A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input)

More thoughts about grounding :

I guess your PC is connected to earth wire (except you use a laptop with power supply not connected). If the DAC or one of the devices in the DAC chain connect audio gnd to erath wire you have a ground loop via earth wire. Also check your chassis vs. screw / bolt of Audio Device (e.g. DAC) PCB's, they could also connected audio gnd to earth wire.
 
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with things like this try a number of things, deduction is the key word here... so try another cable, use another computer, connected another amplifier and so on. change power supplies for batteries, audiophile quality is not important, just find a way to isolate a possible cause. my suggestion is to try all this options to find out. be creative and think outside the box.
 
But you get noise when you plug in the usb complete without playing music ?
Yes.

I researched the subject a little deeper in DDDAC site on Quantization noise floor:
DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit

So played HI-REZ music in 96Khz and the noise is gone! And certainly this converter sounds better in HI-REZ.

Played regular 44.1Khz files, and the noise came back.

This appears to be the Quantization noise floor!

It turns out that the DT880 headphones is very revealing.
So maybe I'll have to sell this wonderful converter. :(

Most of my music is in 44.1Khz and I use headphones quite a bit.
 
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Yes.

I researched the subject a little deeper in DDDAC site on Quantization noise floor:
DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit

So played HI-REZ music in 96Khz and the noise is gone! And certainly this converter sounds better in HI-REZ.

Played regular 44.1Khz files, and the noise came back.

This appears to be the Quantization noise floor!

It turns out that the DT880 headphones is very revealing.
So maybe I'll have to sell this wonderful converter. :(

Most of my music is in 44.1Khz and I use headphones quite a bit.

2 things to try.
1) you can reduce the quantization noise with 4 small film caps. Can't post details from my phone, but do a search and you should find it
2) depending on what computer and software you use, you could try software upsampling with the sox plugin, so you only send high res to the DAC
 
2 things to try.
1) you can reduce the quantization noise with 4 small film caps. Can't post details from my phone, but do a search and you should find it
2) depending on what computer and software you use, you could try software upsampling with the sox plugin, so you only send high res to the DAC

I would try point two first, was exactly my thought....
than use the Hf filter.... 22 nF between POS and NEG will do for 1 dac board