Problems with 12v to amp

[raidersan]

Some background. I know very little about electricals, enough to wire a plug and be dangerous to myself when trying stretch that knowledge.
Nevertheless I decided to go bold to get my son interested in some building activity and here we are with our bartop.
I am sitting with a stripped PSU, motherboard, hard drive all seemingly playing ball even though I got rid of the kettle plugs and soldered the points on the PSU straight to my power cable.
The problem comes when I decided to feed my amp straight from the 12v from the PSU instead of the PSU from the amp. All works fine when the dedicated PSU is used (It's an oldish Creative SoundWorks SW310 with sub). So I know there is no interference from other components. When I try to wire my main PSU's 12V line straight the to amp's points, I get massive interference, to the point that I can't hear normal sounds through. The earth is good and the 12v is dead stable at 12.03v. I am drawing 5v on the same line as well. I also tried to run a separate earth straight from the power cable (instead of the molex from the PSU). Nothing helps. I assume it is something to do with the way I have soldered the line on the MB, but I am at a loss. Could it be shielding required? Any help would be much appreciated!
Regards,
JF

[big10p]

The soldering on the ground wire doesn't look great, TBH. Have you checked it for good continuity? Otherwise, it'll probably be some kind of ground loop issue, I guess.

[BadMouth]

It's generally a bad idea to power the amp from the same power supply as the source audio.
Just asking for noise IMO.  A ground loop isolator may help, but probably won't eliminate the noise.
The ground loop isolator will probably take up as much room as the original amp power supply.

[JollyRoger]

I have seen this before, it is a ground loop issue, . The original power supply to the speakers was probably, just a square block plugged into the wall with 12 VDC out. That is has a transformer which reduces the 120 VAC to a lower level the rectifies it to 12 VDC. Cheap and easy. The power supply of the computer is mostly likely a switching supply, which uses a regulator to "switch" the output to compensate for the change in loads. If you add more load the noise you hear will change.

I'm using a pair of USB powered speakers in mine, plugged into a 5 dollar phone charger from Walgreens.

[stuzza]

As others have said this is most likely a ground loop issue.  I have experienced this issue on my bartop builds when running the amplifier from the computers power supply.  Due to space restrictions I couldn't fit in another Power supply so I installed a Ground Loop isolator and this has pretty much eliminated the issue.

I'm in Australia and bought this one:  http://www.jaycar.com.au/PRODUCTS/Sight-%26-Sound-Car/Car-Audio-Hardware/Car-Hardware-Accessories/Ground-Loop-Noise-Isolator-%28Stereo%29-3-5mm/p/AA3086  You should be able to find similar locally to you at an electronics store or on ebay.

[raidersan]

Thanks all for helping out. I have located an old 12V PSU that I have now added to my build and that has solved the problem. Next time I will buy of one of isolators, they are only about £4 on eBay