Ground loop noise

[FrankenPC]

I purchased some cheap CA powered speakers/sub for my shop and they have a ground loop problem causing hum.  I tore the speaker apart and exposed the amp.  It's a ST TDA7379 supplied by 18V through a 1/2 wave rectified transformer.  What's the quickest way to kill the ground loop hum?  I thought I might try a 18V DC switched power supply.  Or is there a simpler way with a filter?

http://www.datasheetlib.com/datasheet/397755/tda7379_stmicroelectronics.html

[Shredhead]

Depends on if the ground loop is from the signal chain or the design of the amp.  If you can rule out the signal chain and any external interference I would take it back to the place of purchase.  Why pay for a bad design?  I know I'm giving no real answers here and I apologize but in my experience, tracking ground loop hum can turn into a nightmare.

[Paul Price]

If you hear hum in your CA speakers without the input to the speakers connected to the PC, then you certainly have a defective speaker system and get your money back.

On the other hand, if you only hear hum with the PC connected try reversing the AC power plug (if the plug isn't polarized) on the speaker power supply. If that doesn't help, changing to a switching power supply will not help.

The hum is caused by a capacitive connection to the AC main flowing through the ground connection of the speaker's sound input cable to the PC.

Try plugging both the PC and the CA speakers into the same power strip(and reverse one or the others AC power plug  to see if that helps.

 A fairly large wire running from your PC's metal chassis to a ground connection on the speaker would also help, if you think it is worth  that extra wiring hassle.

Finally, at the speaker's end, putting a 10-ohm resistor in series with the ground of the cable coming from the PC (in combination with the extra grounding wire) will certainly fix the problem.

[bingo600]

Might not be relevant here , but i once had 50hz hum in my active subwoofer.
I'd used a cheap 5m RCA cable from the AMP to the SUB , and it was close to some 230v mains wiring.

I solved it by using a 5m coax cable (75ohm tv cable) with RCA plugs on.

/Bingo

[sigxcpu]

I've had the same thing in this scenario:

- connect laptop to amp (no hum)
- connect TV to amp (no hum)
- connect HDMI from TV to laptop (hum started)

Solved by floating the amp (use 2 prong socket).

[G7PSK]

Might not be ground loop problem if the DC is from a half wave rectifier, the smoothing cap might be too small.

[Anks]

Try the speakers with a mp3 player running on battery with a short cable to check. If that is fine im going to guess potential ground difference or too long a run.

If the run of cable is picking up the hum then you will need to convert to a balanced connection. A easy way is to use a D.I box.

[FrankenPC]

Try the speakers with a mp3 player running on battery with a short cable to check. If that is fine im going to guess potential ground difference or too long a run.

If the run of cable is picking up the hum then you will need to convert to a balanced connection. A easy way is to use a D.I box.

The amp is bad.  When I first got it, it was humming only when I completed the circuit by plugging into mains.  Now, the hum is there all of the time.

Furthermore, I removed the AC connection and powered from my bench supply.  The hum is still there.  I even tried moving the cables to eliminate any kind of AC resonance.  It's still there.    It's bad and it's cheap.  I'm loosing interest in it