Amp Powered from PC with Noise Problem. Make use of Optical

Sun Mar 07, 2010 5:02 am   

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
British Columbia

Sun Mar 07, 2010 6:29 am   

In article <058c583e-2caa-45c1-a5bd-d22738546ab3
@g11g2000yqe.googlegroups.com>, stratus46_at_yahoo.com says...

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

Sun Mar 07, 2010 7:16 am   

On Mar 6, 8:02 pm, D from BC <myrealaddr...@comic.com> wrote:

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

Sun Mar 07, 2010 12:46 pm   

On a sunny day (Sat, 6 Mar 2010 20:02:39 -0800) it happened D from BC
<myrealaddress_at_comic.com> wrote in <MPG.25fcc53f6c7872099896c6_at_209.197.12.12>:


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

Sun Mar 07, 2010 11:53 pm   

On Sat, 6 Mar 2010 20:02:39 -0800, D from BC <myrealaddress_at_comic.com>
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.

--
John

Mon Mar 08, 2010 12:29 am   

In article <2m88p5lils6uq8560pmb0g8no3sohkbe4k_at_4ax.com>,
quiasmox_at_yeeha.com says...


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.


--
D from BC
British Columbia

Mon Mar 08, 2010 2:50 am   

On Sun, 7 Mar 2010 15:29:28 -0800, D from BC <myrealaddress_at_comic.com>
wrote:


I hope you're right.

--
John

Mon Mar 08, 2010 9:33 am   

On Mar 6, 8:02 pm, D from BC <myrealaddr...@comic.com> wrote:

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.

http://forums.slimdevices.com/showthread.php?p=205598
I've been meaning to try this out. Note that the buffering will not
allow the video to sync with the sound.

Mon Mar 08, 2010 6:51 pm   

On Mar 7, 3:29 pm, D from BC <myrealaddr...@comic.com> wrote:

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.

G²