Hum 50Hz from USB notebook port into DAC

Hi all,

Hopefully somebody can give me some advice in getting rid of some hum from my USB port. I just finished building a test setup of a Broskie cathode follower which is connected to the balanced V out of my DAC. The DAC is a IFI nano DSD with a Burr Brown 1793 DSD chip. First impression are ok, it sounds pretty good. However the hum/noise from the USB port is spoiling the fun a bit. Some symptoms:


  • Disconnect USB cable from the notebook, hum is directly gone, tube stage very quiet
  • Hum is in left channel more present than in the right channel
  • Disconnecting the power supply from my notebook result in stronger hum
Does anyone have an idea how to get rid of the hum probably caused by a ground loop?

I myself found following device which might help:

Intona ? Products (USB high speed isolator)

Does anybody has experience with this isolator?

Sorry for the bad english, it is not my native language :rolleyes:
 
Fifty-Hertz hum is the ground loop and I think the source is the notebook. Did you try a different digital source? People have found different solutions to the mains hum: isolating 5V in the USB cable, even lifting the ground from it (in most of cases, DAC will not see the source anymore). The last resort would be the ground loop isolator.
 
use the battery, i.e. unplug the charger. see if it helps. report back

Sorry, i just realised after more testing my title is a bit misleading. The funny thing is that when i unplug the power of my notebook the hum/noise is getting worse. Therefore i do not think it is the 50Hz noise from mains, but computer noise. I can even hear (when not playing music) the copying of a file via wifi. This gives a sort of rattling sound. The noise reduces a little bit when i plug in the power cord which has a ground lead.
 
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silverprout

Member
2008-04-03 4:19 pm
Thanks for the tip! That does indeed sound logic. Which would you recommend for my DAC chip BB 1793 DSD V out? Will 600/600 ohms suffice?


SOWTER 9150 Coarse control 10 dB steps... you should contact them they have better alternatives for your application.

You should also use an APTX Bluetooth adapter and suppress your cable ;) if you don't care about HD music.
 

Extreme_Boky

Member
2003-12-07 11:57 am
we need to see the photos. how is that dac connected to tube amp? is the dac embedded within a tube amp box, or a separate component?

does DAC have balanced output?

my guess is that DAC is a separate unit, and it has its mains cable ground connected to the case - same with the tube amp - and there's a problem... through interconnects shield

to start the process of elimination, the USB cable should have the shield disconnected from the UBC connector metal shell at laptop end. this will help you locate the problem... i still think it is a sensitive nature of the tube input stage, with enormous input impedance, together with that double grounding of DAC case and tube case a mentioned above

you need to send the photos...and be prepared to open everything.

otherwise, sowter suggested above will fix the problem most likely (if it is not laptop - DAC coupling related)
 
Hi all,

Before i got the advice of using an isolation transformer i had already ordered below device :innocent: and yesterday it arrived. Bit expensive and ugly as hell :)

Intona ? Products

Despite being beautiful weather here i could not resist to try and test it. Result?

Dead silent! :) I can open up the volume of my amp to maximum and no more computer noises and/or hum (without playing music). And according the specs this device can handle all USB2 speed modes, so high res music should not be a problem (sorry, did not test this yet).

Please note i also ordered some transformers out of curiosity, so i will try this as well with the dac (without and with the USB isolator) and share the results.

Thanks all so far for your advice and suggestions!

PS Photos will follow later when i have it boxed and a bit tidy. Currently it is just in test setup on a wooden plank.
 
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For future reference ;-) i would like to share following quote from Analog Devices:

'Traditionally, to avoid this problem many types of industrial communication standards, like RS-485, use differential signal transmission with no ground connection to avoid potential ground loops minimizing induced noise in the system and potential for data errors.

However, with standard USB cables the ground connection cannot be avoided. The only alternative is to break that ground loop with a USB isolator, such as the ADuM3160 or ADuM4160. Both of these devices will galvanically isolate the host and peripheral USB devices while seamlessly allowing communication between the two systems.'


Source: http://www.analog.com/media/en/tech...rticles/NAppkinNote_Breaking_Ground_Loops.pdf