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Plugging laptop power supply in gives interference.

#1 User is offline   armaros 

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Posted 14 January 2005 - 08:58 PM

Hi I ran a show Sound on a Show last night, we was using a Panasonic projector as we was playing a DVD on power DVD of a laptop (the laptop is 6months old) from the laptop we ran a mini jack 2 jack from the laptop in to the sound desk but no matter what I did on the desk all I got was like a cricket noise In the background this was even when I muted the channel, any suggestions why this was happening?
Thanks Martin

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#2 User is offline   PoppaDom 

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Posted 14 January 2005 - 11:56 PM

Two things need answering here to help you,one was the mini jack to jack configured for stereo (ie: shield to shield on both jacks, ring to tip on jack no1, tip to tip on jack no2) or were you using a standard stereo jack to stereo jack lead into the desk?

In this configuration the left and right signals feed the balanced input on the desk with reversed phase (no rants about phase versus reversed pins and the fact it isn't actually reversed phase blah blah blah please....trying to explain simply! :D )
Result is bad and sometimes no sound at all!

(Note: the onboard soundcards on laptops arent particularly good when it comes to sampling quality and s/n ratios)

The other question is which make of laptop are you using?
I ask this question as a lot of power supplies for laptops generate a lot of mains interference and noise that amplifies itself through groundings and the actual signal lines themselves. Try running it off the batteries! If that stops it, the psu is too blame.....Get a mac! lol (or replace the power supply with one designed to generate less mains interference)

As usual hope my ramblings make some kind of sense!

Poppadom

#3 User is offline   Pete Alcock 

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Posted 15 January 2005 - 09:25 AM

Martin,

Presumably when it was playing in the PC with nothing connected, you could hear sound in the laptop speakers?

If so, what if you plug a pair of headphones in with a mini jack on. Sound in the headphones then?

If yes, it has to be a cable issue or something is set wrong on the desk (routing OK? mutes off? Channel on?)

You need one of these: Jack lead in CPC catalogue to start with.

When you do eventually get sound through the desk and there is a buzz, hum or other noise in the background I always find I can cure this using a stereo transformer box (two 1:1 audio transformers in a tin, which provides complete isolation of the signal earth from input to output).

All the best,

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#4 User is online   henny 

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Posted 15 January 2005 - 10:36 AM

hi...

let me guess it was a hp laptop, the problem with the hp laptops are their PSU's they are verry verry noisy, what u need to do is before each cue before you open the channel on your sound desk, is pull the psu from the laptop,

most hp laptops seem to have the same problem even when useing external usb or firewire sound cards.

henny

#5 User is offline   Matt 

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Posted 15 January 2005 - 04:14 PM

I second the power supply theory.

I have a dell inspironn 5100 and whenever I am playing sound from it into a mixing desk over a PA at loud volumes, I always get this interferance.

..and indeed just disconnecting it from the laptop does solve the problem. We also had a problem with this in a show not too long ago, and the tech person that was running it was adament that it was the poor quality of the sound desk, until I provedd him wrong - he never spoke to me since.
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#6 User is offline   slim_mcslim 

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Posted 15 January 2005 - 05:38 PM

It is the noise created by the switch mode powersupply in the laptop, in conference its something we come up against all the time, I wouldn't say any laptops are particularily good for it.

We normally fix it by using a passive DI box on the line. The best thing to do is pull the powerlead on the laptop whilst playing the DVD and it should be quiet for the duration.

You get this switch mode noise with some other equipment, including the old behringer stuff.

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#7 User is online   paulears 

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Posted 15 January 2005 - 05:52 PM

My old Dell did the same thing - my new Fujitsu is not quite so bad, but I just discovered a sp/dif output I didn't know about - VERY useful.

#8 User is offline   Matt 

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Posted 15 January 2005 - 06:55 PM

why does it do this though, becuase the psu is completely seperate from the laptop and only has a + and - connection to it. It is something to do wiith the earthing on the psu and cross interferance in the cables?

...if so would sehiled cables of "mains cleaning devices" help the problem???

causes havoc when doing sound work on my laptop at home grrrrrrrr :@

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#9 User is offline   PoppaDom 

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Posted 16 January 2005 - 12:25 AM

Matt, on 15 Jan 2005, 06:55 PM, said:

why does it do this though, becuase the psu is completely seperate from the laptop and only has a + and - connection to it. It is something to do wiith the earthing on the psu and cross interferance in the cables?

