Additional measurement for USB DACs - immunity to a ground loop

[miero]

@amirm I suggest a following measurement for detecting possible USB interference issues for USB DACs. Sine 942Hz at various volume levels (e.g. THD+N vs. dBFS) for two highest sample rates supported, e.g. 352.8 and 384kHz.

The 942Hz and its harmonics should not hide possible peaks of USB low-speed frames (1ms ~ 1kHz) or USB full-speed microframes (125us ~ 8kHz).

Why? I tested Pro-ject Pre Box S2 digital that behaves very badly when it was connected in a middle of a ground loop - PC with ATX supply and Behringer B2031A monitors, that have connected singnal ground to PE.

I must admit that I my PC and monitor speakers were connected in different wall power outlets and although these sockets were next to each other, each one has a separate circuit breaker 10 metres away, which is extreme situation not suited for unbalanced audio.

But that DAC playing over USB has been producing a well hearable and very annoying 8kHz and 16kHz tones in both channels, in the left one it was louder. And on higher sampling rates it was even louder and not bearable.

I decided to not review that DAC anymore, because ODAC (USB 1.0) was in the same situation silent.

Unfortunatelly I found out my problematic power wiring just after I returned that DAC, so I don't know how it would improve after connecting to a single power outlet, but keeping it in the middle of ground loop.

 

[amirm]

To clarify, you are asking for two changes: one is from 1 kHz to 942 Hz and another is to test high sample rates of 352 and 384?

 

[miero]

Yes, two measurements of 942Hz sine at high sampling rates. Maybe it would be enough to test it once, just the highest fs - because that will generate the most of USB traffic.

 

[Blumlein 88]

I don't find the level of 8 khz spikes from USB to be related to sample rate or the level of traffic on the USB signal going to the DAC. Have you found that to be the case in some measuring of different DACs?

 

[miero]

I made an experiment with an integrated amplifier NAD D3020 and its USB input. Here are two measurements with a just one difference. A grounding wire is added between notebook #1 and notebook #2 in the second case.

Measurement setup:
Notebook #1 on battery playing 942Hz sine on 96000fs -> USB input of NAD D 3020 playing into headphone output -> Audio line in to Notebook #2 on battery.

Without a ground loop:

With artificial ground loop using a wire between USB ports of two notebooks:

@amirm it seems that a ground loop is required to trigger this issue, at least with NAD D3020. I guess some DACs might be more immune to ground loops than others. I think that it is worth to try to measure some. It needs just a single wire to be added.

 

[amirm]

@amirm it seems that a ground loop is required to trigger this issue, at least with NAD D3020.

I have been experimenting with this also. Some devices don't care, some get worse, some get better. For my dashboard view, I have started to optimize the situation and show that.