laptop supply hum ??

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

Jaycar Part# AA-3085.

That will solve your problem.

--
Trevor Wilson
www.rageaudio.com.au

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

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.

"cth"

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

..... Phil

Correct, they're earthed.

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.

** So that one will have a nasty noise injecting cap from the AC side to the output ground. ** If you are lucky and if the audio signal level is kept high - like 1 volt rms.

.... Phil

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.