noise from laptop psu on audio out

[glowinganode]

Oooops, ferrite chokes on the audio leads too.
These will help cure hash and buzz, but not hum.
I doubt it's an earth loop though.
The Maplin p/s may have a linear regulator rather than switch-mode.
Like Mark says, run the o/p high and attenuate at the sound-card.
Rob.

[tiv48]

not an earth loop problem .the maplins unit is a smps . if the volume on the laptop is turned dowm the buzz goes. the buzz alters in frequence depending on what the laptop is doing. i have scoped the output of the psu and its clean off load. i have tried 2 inch ferrit rings on ac input and dc output .makes no difference. so i am still at a loss to explain this
rob t

[Mark Hennessy]

It might not be an earth loop as such (although being a SMPSU doesn't rule that out by any means), but SMPSUs can have high leakage currents between input and output, so if the laptop chassis isn't connected to earth via the PSU, this could be a factor. Otherwise, try an external USB soundcard, as I mentioned previously, because many laptops suffer this problem and there is no cure short of redesigning the motherboard...

Good luck,

Mark

[tiv48]

hi mark im ahead of you i tried a cheep usb sound card (no good) and am told by a dj mate that the latest maplin psus wont cure the problem ,the lanlord is now going to try and get pc world to sort it out as he told them what he wanted to use the laptop for. i wish him luck.
rob t

[Mark Hennessy]

Very strange indeed. If I understand correctly:

1. OK when running on battery
   
2. Noisy on mains using internal soundcard
   
3. Noisy on mains using external soundcard
   
4. OK with the Maplin PSU
   
5. Haven't tried using audio isolating transformers.
   

So what's different about the HP mains power supply? It's got to be leakage (between the windings of the transformer, and via the filter components) finding its way to the DC output. Have you tried earthing the laptop chassis yet?

The fact that both soundcards present the problem reinforces this for me; some strange leakage current, modulated by the laptop activity, somehow getting into the earth system of the PA gear. Option 5 might provide a cure, but it's worth trying the separate earth on the laptop chassis first.

This true story might seem like a diversion, but bear with me:

Some years back I had a problem that turned out to have a similar cause. When doing a breakfast shift, the presenter complained about mild electric shocks from her microphone. This was a small personal clip-mic attached to the lapel of her jacket, which made occasional contact with the skin on her chest. Of course, this microphone was resolutely connected to earth, so was quickly eliminated...

Watching carefully, we noticed that she only got the shocks when she touched the lectern which held the laptop (reading the news while standing up was the fashion back then). Said lectern was made from plywood with a brushed aluminium laminate applied - a very high-tech looking thing. None of the aluminium panels were bonded to earth as it would have been near-impossible to do this without spoiling the appearance...

Anyway, the laptop wasn't earthed, meaning that the chassis was floating at half-mains (completely normal). The chassis was capacitively coupling to the aluminium sheet on the top surface, and every time she touched this, she felt a shock.

Why was the shock felt at the earthed microphone on her chest? Quite simply, and not to put too fine a point on it, the skin there was much more sensitive than her fingertips

To get through the rest of the shift we unplugged the PSU during the bulletins, and the fix was to earth the laptop chassis - we used a 9 way D-type attached to the serial port, but no-doubt a modified USB cable would be simpler these days...

Anyway, may I join you in wishing your friend luck with his dealings with PC World :argue:

Mark

[Tedzed1]

Hello rob, had a similar problem with my Toshiba satellite pro, when used direct from headphone socket to a gram input on ANY of my old radios,
It turned out to be a nasty power supply (cheap chinese copy), I purchased a genuine second-hand toshiba supply from Ebay, and now it is perfect on all my radio sets, no hum whatsoever.

[tiv48]

this was a real hp psu will know what pc world say on wenday
rob t