Car Speaker & small amp problems

[EvilGreenDevil]

Building my first cabinet and ran into a little bit of a disappointment when my audio orders arrived yesterday and I couldn't get things working. I was hoping someone could help. Building a bartop cabinet that will use 4" speakers as most do. I found a link to a cheap "mini/motorcycle" amp through researching other threads that is suppose to run on 12V. I'm thinking that wiring a molex connector from my 450w PC power supply should be enough to power it.

I did the wiring for the power and connected it to the amp, success (at least I thought so given the red power light on the amp went on). I used the 12V line from an extra molex connector and the ground wire next to it. The 12V/ground were then spliced to the corresponding ones to the amp harness.



I went on to connect the +/- terminals to the corresponding wires that came with the amp by a simple connection of the wires and wrapping each one in electrical tape. When I pugged in the speakers i'm getting a faint/dirty hum and no sound from the pc is recognized. I know I'm doing something wrong.

The amp is connected to the audio jack (I've tried front and back) to the PC using a 3.5mm --> RCA split

Speakers: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=269-178

Amp:http://www.aliexpress.com/product-fm/369889204--New-Red-MP3-Stereo-Amp-Motorcycle-Amplifier--Made-in-China-Cheap-Accept-Paypal-wholesalers.html


Back of amp




Any help would be appreciated

[EvilGreenDevil]

It was and right after posting my question I went back to the cabinet to check the audio card settings in windows. I think I had an audio setting checked that I didn't new for stereo/analogue audio.

I get sound now but I'm still getting a minor hum whenever sound isn't being played. I'm assuming it's a grounding issue.

[Drnick]

Indeed it is a grounding issue.  I have the exact same amp. You could either try a grounding loop or you could use a separate power supply.  I used a 14V 3a or something very close to and that works OK.  Ultimately I decided against using it in the Kiosk I was converting and went for a set of Logitech x530's instead.  I am now using the amp to run a small sub I had kicking around from an old 5.1 dvd setup.

[EvilGreenDevil]

Not sure I'm going that big, mine is a small bar top so I will just be using the car speakers, no sub.

What at your suggestions for a ground loop and where to attach it in the cab if I'm gonna stick with the pc psu?

[Drnick]

Do a search for Ground Loop Isolator and see what comes up, you should be able to find something fairly cheaply.  I can't really advise on the best one as I just used a separate adadpter to power the amp in the end. 

[matsadona]

Drnick is right. This is a common problem, which I share using my laptop for DJ’ing. You need an isolator and they are pretty cheap.
Here is one found at Radio Shack, which I assume you would find close to were you live.

http://www.radioshack.com/product/index.jsp?productId=2062214