Laptop earth-buzz audio fix

[lecnoob]

Ok - long story short. I do music shows and have power trouble using laptops for audio - earth buzz issues. My usual fix has been to find a replacement power supply designed with no earth pin on the plug - no more buzz. This has worked for the last few Toshiba laptops I had and is way cheaper than messing around with earth-loop killers or whatever.

So, my new Samsung has the usual problem, but now I can't find a 2-pin replacement power supply. I've been warned off just disconnecting the earth on the OEM power supply as apparently it might need it by design - though I have no idea why it should do when a 2-pin power supply works totally fine. Please correct me if I can just do this!

In desperation then I'm considering just swapping the tip over on my old Toshiba replacement 2-pin supply and using it with the Samsung - it's 19V and has enough amps. Is this a dumb idea? If not, can anyone tell me where I can buy a solder-able DC tip?

Many thanks!

(Tried posting this here first but doesn't seem like anyone there knows)​

 

[Specialist]

If the Power supply Definately has an Earth wire to the Pin on the plug the I would'nt disconnect it, a Manufacturer won't won't waste money puttiing anything in that does'nt need to be there.
As for the Solderable DC plug then Maplin would be your best bet.

 

[lecnoob]

Thanks for your suggestion Specialist. I can't really tell if the earth goes to the pin - but I doubt it, looks like only 2 connections are possible here. If so, is it a pretty low risk thing to remove the earth from the plug to the transformer?

I attach a picture of the pin below ... couldn't find anything like it on the Maplins site unfortunately.

 

[Specialist]

No mate: As I said earlier, if the Earth does go to the Earth pin on the mains plug, it will be there for a reason.
For the DC plug, you could try taking a look at www.cpc.co.uk they used to supply a lot of different plugs / Adaptors for Laptops.

 

[CrazyScozzese]

The earth on a AC/DC adapter for a laptop is typically to provide a noise filter and basic SPD functionality. The noise filter will be along the lines of a Shaffner filter so cutting the earth will allow the internal earth to float at around half line potential, this could affect other areas in the adapter, including the SPD function. I wouldn't advise on it so you would be better sourcing a two pin adapter, you could use the jack from the original unit.

However, to decouple the buzz, try also connecting a USB cable in parallel with the audio cable, that can stabilise the ground level of both items of equipment and leave the audio alone. Not guranteed, but les hassle.

 

[lecnoob]

Ok thanks to both ... I'm thinking then I might be best swapping the connector over so I can use my old 2-pin no-earth Toshiba adaptor. I'll have a look to CPC, cheers!

Oh and thats the first decent info I've had on AC/DC earths, nice one CrazyScozzese !

 

[lecnoob]

BTW if I were just to cut off the end of my Toshiba lead to swap it over, whats the best way to attach it? I'm assuming I'm not going to be able to solder it, but could I use one of those screw-in cable joiner things?

 

[CrazyScozzese]

If it is a new laptop, remember you will invalidate your warranty. What I have done in the past to avoid it when I couldn't source a proprietory plug, is bought a broken/faulty ac/dc adapter of the same type from ebay or the local junk/cash shops, used the cable and connector from it and fitted a female jack to the other end to mate to a new generical ac/dc adapter I used as a replacement.

Then you do not need to cut into your new adapter, you have two adapters, one for work and one for home.

You can also buy humbuckers that will opto isolate the audio lead making them electrically seperate and remove the hum.

 

[DNS1]

I've had this exact same problem.

A friend and I run a mobile DJ rig, using a Toshiba laptop and Denon USB controller for video and audio. Isolators and humbuckers made no difference.

In the end I made a very short extension lead with the earth disconnected and labeled in VERY big letters to stop anyone using it for anything other than the laptop. Problem solved.

The way I looked at it, if I was to take that laptop to anywhere that uses 2 prong plugs as standard (USA, Canada, older European sockets etc) and plug it in with a travel adaptor, there would be no problem. I'm just recreating this situation in the UK.

 

[spark 68]

Personally I would leave the DC side well alone, the third pin here can be used for various reasons.

It may be used for voltage/current sensing reasons in the charging circuit, and on some laptops it is used for intelligent battery sensing, ie. what make of battery, temperature of the cell pack etc..etc.

I have come across one charger that uses the third pin for regulation purposes, stepping the voltage up and down depending on the state of charge for the cell pack, and how many cells are present in a given cell pack.

Edit: the Earth is not usually connected directly to the secondary side in SMPSU's, it usually bridged across the isolation barrier via a high value resistor (circa 10Meg Ohms), and sometimes a capacitor in parallel, this is to stop the secondary side from floating to too high a voltage which may be above the breakdown of the isolation components.