01-07-2011, 10:49 PM
Very strange indeed. If I understand correctly:
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
- OK when running on battery
- Noisy on mains using internal soundcard
- Noisy on mains using external soundcard
- OK with the Maplin PSU
- 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
