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Help me sort out USB noise and a ground loop in a rather convoluted setup

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#1
I have assembled a Frankenstein setup in my home office that is very close to having excellent sound, but I can't quite figure out how to finally vanquish the noise in a convenient way. Apologies in advance for the long-winded post, but if I simply listed out my current gear it might not give context for why I have/need this setup and why "get something with a better USB implementation" isn't an option.

The heart of the system is a Dayton Audio DSP-408 that I use for EQ, room correction, and crossover. This DSP has 4 RCA inputs and 8 RCA outputs. Two output channels feed a Crown XLS-1002 that powers a pair of DIY SEAS Idunn speakers (not built for this purpose - they used to be in my living room). One output channel feeds a Behringer iNuke NU3000 that powers 4x JBL CS-1214 subwoofers in a pair of 2x12" prefab enclosures. My concept with this system was to be able to do both critical "audiophile" listening and to be able to crank up the subwoofers to provide club-quality sound for listening to and DJing electronic music. Mission accomplished.

Initially, my source was exclusively a Denon MC4000 USB DJ controller. My old NuForce uDAC-2 had died and my FiiO E18 has no preamp functionality, so I figured for the time being - not needing headphones for critical listening in this environment - the Denon would be just fine as a source since it reportedly has a decent soundcard and can be used with WASAPI/ASIO by Windows. It is not bus-powered - there is a separate 5V/3A power supply.

Unfortunately the Denon is horribly noisy. It has a high noise floor to begin with, and there is a ton of noise that comes over the USB cable from my PC chassis - buzzes, noise when I scroll the mouse, etc.. It's fine when music is playing, but it's not tolerable when idle and it's especially bad for gaming when the GPU spins up. Adequate for DJing, not adequate as an all-purpose source. I know that it's the USB because this noise goes away when I disconnect it. I've tried other cables and ports and it doesn't help.

Months later (now), I purchased a Schiit Modi 3 and Sys for passive volume control (this was dumb - I should have just bought a headphone amp with preamp outs and now have a JDS Atom on order) both to troubleshoot and see if it was a sound upgrade for non-DJ listening. I have the output from the Modi 3/Sys running to the other two input channels on the DSP (NB: I am well aware that the ADC/DSP/DAC part of my signal chain is a bottleneck on fidelity, but I think the signal going in still matters). One cool thing is that I can easily do a level-matched A/B between the two sources. The Modi 3 blows the Denon away - clearly a good DAC is an excellent addition to my system. But the Denon is a great piece of DJ gear and it needs to stay in my system.

And this brings me to the problem: troubleshooting the noise when this is all connected.

- With both fully connected, there is tons of PC noise coming through the speakers. No ground loop hum.
- With the Denon disconnected entirely and the Modi 3 on USB -> PC noise + ground loop hum.
- With the Denon disconnected and the Modi 3 on optical + external power -> no PC noise whatsoever, but ground loop hum.
- With the Denon connected and the Modi 3 on optical + external power -> PC noise, but no hum.
- With the Denon's USB disconnected and the RCAs connected -> no PC noise, no ground loop hum but only when the Denon is powered on.

Now, I am totally fine with only connecting the Denon's USB and accepting the attendant noise when I actually want to use it to DJ. But leaving it powered on with the USB disconnected to eliminate the ground loop hum is really annoying because all of the LED-lit buttons on the controller flash nonstop. I am also troubled by the fact that if the Denon vanished, I'd be left with a ground loop hum that I couldn't solve.

Two questions then:
- How do I eliminate the USB noise from the Denon given that it's already got an external power supply? Do those $40-50 USB isolators/filters actually work?
- Where might the ground loop hum be coming from in the rest of the setup, and why would the Denon eliminate it when powered on?
- Would hum eliminators or DI boxes or some other pro audio gizmo between the DSP and the amps (probably converting the unbalanced RCA to balanced XLR in the process) - which presumably would eliminate the hum - audibly degrade the signal?

Thanks!
 
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#2
Confirmed that connecting the Denon via a powered USB hub instead of directly to my motherboard/PC case doesn't stop the noise.
 

Blumlein 88

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#3
This is probably a too complex question to solve remotely.

It seems as if your Denon is effectively the main ground point when it is powered and connected.

Are all of these devices on a power strip or plugged into various outlets? Your hum is a ground loop of some sort likely.

A good DAC would not let computer noise thru the USB into the audio system. The Modi 3 is probably not good in this regard, and apparently neither is the Denon.

"- With the Denon's USB disconnected and the RCAs connected -> no PC noise, no ground loop hum but only when the Denon is powered on."

