Difficult to understand some electronic equipment

Rocker

For many years the audio system attached to my P.C is as follows:  a Cambridge Audio DacMagic Upsampling DAC, taking the USB output from the computer and outputting a stereo signal into my Musical Fidelity amp.  Worked very well until about a year ago when the DacMagic started to get very hissy and noisy.  By a process of elimination this was shown to be the USB input.  Suspecting the USB cable and/or the USB outputs of the computer, I tried numerous cables and all the USB outputs to no avail.  Hearing of my problem, a friend gave me his unused Gustard U12 USB to coax 'box' and amazingly this silenced the noise when the coax output was fed into the coax input of the DacMagic.

Last week, I unplugged the USB cable from the U12 and plugged it into the USB input on the DacMagic.  Exactly as it was up to a year ago.  No noise and a sound quality that, while not good hi-fi, is still way better than the mini stereo jack output on the computer.

I have no idea what is going on here.  If the computer itself was creating the noise, the U12, which is a top notch USB->coax converter, removed it.  The computer OS has received several updates over the years....

Trying to fathom it out, does my head in.  So it does! 

Emp_Fab

I can't offer advice here but I know what's coming.... :lol

Funkfingers

Better grounding and/or screening on the co-ax cable should reduce the signal-to-noise ratio. So would a hotter signal. 

Proximity of output sockets to noise-generating circuitry is another likely candidate. For example, the 1U rack case of the Roland S-760 sampler obliged the headphone socket to be too close to the LCD and one of the processor chips. Result - weird clicking audio artefacts audible through headphones but not via the analogue or digital outputs of the module.