Laptop power supply hum

[Dallas]

Has anyone come up with a solution for that f#@n' annoying buzz caused by the power supply when using a laptop that doesnt involve running on batteries??

(My mate's specific case - powered monitors connected to USB interface plugged into laptop)

[mylesgm]

try a stereo isolation transformer between interface and monitors.
M

[stosostu]

Yep, that's the only reliable method I've found.

[Futureman]

Or take the earth pin out of your laptops powersupply [Or take the earth pin out of a power douber adapter and ONLY plug the laptop into that ] . Dodgy, probably illegal.. Although considering that the laptops power supply is plastic and insulated, It may not be needed with regard to safety. (Feel free to crucify me for saying the above as I know how reckless it is)

[Ben M]

I've done the earth pin on the double adaptor trick before. Works a treat!

[Chris H]

I've done the earth pin on the double adaptor trick before. Works a treat!

.............Untill you fry your eyeballs........ and your testicals explode........ as the 240 volts from someones favourite ( fatally modded ) vintage valve preamp you are trying out, traverse through your convulsing body at lightning speed on its relentless persuit of the shortest path to earth, the milisecond after your lips contact the grill of your favourite Bees Neez mic ............. who's gold spluttered diaphram is now pitted all over your teeth and whats left of said lips................. as the guitar strings you forgot to trim, dangling from your electric guitar's headstock get caught in the speaker terminals of your studio monitors while you are trying to extent the feedback of the soaring last note you will ever play................

opps.jpg

[Ben M]

Oh ....yeah......Just add pop shield.

[Futureman]

.............Untill you fry your eyeballs........ and your testicals explode........ as the 240 volts from someones favourite ( fatally modded ) vintage valve preamp you are trying out....

But would the same thing happen if the laptop was running off it's batteries?

[Dallas]

Oh ....yeah......Just add pop shield.

[Chris H]

.............Untill you fry your eyeballs........ and your testicals explode........ as the 240 volts from someones favourite ( fatally modded ) vintage valve preamp you are trying out....

But would the same thing happen if the laptop was running off it's batteries?

.........1st law of storytelling " Never let the truth get in the way of a good / bad story!" ?

[waitup]

I lecture electronics at one of the audio schools, and one of the points I always stress to students is how dangerous it is to remove the earth from a power cable. I usually hand out a page long scenario in which a keyboard player is fried as they go to sweep the fully sick bass filter on there moogyweil, or a roady spontaneously combusts as he turns up the marshalls, but from now on I might just use that photo that Chris H posted instead.

[Chris H]

........the pic is from one of my favorite Arnold Schwarzenegger movies Total Recall.........when the bad guy exits the controlled environment on Mars.
Gross.

http://www.youtube.com/watch?v=KOGpgkN2 ... re=related

[wez]

...a keyboard player is fried as they go to sweep the fully sick bass filter on there moogyweil.

i came very close to death many years ago in a very similar scenario. an earth pin had been removed to fix a hum... all was fine until a roady dropped a synth... there was something rattling around in the case, but it powered up ok and seemed to work fine. but during the gig, i received numerous, serious shocks through the metal case of the keyboard that night, and basically everything that was connected to it suffered some damage. when i took it to electric factory the next week, the tech showed me the scorch marks from the arcing caused by a loose screw becoming wedged between the power supply PCB and the metal case.

the moral of the story... earthing saves your life when things go wrong. removing the earth removes a crucial level of protection from the very real possibility of death through electrocution if something breaks, comes loose, or is badly designed. don't do it - there are better ways to deal with hum.

[Ben M]

I've done the earth pin on the double adaptor trick before. Works a treat!

Please do not try this at home. Seriously!

[JulienG]

I've done the earth pin on the double adaptor trick before. Works a treat!

Actually you're usually fine, although getting the US version of laptop chargers may be a more sane alternative.

I do that with my thinkpad's, AU chargers are three pin, US are only two pin.

[Linear]

Is it a mac laptop?

Interestingly enough, there are two adaptors included for the macbook power supplies - one is a long cable with the mains plug attached (with an earth), the second is just the mains plug.

Both of them connect to the power supply, but the latter has no earth pin. I guess it makes sense, as the power supply is made from plastic and if there is no mains cable between the power supply and the wall plug, then there's no chance of electrocution due to faulty wiring.

So if it is a macbook, get him to use the short direct-style plugpack mains adaptor.

Chris

[astrovic]

The "don't pull the earth pin" issue has been vexing me for some time. I get that the earth pin on the laptop power supply is providing an earth return for the laptop, so that if anything shorted in the laptop you've got that safety.

But two things come to mind:

1. The laptop runs on a low voltage, with the transformer being tucked away from the laptop halfway down the power cable. You can't be exposed to mains power on the laptop itself, can you?

2. Does the laptop earth pin isn't providing a path to earth for any other mains gear? Usually the only thing you'll be plugging into the laptop is the audio interface, which either has its own separate power supply either on a wallwart type power supply or with a built in power supply that has its own earth pin, or it runs on low voltage USB or firewire power.

I'm just struggling to get where the exposure to mains voltage would come from that would be fatal. I'm sure it's the case, I'm just missing something, right?

[Damien]

most of the time the laptop/psu noise issue wont be solved by removing the earth pin... its a different noise issue..
you need a good iso transformer in the audio path..

i have r\the radial twin iso for exactly this issue. when the earth WAS removed from my laptop psu the issue didnt change.....

we have many customers with this issue, the iso solves it 80% of the time... incorrect leads/wiring is usually the issue the rest of the time, or rack gear position (one units PSU bleeds into the audio path of another..)

[Stephen C]

Those cheaply made, but expensive 19V switchmode laptop power supplies on Dell's and HP's are noisy bastards. There is a third pin which carries a coded pulse train to tell the laptop that it is powered by a genuine branded power supply. Not sure if it is poor filtering in the switch mode PSU electronics (or equally, poor filtering in the DC input section of the laptops motherboard) or could be the pulse train leaking into the power, and then the cheap laptop audio cards picking up the noise on the power. Either way, as mentioned already, the iso transformers usually solve the problem. There a few cheapies available that do the job.

I worked for a large PA company in Australia... we were warned as trainees that if you removed the earth on a bit of gear, then you lost your job! As a matter of principle I still couldn't cut the earth! (cutting the earth on a double adaptor is a bad bad bad idea...)

If you were really keen you could build a linear power supply for your laptop, would be a bit heavy though! a 4A or 6A 19V PSU would be a monster!