Amp Powered from PC with Noise Problem. Make use of Optical S/PDIF?

[D from BC]

I made an audio amp that's powered off my PC's 12V.

Everything is fine except for some digital noise.
I can hear digital noises when I scroll, move the mouse and when drives
are spinning up('rrrrrrrrrrrrrrrrrrrrrrrrrrrrr').
I suspect it's a ground problem only fixable by some form of isolation
between signal ground(analog from sound card) and power ground from the
switching supply.
If the noise were 1/2 as loud, it would be good enough.

Should I try to find an optical S/PDIF receiver and a codec to fix the
ground problem?
My PC has a S/PDIF optical out.

 

[D from BC]

I got some noise in the analog audio on the ViewSonic monitor I bought
in December very similar to what you're describing. I have an inline
analog pot for volume since explaining to the family how to use a menu
to change volume is just dumb. I broke the ground in the audio line
and inserted a 10 ohm resistor to 'loosen' the ground and greatly
reduce the ground loop currents. Problem solved. SPDIF would work but
costs a lot more than a simple resistor. For the main analog audio out
to 2 other receivers in addition to the ViewSonic, I have a Niles
Audio 6 ouput DA with individual level controls for all analog outs.
Works very well.

G²

Yup.. I'll try quick fixes over making a pcb for S/PDIF ardware..

 

[D from BC]

Anyhow, you might find that the problem won't go away as long as
there's a common electrical connection from the PC to the amplifier.

Yah...Total isolation is king. However, I have some disagreement with
that in this case.
When an audio amplifier is powered by tapping the 12V off a desktop
computer, two ground returns are present, signal ground and power
ground.
The optical connection out of the PC fixes the problem of power noise
current running through the signal ground.
What remains is the power ground to power the amp which is quite noise
tolerant (C bypassed + Amp has PSRR).

I'm expecting good results as long as there's only a power loop between
the amp the PC power supply and not a long electrical signal loop.