Investigation Into Effects Of PC load On DAC Analogue Output

[One and a half]

@March Audio, In revisiting this thread and the graphs on page 1, the PC is shown at idle. 

Most DACs I know of remain muted and if there's no incoming signal to process and display the last current state and don't perform any work.

 

The graphs would be more meaningful if the PC was actually playing a file (doesn't really matter what type) and the DAC is processing and performing what it does. The idle PC shows, predictably no difference in the DAC output cause there's no work to do.

 

Or am I missing something?

 

[One and a half]

Is there a way to measure common mode noise only on the input to the DAC and the output of the DAC?

[One and a half]

4 hours ago, March Audio said:

Sorry but you misinterpreted the plots.  The dac is playing a -60dB 1kHz tone, so it's working away 😀

Tones. No music?

[One and a half]

16 hours ago, March Audio said:

Why?  We are only interested in the output of the dac.

The images of the waveforms don't tell the whole story. A PC is a common mode generator par excellence. There are so many smps and pwm controllers on the motherboard, that anything connected will receive a fair dose of noise, this includes USB, AES3 cables.

The images don't show how the soundstage is set, how wide or narrow the instruments are placed in music. From my humble experiments with the DX ISO filters on ethernet transmissions, there's more up the upper end of the conducted EMC spectrum that causes soundstage errors in DACs. Yes these are heard and are subjective. By removal of upper audio band noise does the audio soundstage improve.

 

Here's a paper on common mode noise on AES3 transmissions for interest.

 

 

AES2003.pdf

[One and a half]

7 hours ago, March Audio said:

Must to add, for soundstage to change the frequency response, phase (timing), or amplitude would need to change.  The tests look at all of those things.

Many a subjective experiment concentrated on reducing common mode noise has made improvements in soundstage. The worst scenario is a laptop connected directly to a DAC via USB (with a printer cable). Common mode noise can ruin an image very easily.