laptop supply hum ??

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
hi, a friend of mine  has setup a conference listeneing using  VOIP .

he has the pre amp out connected straight up to line in on the laptop .
he has the mains adapter plugged in to the laptop.
but there is a hum coming thru  ,, when he disconnects  the mains supply for
the laptop , and the laptop runs on its own batteries , the hum goes away
....

would the laptop supply need some sort of filtering ?
thanks,

mark



Re: laptop supply hum ??



Quoted text here. Click to load it

**Probably an earth loop. Use one of these:

Jaycar Part# AA-3085.

That will solve your problem.


--
Trevor Wilson
www.rageaudio.com.au



Re: laptop supply hum ??



Quoted text here. Click to load it

It's caused by a cheap way of meeting EMC specs which places a safety capacitor
between the rectified DC primary side reservoir cap and the low voltage DC
outout ground. A typical value is 2n2.

This causes a significant leakage current which wants to find its way to ground
if it can. Poorly designed audio equipment with inadequate attention to
grounding and EMC issues will amplify the sound of this leakage current
beautifully.

Either get some properly designed audio gear or run the laptop on batteries.

Oh - or get an audio isolation transformer. Commonly called D.I.s in the music
trade. You should be able to get one in a guitar shop.

Graham


Re: laptop supply hum ??
Quoted text here. Click to load it

FWIW, I'll relate a similar experience - using a laptop for party music.

Using the laptop headphone out I constructed a cable with resistive
divider to go into the line-in of a pair of amplified speakers.

The speakers have balanced (XLR) inputs and at first I used the typical
unbalance-to-balanced wiring i.e

headphone hot to XLR Hot(+)
headphone GND to XLR Cold(-) and GND

Which resulted in much buzzing unless running on battery. As this is a
fairly ancient laptop, the battery's not much good so running on mains
is the only option.

Solution was to disconnect the XLR GND and leave headphone GND connected
to XLR cold(-). There's still a low level of hum but *much* less
noticeable. I don't recommend it for recording or hi-fi listening, but
OK for background noise.

Chris.


Re: laptop supply hum ??

"cth"
Quoted text here. Click to load it


**  Amplified ( ie self powered) speakers are nearly all mains earthed
appliances -  so are highly prone to ground loop hum when connected to an
earthed signal source.


Quoted text here. Click to load it


** Which proves there is an AC supply safety ground on your external PSU
that is carried through to the chassis of the laptop.



.....   Phil




Re: laptop supply hum ??
Quoted text here. Click to load it

Correct, they're earthed.

Quoted text here. Click to load it

The laptop in my case, above, is a Dell which has a 3 pin mains plug
with corresponding connections into the switchmode PSU, so your
conclusion is no doubt correct.

Must try another experiment...

I also use a Toshiba for work which only uses a figure 8 twin mains
cable with no earth pin on the mains side into the PSU.

I expect the Toshie has no safety ground & would sound OK using the
'typical' unbal->bal connection scheme...

Chris.

Re: laptop supply hum ??

"cth"
Quoted text here. Click to load it


**  So that one will have a nasty noise injecting cap from the AC side to
the output ground.


Quoted text here. Click to load it


**  If you are lucky and if the audio signal level is kept high -  like 1
volt rms.



....   Phil







Re: laptop supply hum ??

Quoted text here. Click to load it
Is it a Dell laptop, I have this problem using a headset straight into a
Dell laptop with the power adaptor plugged in. Unplug the power adaptor and
the hum goes away. Voip is unusable with the power plugged in.



Site Timeline