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U3818W, off, audio ground loop/hum/ buzz

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I have a U3818W attached to two laptops, with the audio going from the U3818W 3.5 mm out to a Denon receiver. In normal use it works fine, no noise, no hum, good audio.

However, if I turn the U3818W off, there is a constant loud hum on the audio which may be a ground loop (I'm not sure). Oddly, when the U3818W goes into power saving mode (e.g. leaving the laptop for a while), the hum is intermittent but regular: roughly 2 seconds of hum, half a second of silence, then a brief staccato hum, then another 10 seconds of silence - and repeat.

The hum is present if I have just the audio cable plugged in to the receiver, but disconnected at the monitor end. If I unplug the cable from the receiver, the hum stops. Not sure if the receiver "knows" if a cable is plugged in and otherwise silences the input, but it's just an ordinary RCA -> 3.5 mm cable - I tried 2 different ones and it's the same.

I have two other Dell monitors: a U2515H and a U2413, and they do not have this issue. There is no hum whether or not the monitor is turned on. Another U3818W that I have (temporarily - faulty panel, awaiting collection by Dell) displays the same audio behaviour: silent when the panel is on, but loud hum when it's off.

The reason it's annoying is because if I leave the computer and do something else like take nap or read a book, when Windows turns the monitors off after a preset period to save energy, the intermittent hum starts. I don't want to have to turn the receiver on and off each time I leave the computer.

So (I'm not any expert on this - just guessing) it seems like the U3818W does not ground the audio out when the panel turns off, but other models do. Is this correct?

Could there be a firmware update to fix this behaviour?

Any other suggestions? I've heard of ground loop isolator hardware but I'd rather not have to order extra hardware except as a last resort.

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Re: U3818W, off, audio ground loop/hum/ buzz

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I've bought a shielded cable and it does fix my hum.

Well, the first shielded cable I tried there was still a small hum, so I got another. Seems like it needs to be a high quality double shielded cable.

It feels a bit like a workaround as I didn't need it with other monitors - it wouldn't be a problem in the U3818DW was configured to join the ground and live when the panel is off.

Anyway, I'm OK for now. Thanks.

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Re: U3818W, off, audio ground loop/hum/ buzz

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Thank you! We have received the required details. We will work towards a resolution via private messages to ensure the security of your information. In the meanwhile, you may receive assistance or suggestions from the community members as well.


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Re: U3818W, off, audio ground loop/hum/ buzz

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I bought a ground loop isolator:

https://www.ebay.co.uk/itm/Mpow-Ground-Loop-Noise-Isolator-For-Car-Audio-Home-Stereo-System-w-3-5mm-...

but it does not fix the problem.

The hum becomes slightly quieter / higher pitched when I plug the hi-fi aux cable into the isolator, but the monitor does that as well, so basically the isolator does absolutely nothing.

I tried plugging the hi-fi cable into my phone (Lenovo P2) also, and it has the same (quieter) hum, but it stops when playing audio.

The U3818DW must be capable of cutting it out since it does so when the panel is on. However, the U2515H also does it when the panel is off, or even unplugged from the power. So what is the difference between the U3818DW and the U2515H? Is it the audio circuitry (U3818DW has speakers) or is it a firmware issue?

I appreciate there might not be an easy fix, but it's annoying that such an expensive monitor should perform worse than a much cheaper one from the same brand.

I received a PM from Dell on this forum asking for my service tag a few days ago but haven't heard back.

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Re: U3818W, off, audio ground loop/hum/ buzz

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Cubbers,

 

There are no knowledgebase articles that replicate this low noise complaint. I know we do not have any high-end Denon receiver in our lab. We have not seen this issue in our lab. Monitor firmware is for systemic issues when multiple customers have replicated the issue.  


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Re: U3818W, off, audio ground loop/hum/ buzz

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It's not really a high-end receiver, just a standard consumer-grade item that I'm using to get better sound. Maybe the receiver is faulty to have a hum when the input cable isn't connected to a source (I am no hi-fi buff so don't know if this is standard), but it isn't a problem with my other Dell monitors, which is why I'm asking why it is a problem with the U3818DW (I tried 2 units and it's the same on both) - what's different about this model? And it's potentially something that could be fixed in a newer version of the monitor.

