Stereo outputs annoying interference noise from laptop

When I use headphones with my laptop, I get no interference noise at all. However, once I plug it into either the 'aux' or 'tape monitor' of my (old) stereo (Yamaha CR200), I get an annoying interference noise (possibly at 50 cycles, but have no way of testing other than superstition)

I assume this is to do with the monitor card as it will stop when I drag a window around on the screen. No idea why..

As it doesn't happen with headphones, I assume it has something to do with noise suppression on the stereo somewhere.

Am I heading down the right track? Is there any way to check? Is it possible to add noise suppression somewhere?

Cheers! Rick Measham

Reply to
Rick Measham
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Hello Rick,

It's ground loops. Noise from the switch mode power supply gets splattered into the mix which is why the 50Hz probably doesn't sound like old tube radio hum but more like rat-tat-tat.

Pretty much the only easy way out is a little audio isolation transformer (dual version for stereo). I don't know whether Australia has Radio Shack stores. They call them ground loop isolators:

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Regards, Joerg

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Reply to
Joerg

BINGO!

Thanks for that Joerg ..

I've read up on ground loops (excellent site at

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and have discovered that it is my laptop power supply that is causing the problems. Strangely both the power supply and the stereo are plugged into the same ground point on the wall! So i'm guessing that the powersupply is somehow creating its own ground that is competing with the one in the stereo. Once I'm on batteries, the noise goes away. Unfortunately my batteries wont last past the opening title of a DVD!

Now I know where it's coming from I'll start to try to get rid of it. We don't have RS here in .au, and I can't find similar products yet. I'm a little against buying such a thing and I'm hoping I can find another way 'round it.

Thanks again! Rick Measham

Reply to
Rick Measham

Hello Rick,

It's hard to do it any other way, short of building your own power supply for the laptop. I just had the same problem in the lab when doing an FFT on a laptop. Grabbed a little audio transformer from the parts bin and it was gone.

If you decide to throw in the towel and buy a dual audio transformer I just looked up Jaycar. They seem to have one that (hopefully) might work for $19.75, AU$ I guess. Jaycar's catalog number for it is AA3085 and they call it "Car stereo line isolation transformer". In cars these are used to reduce alternator whine that somehow ends up in a chassis loop, a very similar problem to yours. It is likely that auto parts dealers have these as well, at least they should if they sell those big audio amps that kids parade through town in their low-rider Hondas.

Regards, Joerg

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Reply to
Joerg

It is the switching regulator in the laptop power supply causing this. It appears these supplies drives very large earth currents if you re-earth them via audio cables to something else already earthed.

Either use an earth isolating arrangement (transformer, opto-coupler) or operate the laptop from a more conventional power supply. As you have found it doesn't happen on battery power. Nor will cause interference from a more conventional non-switching power supply.

These switching regulated light weight power supplies can also cause quite horrendous AM radio interference if they are in an earth loop set up too.

Ross

(To get email address ROT 13) ebff snipped-for-privacy@lnubb.pbz

Reply to
RMD

Yes. The equivalent circuit is roughly an AC voltage source in series with a capacitor, connected between the chassis of the laptop and the active / neutral leads of the mains. At the frequencies involved in the switching waveform, active / neutral aren't much different to ground.

This is a real problem with switchmodes in consumer gear. If you put say a

100 ohm resistor between one of the DC output wires and ground, you can measure an amount of switching waveform across the resistor using an oscilloscope, showing that significant current is flowing.

However, if you connect the switchmode DC output to some self-contained device, the noise to ground does not worry you.

Most switchmodes contain a capacitor wired between the DC output common and the mains active or neutral. This is shunts the noise - and if you remove it, both EMI and noise to ground increase.

Roger Lascelles

Reply to
Roger Lascelles

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