Noisy laptop power lead.

Afternoon all,
I'm recording our rehearsals with an acer laptop, whenever the power lead is plugged in there is an awful hum on the PA.
I've changed the power supply and the problem is still there, unfortunately with Acer you have to use their branded lead otherwise the laptop rejects it, and my battery is pretty poor, it only gives me about 1/2 an hour when recording, I don't want the laptop to shut down half way through a recording.
 
Any suggestions (apart from slinging it out the window)?

I'm using a Tascam 1800, with my drum kit mic'ed into it via 7 channels, DI box for the bass, 2 guitars off the line out of the amps, 1 vocal mic and a stereo pair for keyboards all into seperate channels.
The hum I get is there when no channels are connected, and is also present when I use my Mackie Link USB interface with 2 channels.
The monitor output of the interface is plugged into two channels of the Yamaha Pa mixer, but if this is not connected the hum is still there.
When I unplug the power lead to the laptop the hum dissappears.

It's normally about 10' away from the mixer, on a seperate wall socket over behind my drums with a long usb extension to the Tascam interface.

Get an outlet tester at Home Depot. Its possible that the outlet you are using is not wired properly. They cost about $5 and are well worth having. I've had one for over 30 years and I use it whenever I need to check a power outlet. 

You're not using an external display are you (like an old CRT display). Those will create hums in the guitar pickups. 

The thing is Mark is that even with no audio connected between his interface and the PA he is still getting the hum. We need to see if moving the laptop around helps. Sounds like it could be a hum noise induction thing from something if no audio is connected between them.

That is good news in a way curb because then it is more than likely an earth loop situation. If the line inputs on the PA are balanced then there is a way wiring a cable that will go from the Tascam to the PA mixer and the earth loop can be broken. But it does need the inputs to the PA mixer to be balanced. They may be jack inputs but it could still be a balanced TRS connection. You may not be able to tell from looking. But get the make and mdoel number of the PA desk and one can download the block diagram at least and it will show if the line inputs are balanced or not.

The mixer is a Yamaha MX4/12, just looked at the manual and it says the line in's that I'm using are balanced TRS jack sockets, although I'm pretty sure I'm using standard jack plugs from the Tascam monitor out.
Maybe I could try getting a pair of balanced jack leads for next time I go up?
 
Edit;
Also the Tascam manual says;
'Outputs include 4 balanced line, a balanced pair for monitors, digital and headphone.'
 
So I'll try this next time, you mentioned there was another way of wiring the balanced ins and outs to get rid of the ground loop?

Update on the problem;

I went to rehearsals last night, the problem isn't with the link from the Tascam Interface (monitor out) to the mixer, when I disconnected these cables it made a slight difference to the hum, but not much. similaly, with the cables plugged in there is a slight change when you pull back the faders, but again not much difference.

With everything connected, laptop plugged into the mains, USB from laptop to Tascam interface, all mic's and DI's plugged in, Tascam Monitor out to Yamaha Mixer, there is a very loud hum that sounds a little like radio interference, ie not constant.
Switch off the mains to the laptop power supply or unplug the power lead and the noise disappears completely.
Plug the laptop in again and the noise returns.
Boot up Sonar with my recording template set up and the noise gets slightly worse.
Unplug the USB cable from the laptop to the Tascam interface and the noise again disappears completely.
I also noticed last night that the noise also comes out of the bass amp, which is connected to the mixer via a Behringer Ultra DI - DI20, the bass goes into the DI box and then gets split into 2 channels, 1 to the Bass amp and 1 to the Tascam mic pre channel 8.

It sounds like it's putting noise onto the mains loop, is there a product that will isolate the laptop lead to the mains socket?

I use a three prong ground lift adapter on my laptop power supply. Solved the issue for me.
DaveT

yep, mine does the same thing.  a $1.00 ground lift plug fixes everything for me