Stereo outputs annoying interference noise from laptop

[Rick Measham]

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

 

[Joerg]

Hello Rick,

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

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.

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

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:
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=270-054

Regards, Joerg

 

[Rick Measham]

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.

BINGO!

Thanks for that Joerg ..

I've read up on ground loops (excellent site at
http://www.epanorama.net/documents/groundloop) 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

 

[Joerg]

Hello Rick,

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.

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

 

[RMD]

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

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

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