADC circuit (while it certainly helps).

Uh huh all this that rooten stinking hi distortion to start with sub could work as intended (it really makes you wonder).
My 30 years old Audigy 2 SB did all what it needs and good enough (there are Audigy [4-5] PCIe cards out there and are cheap now but they are old with problematic support regarding driver's so I wouldn't exactly suggest such purchase path).

 

[valsidalv]

Yes he is (AC/DC transformer hum EMI).
I already told you going optical is the best approach. Showd you both sides (PC and standalone DAC) propositions as the foundation where to look. You don't really need balanced (normal RCA cable is perfectly fine for home use) but if you wish it then go for it (it won't resolve anything and will cost substantially more). Before buying anything try to barrow a deacent stand alone USB DAC with it's own power supply (cut the dirt in the root) to see if that's all you really need. Even using the separate power supply (like phone charger) to power the USB DAC without it's own deticated power supply line helps a lot. Buying a new PSU also won't fundamentaly resolve the problem (it just might help it become less prominent) and same goes regarding making shielding - cage on any transformer or DAC/ADC circuit (while it certainly helps).

Uh huh all this that rooten stinking hi distortion to start with sub could work as intended (it really makes you wonder).
My 30 years old Audigy 2 SB did all what it needs and good enough (there are Audigy [4-5] PCIe cards out there and are cheap now but they are old with problematic support regarding driver's so I wouldn't exactly suggest such purchase path).

Gotcha
The iDefender+ I am using accepts an external power supply and provides power to my DAC via a phone charger. It eliminates static hum almost entirely (although the whine I mentioned still goes through). So I will start by buying or borrowing another DAC to test over USB. If that fails to eliminate the noise I will buy a PCI-e card with optical output and use that. Assuming it's not the easiest thing to find a DAC with both USB+optical inputs as well as RCA+balanced outputs, I may lose the RCA connectors and just look for something with balanced TRS/XLR. Thank you.

 

[ZolaIII]

@valsidalv plenty of DAC's with optical and USB inputs with both balanced - unbalanced outputs tested hire to choose from. Topping EX5 (400$ and if you find it on Amazon consider your self lucky) being one of my favourite with it's small drawbacks (narrow IR remote and BT receiver only) as a preamp with remote and source switching but different people have different taste.

 

[Audiofire]

The iDefender+ I am using accepts an external power supply and provides power to my DAC via a phone charger. It eliminates static hum almost entirely (although the whine I mentioned still goes through). So I will start by buying or borrowing another DAC to test over USB. If that fails to eliminate the noise I will buy a PCI-e card with optical output and use that. Assuming it's not the easiest thing to find a DAC with both USB+optical inputs as well as RCA+balanced outputs, I may lose the RCA connectors and just look for something with balanced TRS/XLR. Thank you.

Since you seem to have an electrically noisy power supply in the PC, I think that TOSLINK makes sense. If it was me, I would buy RME Digiface USB for that. But it's because I like their good ASIO drivers, and their equipment is also really good for recording music. You can then connect a good audio interface that has TOSLINK I/O to the Digiface USB. There is Creative Sound Blaster Z SE, but I just don't like PCI-e sound cards.

I wouldn't use the iDefender method, since it's not really a solution in my opinion. Keep in mind that if you connect USB and RCA in the same DAC, you won't be able to solve the problem with a noisy power supply in the PC.