I'm thinking it's probably not firmware since the U2515H is fine when powered off so presumably the firmware is not actively doing anything.

But something happens when the U3818DW panel is on, to remove the hum. I really know very little about this kind of thing so I would appreciate if someone with the technical knowledge could advise.

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Re: U3818W, off, audio ground loop/hum/ buzz

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Cubbers,

 

This information came from the Monitor SME. I will ask to see if there is anything that can be done. 


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Re: U3818W, off, audio ground loop/hum/ buzz

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This sounds like a typical ground loop hum,  technically a difference in potential between the grounds on the two units.  Usually "lifting" the AC power ground on one device will fix it.  This can be done with an AC power receptacle adapter that does not have the ground lug.  Use that on either connected device and this, in most cases, fixes the ground loop hum.  This can happen in high $$ equipment or inexpensive, doesn't matter.   I had an "audiophile" grade tube guitar preamp (high $$) that caused a ground loop problem.

One caution lifting a ground.  Make sure the other device is grounded and keep the interconnecting cable connected so both devices are AC power grounded.

There can be other causes, including a defective audio cable.  And, an AC power receptacle that is missing the ground or hot and neutral wires mis-wired.  A $5 AC tester will tell you if the AC wall receptacle (or power strip if you use that) is properly wired.

 

 

 



I am not a Dell Employee

Dell forum member since 2002

Dell Inspiron 15 - 5577 Laptop

Home Built Desktop PC with ASUS ROG Z170, i7 6700K CPU

Windows 10 64 bit Pro and Windows 10 Insider Program Beta Versions. SSD drives. Cakewalk by BandLab and Studio One 4.6 Pro Recordng Studio Software.

Dell S2719dgf Monitor

Member of Nashville based R.O.P.E.

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Re: U3818W, off, audio ground loop/hum/ buzz

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Thanks for the responses. I found a useful comment on Quora in answer to the question "Why do speakers create a buzz noise when the stereo cable is not connected to anything?"

https://www.quora.com/Why-do-speakers-create-a-buzz-noise-when-the-stereo-cable-is-not-connected-to-...

Someone said:

When you disconnect your stereo input cable from the sound source, it acts like an aerial and picks up the 50 or 60 Hz hum from nearby mains electric wiring.

To stop the buzz you can short-circuit the three jack-plug metal connections together by wrapping tightly with, say, aluminium foil. You could add a clothes peg to hold the foil in place.

I tried the foil thing, and it does completely stop the hum. So it looks like the cable is acting as an aerial when the ground and the live connections are disconnected from each other.

It must be that the U3818DW circuitry disconnects the ground and live connections when the panel is off, but other monitors don't.

Not sure what to do next - maybe try buying a shielded cable to minimize the aerial effect? I tried 3 different cables (all probably cheap unshielded ones) and was the same with all.

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Re: U3818W, off, audio ground loop/hum/ buzz

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You need a shielded audio cable.  The non-shielded cables can be used for speaker wire but not the audio connection application.. 

That is definitely one problem.  

 



I am not a Dell Employee

Dell forum member since 2002

Dell Inspiron 15 - 5577 Laptop

Home Built Desktop PC with ASUS ROG Z170, i7 6700K CPU

Windows 10 64 bit Pro and Windows 10 Insider Program Beta Versions. SSD drives. Cakewalk by BandLab and Studio One 4.6 Pro Recordng Studio Software.

Dell S2719dgf Monitor

Member of Nashville based R.O.P.E.

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Re: U3818W, off, audio ground loop/hum/ buzz

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I've bought a shielded cable and it does fix my hum.

Well, the first shielded cable I tried there was still a small hum, so I got another. Seems like it needs to be a high quality double shielded cable.

It feels a bit like a workaround as I didn't need it with other monitors - it wouldn't be a problem in the U3818DW was configured to join the ground and live when the panel is off.

Anyway, I'm OK for now. Thanks.

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