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DX3 Pro Hum issue

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Feb 2, 2019
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#1
Hello

My DX3 Pro has very silent but still noticeable hum in headphones (250 Ohm Beyerdynamic DT 1990 PRO) even with disconnected USB and TOSLINK cables.

The hum is constant and it does not change in frequency and intensity.
Hum does not depend on the volume level.

Hum disappears when I touch metal case of DX3 Pro with my finger.

Is it possible to eliminate this hum?

Europe, 230V / 50 Hz

Thank you
 

NTomokawa

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#2
Sounds like a classic ground loop. There are commercially-available ground isolators to solve this issue.
 
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#4
Ebtech Hum X has only US plug.
I need something for EU.
 

Darwin

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#5
Man this thing is full of problems. Mine was lost in shipping and now I’m glad it was. On top of it you have to send it to China for repairs. My NX4 has been solid though.
Like a lot of people I’ll grab an Atom as soon as JDS makes a matching dac.
 
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#6
Man this thing is full of problems
All audio devices have its own problems.
DX3 Pro is one of the best devices for its price and I like it.
P.S. I also bought JDS Atom recently for my tablet and waiting for it ;)
 
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#7
Man this thing is full of problems. Mine was lost in shipping and now I’m glad it was. On top of it you have to send it to China for repairs. My NX4 has been solid though.
Like a lot of people I’ll grab an Atom as soon as JDS makes a matching dac.
Are you going to consider the tone board DAC? Who knows when and how well it will measure if JDS makes one.

I just returned my DX3 Pro and have the Atom in hand. Waiting patiently for my tone board to arrrive. I have the Aluminum case on hand, just need to drill some holes in it.
 

Darwin

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#8
All audio devices have its own problems.
DX3 Pro is one of the best devices for its price and I like it.
P.S. I also bought JDS Atom recently for my tablet and waiting for it ;)
Not like this which should be evident.
 

Darwin

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#9
Are you going to consider the tone board DAC? Who knows when and how well it will measure if JDS makes one.
I just returned my DX3 Pro and have the Atom in hand. Waiting patiently for my tone board to arrrive. I have the Aluminum case on hand, just need to drill some holes in it.
I looked at their website and wasn't clear on what to order. Will have to look again.
 
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#10

solderdude

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#11
Hello

My DX3 Pro has very silent but still noticeable hum in headphones (250 Ohm Beyerdynamic DT 1990 PRO) even with disconnected USB and TOSLINK cables.

The hum is constant and it does not change in frequency and intensity.
Hum does not depend on the volume level.

Hum disappears when I touch metal case of DX3 Pro with my finger.

Is it possible to eliminate this hum?

Europe, 230V / 50 Hz

Thank you
Is the DX3 connected to headphones only via its own headphone out ?
Not to any other devices such as headphone amps or power amps ?
If that's the case its no groundloop.

Does the hum go away when it is connected to USB ?
That is ... USB from a PC not from a laptop or other non mains connected device ?
That PC must be connected to 3 mains socket with safety ground in that case.

The best way to get rid of this hum may be to ground the DX3 to something like central heating (when used with metal piping) or waterpiping (all copper). When this is not available close by then the safety ground of a mains socket (3rd prong and be careful) is an option.
When the tests above point to the device needing a proper ground I will explain more detailed.
 
Last edited:
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#12
Is the DX3 connected to headphones only via its own headphone out ?
Not to any other devices such as headphone amps or power amps ?
If that's the case its no groundloop.
Exactly. I also think this is not the ground loop.

Does the hum go away when it is connected to USB ?
No

That is ... USB from a PC not from a laptop or other non mains connected device ?
PC with good power supply.

That PC must be connected to 3 mains socket with safety ground in that case.
I live in country where electrical power distribution systems in all houses and buildings don't have any grounding... :)

The best way to get rid of this hum may be to ground the DX3 to something like central heating (when used with metal piping) or waterpiping (all copper).
I tried to ground it to the central heating system. Result: no effect :)

Hum disappears only when I touch the metal case of DX3 Pro with my finger.

P.S. Some days there is no problem with hum. I think the problem is located somewhere in electric power system in my country.
 
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solderdude

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#13
Could be the central heating pipes are not connected to earth (they should be though)

Is the PC connected to a 2 prong mains outlet ?
Mine is also not connected to a 3 prong outlet by the way... old house.
New houses must have 3 prong mains sockets in every room now.

It appears to be a grounding issue.
Would be nice if you can find a ground somewhere .. then move the DX pro there and see what happens when the DXpro is stand alone and when the enclosure is connected to a ground and when touched only.
 
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#14
Could be the central heating pipes are not connected to earth (they should be though)
Pipes are not connected to the earth.

Is the PC connected to a 2 prong mains outlet ?
We have 3 prong mains sockets everywhere but in reality neutral and ground cables are simply connected together...
 

solderdude

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#15
Try to connect it to the safety ground.

Neutral and ground are tied together but only at the point where power comes into the house or somewhere at a distribution point. Depends on how the mains grid is built.

Safety ground should work O.K.

Ensure you do not connect to live or neutral.
See if the hum goes away.
 
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#16
If he checked optical in (thus complete isolation from PC) and used dx3 pro alone (without any extra amps etc) there is no possibility to have ground loop here.

Do one more test - use dx3 pro connected to laptop that is powered only by it's battery (no ac adapter connected) and see what happens (this should eliminate all of your problems, even the one with having 3 prongs and safety prong in the air - not connected, you can use standard usb connection here). If the problem still exists, try connecting dx3 pro in your work office for example to change electrical installation. If that won't help, this may be faulty unit.

Could be also high DC offset on your AC line. In this case adding separating transformer (1:1) could help.

If you touch it and it disappears reminds me something when was the issue with weak ground of the device itself. Maybe it's some cold joint inside or not grounded metal case of the unit properly ? It looks a bit like nothing has stable ground potential heret, thus I would say check if your safety prong in the socket is connected to neutral as you claimed.
 
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solderdude

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#17
The DX3 pro is fed with an external DC supply. Even if the mains is nicely connected to safety ground the amp still will not be.
When he grounds the amp (with his body) the hum goes away.
So that's why I already suggested to ground the enclosure of the amp to safety ground.

Checking for continuity between the enclosure and the RCA ground is indeed a good idea.
Maybe the enclosure is not connected to the groundplane.
 
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#18
Checking for continuity between the enclosure and the RCA ground is indeed a good idea.
Maybe the enclosure is not connected to the groundplane.
That's exactly what I was thinking about. Had some amp in the past that had loose case connection to the ground and acted like this.
 

777

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#19
Hello

My DX3 Pro has very silent but still noticeable hum in headphones (250 Ohm Beyerdynamic DT 1990 PRO) even with disconnected USB and TOSLINK cables.

The hum is constant and it does not change in frequency and intensity.
Hum does not depend on the volume level.

Hum disappears when I touch metal case of DX3 Pro with my finger.

Is it possible to eliminate this hum?

Europe, 230V / 50 Hz

Thank you
Change the stock power supply adapter with another one which doesn't has "Y"-type capacitors in the 230Vac input filter. Or you can choose an power supply with safety ground ("class I" of isolation) . In this case safety ground in your outlet it is a must.
 
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#20
Change the stock power supply adapter with another one which doesn't has "Y"-type capacitors in the 230Vac input filter. Or you can choose an power supply with safety ground ("class I" of isolation) . In this case safety ground in your outlet it is a must.
I have the same, old, crappy installation and my dx3pro was dead silent even when connected to PC USB (not even mention the toslink). Must be something wrong.
 
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