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Help With Annoying Noise Please

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#16 djmick


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Posted 09 August 2012 - 09:29 AM

I used to use isolaters when i first started using laptops, (acers), but having since bought some new lappys the problem has gone away, must be better quality power supplys i reckon,,

#17 Megasong


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Posted 17 August 2012 - 11:21 AM

One of the most common problems we come across with noise is caused by the switch mode power supplies so often used with lap tops as well as computers in general and sometimes big time with iPads.

Depending on the PC this can be really intrusive. It sounds a bit like a whispering and will often increase when the PC is under load. If you are using a laptop or other battery device and the noise disappears when you unplug the PSU you know what it is.

Sometimes you get a 'beat' frequency between two switch mode PSUs supplying different pieces of equipment. This can sometimes appear at a low frequency and be reminiscent to mains hum. It's very easy to go off on a wild goose chase here trying to find a non existent earth loop. You can easily see if this is the case with a scope on the input to your amp (assuming it's not a switch mode PSU in there too, because these can beat with other signals as well - a nightmare to trace and cure without the right equipment).

Sometimes the signal will creep in through the mains from a completely non sound related piece of kit. All you can do is plus things in one by one until you get the interference then scope it to see what's going on. Not a job you can do without the right test equipment and knowledge unfortunately.

There are ways of getting rid of the noise but often it is drowned out by the music and people are unaware of it so I guess to some it doesn't matter. We always try to get our set-up silent and would recommend it as the right way.

It is possible to get well smoothed PSUs for Desktop PCs, they tend to cost about triple the price but are worth it. If you are electronically inclined you can also build your own smoothing circuits, basically a choke and capacitors in each of the DC voltage lines. We have done this for Dell and Acer Laptops as well as iPads which seem to be really bad for interference. It involves chopping the supply line to the PC and introducing the smoothing stuff in a box in the line so will invalidate any PSU warranty but it totally efffective.

The point of all this is that it is VITAL to check your proposed purchase with your other equipment BEFORE shelling out your cash, or be prepared to add smoothing afterwards. Get friendly with a local electronics person if you can unless you have your own people available. It's a big subject and fortunately not one everyone comes across as most noise is easily disposed of, but when it does bite you it can be a real problem.
Megasong A little knowledge is a dangerous thing. In theory it'll be fine in practise.... In practise it was fine in theory.

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