still massive hum in setup!

[midge]

Hi

I'm still getting a massive hum in our live PA setup. It's definitely both the laptops.

I sent the problem to a transformer place and this is where it's up to:



Dear Midge

thanks for your message. I'm trying to imagine the problem...a couple of questions:

Is the hum still there when you run the laptop from batteries? (i'm guessing no)

Does the laptop AC adapter have a ground lead? (i'm guessing no)

Is the laptop AC supply internal or external?

Are you sure it's a hum (50Hz or 100Hz), not a higher frequency buzz?

Does the USB audio device have its own power supply, or it is powered from the USB bus? Is there provision for external power supply?

If it's not a ground loop, it may be poor power supply filtering.



I replied:

Hi David

Thanks for your reply!

The hum disappears when I runt helaptop from batteries
The laptop power sup has no ground lead
The power sup is external
I'm pretty sure it's 50hz
USB device is powered via USB bus

I tried setting up the rig inside (I originally had it in the garage), with no difference. I used a filtering powerboard, but no difference.

It's all strange because I have never noticed the hum before - it's definitely a new thing. The mixer is fine - I tried plugging a cd player into the same RCS input, and there was no hum. I have tried different cables, and swapped speakers. There is a definite hum coming through even when the speaker is turned off.

We actually use two laptops, and I am having the same problem with both of them, only the one near the mixer has a quieter hum! I have moved the power sups and laptop away from the mixer with no difference. And there was absolutely no change when I cut the ground on the RCA cable.

I don't think it's the USB device, because I tried pluging the mixer in via the laptop line out, with the same buzz.

Please help!!

 

[knightfly]

Midge, check your other post - I read nothing here that would eliminate the "line lump" from the list of suspects... Steve