So the ground loop may be mostly between PC and everything else. Try some grounding differently with the PC. Or change the outlet the PC is powered from.

Sorry I couldn't be of more help.
 
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#4
Everything is connected to the same power strip. I've tested the outlet and it seems to be grounded correctly.

I'm not really sure how to improve the PC's grounding. I opened up the case and didn't really see any cables amiss. The motherboard and CPU are pretty old at this point, but the PSU is a nice EVGA one from last year. I will be building a new PC this year - are there any brands of PSU particularly known for being less likely to cause grounding issues?

I guess I will try one of those USB isolator gadgets and see if that at least lets me keep the Denon connected to the PC all the time. I suppose I should consider myself lucky that I finally figured out any way at all to get hum/noise-less sound from this setup, even if it's not ideal!
 
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#5
Firstly try plugging the USB cable into socket furthest from the power supply(at the back of the PC).

If that fails read on...

I found myself in a similar situation a couple of months ago. I also had an EVGA power supply at the time. Recently though after four years the power supply stopped working and so replaced it with a different model (maybe a different brand too, can't tell without looking through my purchase history). Anyway having changed the power supply the problem has gone. Thing is I didn't heed my first suggestion from above before the power supply broke so without fiddling with my own cables I can't be sure whether it was simply re-socketing the USB cable or changing the power supply that fixed the issue.

Good Luck - I know how frustrating these things can be.
 
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#6
Yes, the usb isolators work. I had a similar problem, no hum, but a ton of noise from inside the computer. I can’t remember which one I bought offhand, I got one from Mouser I believe.
 

AnalogSteph

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#7
+1 for a USB isolator. That, or a line isolator.

A ground loop is inevitable in this setup - the PC and the amps are connected to PE, and there is a ground connection running over USB shield and RCA cables. The unbalanced connections then pick up the ground loop crap.

I would try borrowing a notebook with a working battery and connect the Denon's USB to that (on battery). Silence, I bet.
 

Sylafari

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#8
I have the same issue and most of the noise occurs when the GPU starts to be put under a load (playing games, etc.),l and when everything is idle their is only a tiny background hiss.

For my setup an optical cable between my PC motherboard and the DAC solved my ground loop noise problem with my setup but the PC optical port is a bit noisy itself naturally or something as I still had the slightest bit of noise. Tried an iFi USB isolator thingy after with the USB DAC, it removed the ground noise but introduced a high pitch noise instead so I returned that. Going balanced between source and amp solved all my problems noise wise (perfect silence but cost a pretty penny to get a balanced amplifier!).
 
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#9
Further tinkering yesterday revealed that I actually do have some faint hum in the "Denon USB disconnected but RCA connected and powered on" scenario above, but it's faint enough that I can only hear it when I put my ear up to the woofer on my mains.

Both of my power amps have balanced inputs, and the Denon DJ controller actually has balanced outputs, but sadly the Dayton Audio DSP-408 that I have everything running through is unbalanced. That's four channels to convert to balanced... sounds like a real mess of cables and boxes, and I worry about the boxes further degrading a signal that I'm probably already stepping on too much with the standalone DSP and the Crown amp's built-in DSP (which I don't use, but I understand to be part of the signal path no matter what).

For a USB isolator, is something like this what I am after? https://www.mouser.com/ProductDetail/Olimex-Ltd/USB-ISO?qs=sGAEpiMZZMsyM1rGGtQxrO1HI6bIq7Ze

Or this, which has two downstream USB ports and would thus cover both a DAC and the Denon. https://www.mouser.com/ProductDetai...EpiMZZMve4/bfQkoj%2bLuz%2btxKJfHsCFKSracdjBU=
 

L5730

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#10
That Murata unit doesn't look to have any USB ports, and is a part meant for a DIY solution, I think.

I've bought an ADuM4160 based isolator from ebay (China/HK).
These can be had damned cheaply <£10, and although they should work 'as is' they only provide about 200mA of power to the external device.
The DC-DC transformer is crap on these boards anyway, so it's best to just de-solder it, cut the traces and put your own external +5v supply to the board. Arguably easier to do that solder USB ports onto that Murata unit.

All of these cheap isolator solutions will only support 24 bit 96 kHz 2-channel audio because they can only handle Full-Speed (12mbps) data transmission. Bear that in mind if you want to stream DSD or multi-channels over USB.

Can you list the devices again, but which ones have a connection to mains ground.
Most of the AC/DC transformer wall-wart blocks don't have a mains earth connection (because they are double insulated and don't need it) but some do.

A potential problem I can see with the ifi iDefender product is an increase in 8 kHz (and multiples) which is due to the packet data.
This, of course, when using an external power supply to 'break' the ground loop. Else I don't see it as doing anything.
 
