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hello!

this is a problem i've had for a long time, but was just reminded about it just now, as i set up 2 rooms of laptop-based DJs in my club. the power transformers make a really bad whining noise through the p.a.

the easiest solution i've found is to make the DJs run from battery when they are playing, but sadly this isnt possible tonight as the laptops are used constantly so the battery wouldnt last.

has anyone found a way round this?

Have you tried isolating the earths from the laptop soundcard output and the PA?

You should hopefully be able to do this with a DI box, or if you have balanced outs from the laptop then lift the earth at one end.

Yep. Lift the ground.

Those power supplies are switched mode which means they have an internal inverter that runs at a much higher freq than 60 Hz. This is to gain efficiency, better regulation and lighter weight. (120-240VAC --> 170-300 VDC -->high freq DC to AC inverter --> small xfmr-->low voltage AC -->rect/reg ckt w/inverter feedback loop--> DC output)

The problem is the switched mode inverter generates all kinds of EMI/RFI that is creating all that electrical noise that your are having problems with. One solution is to find or build a conventional linear regulated power supply that has a larger current capacity and exactly matches the voltage your laptop needs (some laptops are finiky about their supply voltages). You may be able to find such a supply that has the correct voltage or is adjustable at DigiKey, Mouser, Jameco or some place like that. They might also have the power coax plug so you don't have to cut up the original cord.

Aside from lifting the ground as mentioned, you could try adding a good AC line filter at the AC plug and pregnant snake ferrite filters around the cable at the computer power coax plug and use a lifted DI at the audio out, but I wouldn't be surprised if this did little or nothing. Worth a try though.

Just put a 57 on the laptop speaker and call it a night.
You could do stereo if you want I guess.

japmn wrote:Just put a 57 on the laptop speaker and call it a night.
You could do stereo if you want I guess.

ha! sold! worst thing is? i have actually done this before!!!!! many moons ago, somebody's internal sound card blew up. how we laughed.

i could, of course, tell them to shut up and buy some records like proper DJs?

This is not a problem if you are using a external audio box like a firepod.

The output on computers and DIing can be a little much for some PA systems. I was at a show a couple of years back where Merzbow used two laptops from thier outputs and set the PA speakers on fire with the signal.

ktone wrote:pregnant snake ferrite filters

I'd go see this band

Big John wrote:This is not a problem if you are using a external audio box like a firepod.

The output on computers and DIing can be a little much for some PA systems. I was at a show a couple of years back where Merzbow used two laptops from thier outputs and set the PA speakers on fire with the signal.

sadly, these DJs are not organised enough to own such fripperies as external audio boxes. one of them even 'DJs' on 2 winamp players! feel my pain.