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Topping DX3 pro plus: noise in stand-by, possible grounding issue?

Greenv

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Jan 4, 2024
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I have bought DX3 PRO+ recently. Came home and connected the DAC to the power supply, inserted 35 Ohm IEMs and immediately noticed very audible noise even before I turned the device on, i.e. when it was in stand-by mode. The noise doesn't depend on volume level and is almost or completely gone when I'm touching the metal body of the DAC. Similarly noise level is reduced significantly when I connect the DAC to my PC via USB (which might provide some grounding I think). I've tried another 15 V / 1 A power supply, and the issue remains. RCA out line is also affected, giving very loud background noise to active speakers compared to pairing them with other sources. The only way to completely remove the noise was to power up the DAC using a UPS disconnected from the power line, i.e. when UPS works on battery.

I've tried 3 sets of IEMs - the lower the impedance the lowder the noise. Tried different power sockets in my flat - still noisy. Unplugged all other devices, such as PC and router - no change. The last experiment - I connected RCA line to active speakers, inserted power supply into the socket, toched the minus line of power supply and touched the metal body of the DAC (using my body as a conductor of sort) - and voila - the very same characteristic noise. I think this all indicates toward grounding issues. (Power supply unit doesnt seem to have grounding contacts, btw; don't know if this is important)

I was absolutely sure that my unit is defective. However my friend also got this DAC last year directly from China. I got it for tests and it got similar noise problem, though as far as I could tell a bit less prominent. My friend might have not noticed the issue because he used the DAC exclusively for high impedance heaphones.

I don't know, mayby it has to do something with current voltage / frequency (220 v /50 Hz) or quality of powerlines in my country, but I'm still very much convinced that this is not normal and the unit is in fact defective (despite similar results with another unit).

If you have the DAC could you please do this little test for me - disconnect all lines, connect to power supply and insert low impedance IEMs or active speakers through 3.5 jack or RCA - if you hear the noise, touch the metal body. Please post your results here.

Or if you have the knowledge about this kind of things - is this can be easily repaired, like by removing the metal body or maybe applying insulation to power line input? Or is this a clear case for refund (by local law I can only refund this item if it is defective)?
 

MaxwellsEq

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Aug 18, 2020
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It might be something faulty, but it also sounds like a ground issue. It's unrealistic to listen to a device when it isn't connected to the rest of the chain since it might get its ground from proceeding devices.

If it has no earth connection, it's probably a class2 device, which "floats". You are acting as both high impedance ground and antenna when you touch it.
 

sofastreamer

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Dec 30, 2021
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mine is having a constant 50hz hum that disappears when touching the unit. even when its only plugged directly to the wall without having attached usb or other inputs. might be a problem of my appartement tho.
 
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