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#12
- PC w/750W EVGA power supply (grounded plug)
- Schiit Modi 3 (currently connected with optical and powered with Schiit's included external switching power supply, essentially a USB charger)
- Denon MC4000 DJ controller (USB data connection, external two-prong DC power supply)
- Dayton Audio DSP-408 (USB connection optional - only needed for programming, external two-prong DC power supply)
- Crown XLS 1002 amp (grounded plug)
- Behringer iNuke NU3000 amp (grounded plug)

Everything is connected to the same Belkin power strip. I think the only other grounded plug on the strip is my monitor.

To reiterate, the quietest configuration for listening via the Modi 3 is (a) Modi 3 connected with optical (b) Denon powered on with USB disconnected and RCAs connected. There is a ton of noise when either the Modi 3 or the Denon is connected with USB. I don't hear any noise from the DSP when it is connected to the PC with USB.

In addition to the USB issues - which seem somewhat clearcut, even if the best solution isn't - what's throwing me for a loop (ahem) is why I get hum when the Denon is powered off.
 

L5730

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#13
In your quietest config, you have a mains ground at the PC, then an electrically isolated connection between PC and Modi. The Denon and Modi will be sharing ground via the RCA cable to whatever the Denon connects to.
The Denon may have relays that switch the outputs off if not used. Obviously, if it doesn't have any power it can't activate the relays, and leaves the ground/shield open to the connected gear.

Having everything on the same power strip will just reduce the mains ground potential difference between devices (it won't necessarily be the same). However, how the device ties it's audio ground can cause internal potential change on the ground seen between two devices and their audio connections. Common isn't so common because wire isn't zero ohms.

Can you disconnect either the Crown or Behringer from the socket and see if the noise goes, with the Denon turned off and the sound is humming.
 
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#14
- PC w/750W EVGA power supply (grounded plug)
- Schiit Modi 3 (currently connected with optical and powered with Schiit's included external switching power supply, essentially a USB charger)
- Denon MC4000 DJ controller (USB data connection, external two-prong DC power supply)
- Dayton Audio DSP-408 (USB connection optional - only needed for programming, external two-prong DC power supply)
- Crown XLS 1002 amp (grounded plug)
- Behringer iNuke NU3000 amp (grounded plug)

Everything is connected to the same Belkin power strip. I think the only other grounded plug on the strip is my monitor.

To reiterate, the quietest configuration for listening via the Modi 3 is (a) Modi 3 connected with optical (b) Denon powered on with USB disconnected and RCAs connected. There is a ton of noise when either the Modi 3 or the Denon is connected with USB. I don't hear any noise from the DSP when it is connected to the PC with USB.

In addition to the USB issues - which seem somewhat clearcut, even if the best solution isn't - what's throwing me for a loop (ahem) is why I get hum when the Denon is powered off.
Long shot but I was chasing a ground loop on my system for months after I moved and it turned out if was the ground for the cable TV/ modem vs the mains ground. To test I ran a heavy wire from cable ground to mains ground.... instant silence... then wasted time on cheap cable isolators that either killed TV or didn't break ground loop. Finally ended up with a Jensen one that works. Ground loops are very hard to figure out because regardless of the actual cause they pick up all sorts of crazy noises and isolating the cause is not obvious. Good luck!
 
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#15
Thanks for the feedback, guys. I'll try some more tinkering tonight. I certainly feel like I'm learning some things with this thread.

The Denon has a screw on the back for a ground wire. Would this be useful? At some point I tried running a piece of speaker wire from this screw to a screw on my PC case, but it didn't make a difference.

One thing I neglected to mention (because I forgot about it) is that at that same time, I ran speaker wire between two chassis screws on the Crown and Behringer amps. This DID make a difference and eliminated a buzz and most of the hum. This wire is still present.

Should I consider running a wire from the Denon's chassis screw to the amps?
 

L5730

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#16
That ground screw will be for connecting a turntable/record deck (vinyl playing thing) to, if required. Don't use it for anything else.

Running a low resistance/impedance heavy wire from the chassis of devices can provide an easier path for current to move along, and so allow each device to share the same the same ground potential (it's never exactly the same, because the wire is never 0 ohms - but I'm splitting hairs).

Linking the chassis together may cause issues if the ground isn't removed from all BUT ONE place. There is a studio practice of doing this, creating one place to link to actual earth ground.

Please don't follow anything I say regards mains earth and stuff. I am not qualified in this. I am just regurgitating stuff I have been reading on the 'net over the past week on ground/ground loops and the royal pain in the bum that they can be.
 

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