noise from laptop psu on audio out

[tiv48]

last saterday the local pub wanted to run a music quiz, so i dug out my old disco gear for them .iset it up using my old laptop and it sounded great ,the landlord said he would use his own music sauce so i left him to it .at 8.30 i got a call to come and look as the system was making a horrid noise.when i got there the noise was obvious a loud buzz at about 80hz ,this was proved to becomming from his hp laptop .the noise altered whith what the laptop was doing.while checking it was found that unpluging the laptop psu stopped the noise, as the battery was fully charged we left it like that. i have since found out that this is not uncommon and that some newer laptops are more prone to the problem than others, does anyone know of a cure
rob t

[DODGERN]

I only use mine to put my food on while I'm watching the telly......
But being serious, this may be of help and may be worth a try.

How to fix high pitch buzzing noise from intel based laptops
by ddubness - 10/12/07 12:20 AM
In Reply to: Buzzing noise using HP DV6000? by Cocoagirl

I recently purchased an HP dv6000 laptop that emitted a high pitch noise.
After some research I figured out that it was actually caused by the cpu going into a low power mode to conserve battery life.
I managed to fix the problem by turning off the abiltity for the cpu to go into a low power state in the bios.

Press f10 at start up.
Use right/left key to access system configuration
Use up/down arrow to select CPU C4 FUNCTION
Disable CPU C4 FUNCTION

Obviously battery life won't be as great but at least the annoying noise will be gone. If you decide later you need to, you can always turn it back on.


Sounds as if the power supply is not up to the job. You won't get a manufacture to admit it though. Hope it works.

Roger.

[Jamie-Skinner]

If a laptop has a metal earth pin you get the hum is what a DJ told me.. Mine is plastic so minimal hum..

[tiv48]

thanks roger but the buzz only comes from the pa sytem not the laptop
jamie its worth a try but on asking around it seems to be a common problem
rob t

[Jamie-Skinner]

I've not had a problem.. yet with my Fujitsu Siemens Amilio Li 2727 (Plastic Earth)

[tiv48]

just checked my old tosh has meal earth and gives no problems are spare compaq does have the problem ,not as bad as the landlords hp (through my stereo not a 500 watt pa)that has metal earth pin
rob t

[glowinganode]

Sounds like you need some filtering between the p/s and the laptop. Try some ferrite rings or split cores around the cable, also on the mains cable too. If you can open up the p/s, add a ceramic filter cap across the output.
Rob.

[tiv48]

i think filtering may be the answer i have just thied the compaq on a universal psu from maplins this cured the problem so would you say its a design fault with the psus?
rob t

[Mark Hennessy]

Complicated problem with a number of possible root causes.

First, the advice Jamie received about hum was about earth loops. To be honest, the fact it's a laptop is probably not an issue; any source that is earthed could be prone to causing hum in a system. But, it's completely impossible to generalise because so much depends on the design of the following equipment, plus any other equipment that might be connected to the system! Ask any audio pro: hum loops are a real problem, and it's not always restricted to consumer-grade kit... One way to prove that it's an earth loop is to turn off the source equipment, but leave it connected - the hum will still be there.

One possible solution to prevent an earth loop is a 1:1 isolating transformer. These can be very good indeed, although the very cheap ones that CPC sell aren't so nice. Working where I do, it's normally easy to find good quality "rep coils", as they're called, and building them into a box is an easy task. Use a diecast aluminium box, but make sure at least one set of phono sockets are isolated from the case (at the risk of stating the obvious).

As for the noise at higher frequencies, the causes are normally power supply system design. This doesn't just include the external PSU, but can also include the multitude of on-board DC-DC converters that take the single DC rail from the PSU or battery and convert it to the various voltages required (5V, 3V3, 1V5 or whatever the CPU core requires). Also, perhaps bizarrely, PCB design can have an effect; not just the layout (in terms of how close the audio DAC and amplifier stages are to "noisy" things, but how supply and earth returns are routed. The PCB is always a component in any design). So basically, if your system components are taking current in "spikes", according to say CPU load, these currents can get turned into voltages as they travel along PCB tracks (which aren't zero Ohms in reality), and can get added to analogue signals in the audio department.

An external USB soundcard can be an effective cure, and they can be very cheap as well. Though note the use of the word "can". Perversely, these two effects can combine, so that although you've got the soundcard CODEC outside of the noisy environment of the laptop itself, you haven't got isolation between the laptop and the audio system. So, stray currents from the PSU systems can flow in the earth, causing voltages to be developed and added to the audio signal. If your original problem was an earth loop, an external sound card won't help...

To put this into context, if the maximum signal from your soundcard is 1V at 0dBFS, at -60dBFS, the signal is 1mV. Very good vinyl might manage a signal-to-noise ratio approaching that, perfect FM might also be in the same ballpark. So if you're used to the background noise from records and decent FM broadcasts like Radio 3, you've got an idea of the harm just 1mV might do...

It's worth running the laptop as loud as you can without causing clipping (use a 'scope to check) to maximise the signal to noise ratio...

All the best,

Mark

[Mark Hennessy]

Oh, and another problem is HF hash on the audio outputs might be demodulated by a following stage that lacks input filtering (like, most hi-fi, perversely, although EMC regs have helped in recent years). Again, though, so much depends on the following gear so it's impossible to generalise...