...if  so would sehiled cables of "mains cleaning devices" help the problem???

causes havoc when doing sound  work on my laptop at home grrrrrrrr :@

Matt.
<{POST_SNAPBACK}>



The problem is caused by the "switched mode" part of the power supply and so no amount of mains interference cleaning equipment would help! The noise is created in the psu, after the point of filtering. Although base unit computers utilise the same technologies the size considerations of laptop power supplies leads to less cleaned and "rougher" power supplies.

This Switched Mode Power Supplieswebsite contains information on switched mode power supplies...

I would suggest there are four options:

1, Buy a new PSU that is designed for your laptop but doesn't create as much mains bourne interference.
2, Buy a laptop with sound in mind? (ie:macs don't do it as don't laptops made by companies designed for sound editing)
3, Pull the plug! (Not practical for long editing, and running on batteries slows most laptops down without editing settings)
4, Filter the output from the laptop going to the sound desk either using a 1:1 isolating transformer or (and not always as successful) a DI box.

Once again hope these ramblings make sense

Poppadom

#10 User is offline   Pete McCrea 

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Posted 16 January 2005 - 10:14 AM

The output from my powerbook is nice and clean, so I can vouch for the Mac option......
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#11 User is offline   armaros 

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  Posted 17 January 2005 - 08:57 PM

I don’t know what type of laptop it was, it was one of the tutors at college... she knows nothing about tech and all she said was "...its not MY laptop...its only 6months old.....ect ect...* I turned off after about 10mins...
And as for the jack Q, I don’t know... im a 1st yr doing tech theatre at college and the 2nd yrs set it up and at the college im at they are considered "gods"...lol
I was thinking could it be possible that the DMX/dimmers were causing interference…would they be able to do that? As when ever the lights was up it cut the noise in half… as im a “student” to technical theatre im still trying to get to grips with all the terminology ect.. thank you for all your replies and help
Martin

#12 User is online   paulears 

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Posted 18 January 2005 - 09:15 AM

Aaaaa......

The switch mode noise is a common problem, but your comment "As when ever the lights was up it cut the noise in half…" makes me think we have misinterpreted your cricket noise. It sounds like good old dimmer buzz. If it's worse when the lighting is around half up - it almost certainly can be put down to this - especially if it virtually goes when they do a full up.

Semi-conductor dimmers really chop up the mains waveform when they dim. The mess gets filtered a bit, depending on the manufacturer, but this problem is very very common. Solutions? Well, trying a different power outlet that may be on a different phase of the mains may help, along with making sure the dimmers are well away from any audio kit. Having dimmers and audio running off the same distro is asking for trouble.

In my experience mains so-called cleaners are universally rubbish and won't help here. If it is unuseable, then maybe you could run the laptop off a computer UPS, if it really is the computer that is susceptible to the dodgy waveform.

#13 User is offline   NM13 

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Posted 18 January 2005 - 10:46 AM

if you are getting the buzz even with the channels muted then I would think the problem is with the amp or interference with the speaker cable, do you get any buzz when you use the sound system normally. If you think it might be the dimmers interfering then turn them off and check if the noise is still there. The dimmers are on a different electrical phase to the control room sockets so shouldn't cause a problem but in your theatre you never know. I've got some free time so if you want me to help you solve the problem email me.

#14 User is offline   Andrew C 

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Posted 18 January 2005 - 11:00 AM

stormwolf013, on 18 Jan 2005, 10:46 AM, said:

if you are getting the buzz even with the channels muted then I would think the problem is with the amp or interference with the speaker cable, do you get any buzz when you use the sound system normally. 
<{POST_SNAPBACK}>


Interference is not at all likely in the speaker cable; in mic. or signal cable then yes. This will be particularly sever if you have runs of power and signal cables close together and parallel.

That aside, checking for buzzes in various operation modes is an excellent way forward.
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#15 User is offline   NM13 

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Posted 18 January 2005 - 11:23 AM

Quote

Interference is not at all likely in the speaker cable; in mic. or signal cable then yes. This will be particularly sever if you have runs of power and signal cables close together and parallel.



normally no it's not, but the speaker cable isnt of a very high standard, (I installed the speakers) so for trouble shooting I wouldn't rule it out until I had checked to make sure it wasn't the problem just in case.

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