Laptop generating noise thru PA

There can be a LOT of sources for noise in the audio stream of a laptop. Your mention of it only happening when the power cord is connected would lead me to look at where you are running your audio cables. Keep them away from the power cord (that's a #1 rule of sound, keep Audio and power lines as far away from each other as possible, if they have to cross cross at right angles...). Another thing could be the transformer box on the power cord, I converts the line voltage (120 volts AC or so here in the US) to what your laptop wants (IIRC around 12 volts DC) and by nature they generate electromagnetic interferrence. If that doesn't reduce your problem, it's very likely that there was a compromise in your laptop's design. There is usually another small transformer in there somewhre to convert the 12 volts to 5 volts for some of the componenets, as well as a LOT of other things that can generate EM interference (just about everything the laptop is HD, Processor, especially the new high frequency ones, etc). If they are in the area of the audio signal they will cause interference. Some Laptops have processors that work at different clock speeds depending on if they are running on battery or power cord (reducing the clock speed saves battery power) and it could be that the internal shielding is OK for the lower clock speed, but not the higher one.

 

Clock speeds for a laptop and its supporting ports are in the mhz/ghz range. You are not going to hear that through a p.a.system. You will hear a poorly designed shielding system (or lack of one) in you laptop. LajoieT is right on the transformer and power wires. My advice, buy a new power cord with transformer on it. See if that helps. Or, get a copper box, and put it around the adapter.

 

No, your power adapter most likely already HAS a transformer on it. It's that square box in the middle of the cord. There are a few, but not many laptops that don't use these, but I'd bet money we aren't talking about one here. A UPC is just going to increase your number of transformers. It's also likely that it could be ground noise between the laptop and the PA. They make ground lifting or filtering adapters you can use in the signal chain to reduce this if it's the case.

And yes, you won't here the gHz frequency through a PA, but it will cause electromagnetic interference which will show up in the audible frequency range. The interference does not only exist in the frequency of the offending signal.

 

I'll bet it's ground noise. What are you using for connections? See if you can find a ground isolating adapter for the connection to see if that helps.

{APC, UPS, UPC, they're all the same. They take the 120volt AC and use it to charge a bank of batteries, then they take the battery current (DC, but I've never taken one apart to find out what the battery bank's voltage is...)and run it through a transformer to convert it back to 120v AC. Which your laptop's power cord will just turn around and convert to 12v DC again. You should see no difference in this setup, other than slightly better regulated power.}

 

No, none of those are going to change anything IMO. Have you tried any other signal sources with the same cables? Like an IPOD or someone else's laptop? I've used my laptop for a lot of big presentations and never had the problem you describe, but I have had similar problems when recording to the laptop.

I just don't want to see you drop $50-80 cleaning up your power supply just to find the source is the laptop itself or the cables. There's also something to say for high quality cables with as few connectors as possible. You can get cables that have mini plug (3.5mm, standard computer audio output) on one end and 1/4 inch, RCA, or XLR on the other (depending on what your PA talkes for inputs. Even Radio Shack has decent cables in most configurations, but avoid using barrel connectors as much as possible, get a long enough cable so you don't need one, but not so long that you've got rolls of it looped around for you to trip on and to collect any available EM interference.