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

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.

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D from BC
British Columbia
Reply to
D from BC
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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.

G=B2

Reply to
Glenn Gundlach

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

Reply to
D from BC

On a sunny day (Sat, 6 Mar 2010 20:02:39 -0800) it happened D from BC wrote in :

Cheap audio transformer? Modulated current source driving opto? FM modulator - > HF transformer -> FM demodulator?

Reply to
Jan Panteltje

On Sat, 6 Mar 2010 20:02:39 -0800, D from BC wrote:

I ran into something like that, though not exactly the same. I wanted to play my hard-disked record collection on a laptop through an older stereo amp. The laptop is the same one I'm using for a digital piano. The piano system uses up the main audio system on the laptop, so I decided to use a USB audio card add-on for the music player app. Well, when I hooked the analog output of the USB audio card to the stereo amp, there were all kinds of little squirrely noises, especially when repositioning things on the screen. I thought the problem might be due to using the PC power supply to power the USB audio interface, but opening that interface up and substituting a clean 5V supply as power to the interface didn't make any difference at all. I found the same problem occurred with two other stereo amps, but didn't happen at all on a new Mackie monitor amp/speaker that claims high RF immunity. So, I figured it was due to RF from the laptop's video system (it would get worse if I externally jumpered the HDMI ground on the laptop to the audio ground). I didn't want to throw away an otherwise perfectly good stereo amp and buy a new digital receiver for n-hundred dollars, and the Mackie speakers were dedicated to the digital piano, so I looked for another solution. I tried a lot of fussing around with extra ground resistance, cable routing, heavy filtering on the audio input of the stereo, and I don't remember what else, and nothing really improved it, so I went the optical route. I found the $15 USB audio interface had an SPDIF signal on the chip inside that wasn't brought out, and connected it to a TOSLINK transmitter. Then I made an SPDIF-DAC board using a TOSLINK receiver, a UDA1351 SPDIF receiver-DAC, a couple of power regulators and miscellaneous glue stuff. A 12-foot TOSLINK fiber cable only cost a few dollars. Optical fixed the problem entirely. The music player audio is very quiet.

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.

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John
Reply to
John O'Flaherty

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.

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D from BC
British Columbia
Reply to
D from BC

I hope you're right.

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John
Reply to
John O'Flaherty

I have that feature standard on my mobo.

You may find the old Logitech Squeezebox on the used market. It is a bit more versatile than an optical based D/A. It is meant as a stand alone internet radio or music player, but I'm pretty sure there is a software hook to make it act like a sound card.

formatting link
I've been meaning to try this out. Note that the buffering will not allow the video to sync with the sound.

Reply to
miso

I had servo/power supply noise in a car stereo connected to a CD player. The cure - which worked outstandingly well - was to use balanced line input from the CD player. The two channels of '+' phase go to the signal ouputs. The two '-' inputs both connect to the 'ground' of the CD player which did NOT have balanced outputs. Any ground loop noise is on the signal line as signal + noise. The ground noise gets subtracted from signal + noise leaving just signal. IOW, treat the system as multiple grounds because it is.

G=B2

Reply to
Glenn Gundlach

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