OK, I'm asking this on behalf of a friend with no ng access so be gentle
with me if this is a really stupid question!

He's got a Quad 33/303 based system and wants to connect his laptop to it to
play mp3s. He tells me that everything works fine when the laptop is on
battery power but as soon as he plugs in the power supply, there's a loud
humming audible through the stereo speakers.

It's obviously something to do with the presence of the mains power but what
can he do to stop it?

TIA

He tells me that everything works fine when the
laptop is on battery power but as soon as he plugs in the power
supply, there's a loud humming audible through the stereo speakers.

It's an earth loop hum.

Googling reveals loads of hits, including:

http://www.soundonsound.com/sos/nov0...lation1102.asp

Al.

On 5 Jan 2005 19:51:44 GMT, Al wrote:

He tells me that everything works fine when the
laptop is on battery power but as soon as he plugs in the power
supply, there's a loud humming audible through the stereo speakers.

It's an earth loop hum.

Most unlikely. All the laptop PSs I've seen have been double
insulated.

Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

Bill

In article ,
wrote:
He tells me that everything works fine when the
laptop is on battery power but as soon as he plugs in the power
supply, there's a loud humming audible through the stereo speakers.

It's an earth loop hum.

Most unlikely. All the laptop PSs I've seen have been double
insulated.

Yes.

Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

However, surely the PS would be a switch mode type, so producing no
audible hum?

It's an interesting problem...

--
*Strip mining prevents forest fires.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
wrote:
He tells me that everything works fine when the
laptop is on battery power but as soon as he plugs in the power
supply, there's a loud humming audible through the stereo speakers.

It's an earth loop hum.

Most unlikely. All the laptop PSs I've seen have been double
insulated.

Yes.

Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

However, surely the PS would be a switch mode type, so producing no
audible hum?

It's an interesting problem...

Update: Apparently it's an HP laptop brought over from the US. Would there
be a difference in the earthing arrangements for a US-spec machine I wonder?

On Wed, 5 Jan 2005 19:29:36 -0000, "Stimpy"
wrote:

OK, I'm asking this on behalf of a friend with no ng access so be gentle
with me if this is a really stupid question!

He's got a Quad 33/303 based system and wants to connect his laptop to it to
play mp3s. He tells me that everything works fine when the laptop is on
battery power but as soon as he plugs in the power supply, there's a loud
humming audible through the stereo speakers.

It's obviously something to do with the presence of the mains power but what
can he do to stop it?

hi yes i agree its a ground loop, good cheap and simple cure is
available from maplins ground loop isolation transformer this is 2
transformers in one (stereo) comes fitted with phono plugs and sockets
on short wires these plugs will have to be changed or adapted to suit
your equipment Maplin code VW43W, cheap enough to give it a try around
£7
regards
bob

In article ,
Stimpy wrote:
Update: Apparently it's an HP laptop brought over from the US. Would
there be a difference in the earthing arrangements for a US-spec machine
I wonder?

Is it a wall wart or free standing unit? If free standing, has it got an
earth connection? You could test for this with a DVM by measuring the
resistance between the earth pin and the low voltage cable screen. With no
earth connection it will be infinite.

--
*How do they get the deer to cross at that yellow road sign?

Dave Plowman London SW
To e-mail, change noise into sound.

On Wed, 05 Jan 2005 23:46:28 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
wrote:

Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

However, surely the PS would be a switch mode type, so producing no
audible hum?

Most recent ones probably are. But, judging by the weight and heat
they produce, older power supplies may well be linear.

It's an interesting problem...

The only explanation I can think of is that the return connection in
the PC to Amp lead is not continuous, than you wouldn't need much
coupling through the PS to generate lots of noise, but it may well be
O.K.with the PC operating on batteries.

Bill

In article ,
wrote:
Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

However, surely the PS would be a switch mode type, so producing no
audible hum?

Most recent ones probably are. But, judging by the weight and heat
they produce, older power supplies may well be linear.

If it's linear, there's a good chance there wasn't room for adequate
smoothing if a wall wart. Anything over about 15 watts becomes tricky. And
the main functions of a computer might well be immune to 100 Hz ripple
that would annoy on an audio circuit.

If it's not a wall wart, should be room inside for a larger cap. Be
interesting if the OP could open it up and say what value the existing one
is, along with details of the PS voltage and current.

It's an interesting problem...

The only explanation I can think of is that the return connection in
the PC to Amp lead is not continuous, than you wouldn't need much
coupling through the PS to generate lots of noise, but it may well be
O.K.with the PC operating on batteries.

Could be Mini Jacks are notorious for high impedance connections. Nasty
things and should be banned. ;-)

--
*Money isn't everything, but it sure keeps the kids in touch *

Dave Plowman London SW
To e-mail, change noise into sound.

wrote:

On 5 Jan 2005 19:51:44 GMT, Al wrote:

He tells me that everything works fine when the
laptop is on battery power but as soon as he plugs in the power
supply, there's a loud humming audible through the stereo speakers.

It's an earth loop hum.

Most unlikely. All the laptop PSs I've seen have been double
insulated.

Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

For some reason newer laptop power supplies are earthed and the earth is
connected to the signal ground of the audio output. If I hadn't recently
bought an Acer laptop recently then I would probably have thought the
same as you, as most older Toshibas, Sonys and Acers that I've used were
double insulated.

I got around it by feeding the output into a decent balanced input and
measured something like 80dB signal to noise ratio - not as good as a
decent 16 bit output should be but OK considering the price of the
components they probably use.

Cheers.

James.