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Computer Hum

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opus jody View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote opus jody Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2012 at 4:22pm
Isn't the problem with computer buzzes the fact that they're earthed twice? through the power supply, and through the audio cable, via the mixer? and it's some slight difference in the two earth routes that causes the buzz?
If so isn't it the case that if you break the earth on the computer PSU ONLY (as you can't break the earth on the audio without also killing the audio) then it is still earthed, just through the mixer?
I appreciate that to do this involves a 'special' power cable (earth lifted) which could obviously be very dangerous in a different situation, so no-one's ever going to suggest that one should ever be built, just as sure as I am positive that no-one could possibly have one stashed away in some special place, and labeled up to fuk, for use only if 'one' happened to have a laptop on-stage that was causing his rig to buzz like a b'stard!
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James Tengo View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote James Tengo Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2012 at 4:40pm
Read

http://www.jensen-transformers.com/an/generic%20seminar.pdf


Edited by James Tengo - 23 December 2012 at 4:47pm
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opus jody View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote opus jody Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2012 at 9:19pm
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James Tengo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote James Tengo Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2012 at 10:54pm
In my work as a corporate lampy (ugh... 9 years at an AV company, what was I thinking?) I was so often the first port of blame whenever the noise boys or videots experienced hum or noise (cue shouts of dirty mains, another pet hate) that a litte education session would follow, using flipcharts, of roughly the contents of that paper. Amazing how little knowledge is required to be an AV tech, and what good money you can earn with that half a clue
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b grade View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote b grade Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2012 at 11:15pm

Very interesting read for me.  It seems I must go back to some of my friends who are Ground lift happy and make them ground lift sad.  Good read and thank you for posting it.

Edited by b grade - 23 December 2012 at 11:16pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dlyxover Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2012 at 12:49am
In the Truth there is no news, and in the News there is no truth
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevinmcdonough Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2012 at 8:35am
Originally posted by James Tengo James Tengo wrote:

Read

http://www.jensen-transformers.com/an/generic%20seminar.pdf


Had a quick flick through, but saved for later Smile


However yeah I'd agree, the problem I've always found in these cases is just really cheap transformer circuitry in the power block of most PC laptops, seems to introduce a huge amount of noise into the audio outs, and not just a 50hz hum sometimes quite a wide or fairly HF range of noise.

As PC laptops are all so different, its a bit of trial and error to fix it. Sometimes an external sound card is enough removed from the power supply to do it, sometimes those isolators people have linked to will work, or you can invest in a much better quality power supply/cable.

Mac's seem to be much better at this, not sure whether by design as its known they're used by lots of audio/video/media professionals or just cos the powersupplies use better electronics by default.

k
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Generalhammond View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Generalhammond Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2012 at 9:30am
Originally posted by James Tengo James Tengo wrote:

I use these it's a di, on which you can mono sum, attenuate and ground lift the signal. Never remove a protective earth!



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dlyxover View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dlyxover Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2012 at 7:59pm
I really like sowter transformers. There British too, always a bonus !
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shagnasty View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shagnasty Quote  Post ReplyReply Direct Link To This Post Posted: 26 December 2012 at 3:09pm
I have to say, I always use isolating di boxes but I haver never got rid of a hum like this on the signal side, if you 'scope the output it appears in the waveform when the psu is on, connected but not on seems fine ( negating earth (ground for those across the water) loop as cause) I am guessing a few ferries inside the smpsu would do it, or cut the DC wires and add some smoothing/decouple caps....

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wires View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote wires Quote  Post ReplyReply Direct Link To This Post Posted: 26 December 2012 at 4:32pm
Ground loop isolator will do the trick or something like a behringer hum destroyer if you wanted. Always keep one handy just in case.
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