AC Line hum problem??

fmksey · Post by **fmksey** » Tue Dec 12, 2006 1:47 pm

I hope someone can help me out with this problem or point me in the right direction. We are removing cassette machines from 2 churches who
do weekly shows and replacing them with laptops. The laptops are connected to the line out on the mixers. We get good clean audio using
the battery packs in the laptops ---- but when the power supplies are
connected there is a bad hum. Is this AC line hum?? What ever it is -
any ideas how to get rid of it?

Thanks

Lester · Post by **Lester** » Tue Dec 12, 2006 2:12 pm

Re-wire the church.

Plug the laptops into the same/different circuit as the audio equipment. One of those two will work, but I don't remember which. (Same problem here at work.)

sportsvoice · Post by **sportsvoice** » Tue Dec 12, 2006 2:46 pm

I had this problem before we went to using Circuitwerkes and DTMF to fire the underwriting spots in our sports feeds. We were using a laptop and feeding them down the phone line. Plugging the laptop into my sports mix was causing a hum when the power supply was plugged in.

I fixed it by using a ground loop isolator like this one:
Rat Shack car audio ground loop isolator

This might also work:
http://www.jkaudio.com/pureformer.htm

The-Hammer · Post by **The-Hammer** » Tue Dec 12, 2006 3:45 pm

Also check the laptop power bricks. I've heard of others having problems when the cord from the wall to the brick is 2-wire instead of a 3-wire connection. A switch to a generic power brick with a 3-wire cord solved the problem.

Clay JD Walker · Post by **Clay JD Walker** » Tue Dec 12, 2006 5:44 pm

I bought a SoundBlaster Live external 24-bit sound card to solve that problem...most laptop onboard sound sucks to begin with, as well as adds extra work to the processor and crap...

Clay JD Walker · Post by **Clay JD Walker** » Tue Dec 12, 2006 5:45 pm

Cameron always said the best way to get rid of a hum...

teach it the words...

</rimshot>

Waveguide · Post by **Waveguide** » Wed Dec 13, 2006 10:26 am

Sounds like some isolation transformers are needed.
These damn laptops... ( and desktops too ) do funny things with grounds.
Don't depend on the chassis actually being true ground.
Desktop PC's don't usually have a power transformer in the power supply.
From the line cord right to the switching supply input. ( how they are made cheep)
Take a DVM and measure for volts (and current too) from the chassis ground,
or the common return on your sound card, between your computer of choice
and the ground and/or audio return of the sound system.
You may find a suprise....... ( can we say "ground loop"???)

....... a lot of open discussion now begins.......

genlock · Post by **genlock** » Wed Dec 13, 2006 10:53 am

Waveguide is right, I have actually seen this happen.
Once or twice.

fmksey · Post by **fmksey** » Wed Dec 13, 2006 2:00 pm

Thanks!

The Ground Loop Isolator took care of the problem. I had one on the rack, but don't know why! Lester, your idea of going to another outlet, one the mixer isn't connected to, helps alot. But we couldn't have an
extension cord draped across the alter - so that wasn't an option.
And ---- the problem is with Dell. The IBM laptop works fine.

Lester · Post by **Lester** » Wed Dec 13, 2006 2:46 pm

For my purposes, can you confirm or deny which laptop(s) had a 2 prong or 3 prong wall plug?

fmksey · Post by **fmksey** » Wed Dec 13, 2006 2:55 pm

Lester,

The IBM has 3 prongs. I Don't know about the Dell, it has been installed -
but I'll find out.

Lester · Post by **Lester** » Wed Dec 13, 2006 3:04 pm

You da man!

sportsvoice · Post by **sportsvoice** » Wed Dec 13, 2006 3:56 pm

fmksey wrote:Lester,

The IBM has 3 prongs. I Don't know about the Dell, it has been installed -
but I'll find out.

The machine I was using was a Dell Inspiron 1100 that had a 3 conductor cord, both on the mains side and the low-voltage side.