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.

 

[CrazyScozzese]

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.

I presumed from the description/photo the third pin discussed was on the AC side of the adapter, not the DC, so I expect it is more or less a functional earth for filtering as mentioned. OP please clarify this. That said, the advice from S68 remains correct. I would keep the original PSU intact, and would not advise disconnecting an earth. Some nutter will use the extension when you are at the toilet and get the 120V tingle or more as a surprise. May improve his dance moves.

 

[Specialist]

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.

I hope for your sake you've got really good Insurance, if someone else should happen to plug something into this Lead & receive a Shock or get Electrocuted due to you cutting off the Earth. It won't matter that you Labeled it in very big letters or that in your opinion it was ok because they only use 2 pin plugs in other countries, your the 1 who will carry the can.

 

[lecnoob]

Ok well I managed to find a replacement plug on ebay (link) that I think will fit, so I'm going to have a go at soldering it onto my old 2-pin Toshiba supply and see what that does.

Still a bit of a mystery to me why DNS1's solution is dangerous (does this mean using any 3-pin gear abroad in places with no earth in plugs is a big problem?)
:46:

Also CrazyScozzese I'm already using an external USB soundcard so not sure what you meant about the USB in that earlier post...

 

[Specialist]

Still a bit of a mystery to me why DNS1's solution is dangerous (does this mean using any 3-pin gear abroad in places with no earth in plugs is a big problem?) .

Please Re-Read my post, Earth wires are there for safety. Forget what goes on in other countries. If someone was to plug a Normally Earthed appliance into his extension lead & due to a fault the case became live, then whoever touches it is Basically going to take the place of the Earth & the current will flow through them instead of the Earth wire. At the least they'll get a nasty shock & at worst Electrocuted. i.e DEAD.

 

[CrazyScozzese]

Equipment that is designed to require an earth, needs an earth, disconnecting it risks electric shock and does not neccessarily require a fault to do so. In laymans terms.

The point about the USB connection was to create a simple chassis ground link between the laptop and the sound gear, outside of the analogue audio line. The metal shroud of the USB ports via the cable may shunt any noise looking for a path to earth causing the hum. Not guaranteed, but worth a try.

If you do this a lot, invest a few quid in an optical isolator, more pro, less hassle.