When using our laptop on mains we suffer from the ground loop hum, unplug mains and run on battery and it goes.

Now the normal answer would be to buy a ground loop isolater, they're cheap and straight forward to use. But our laptop connects via lan cable to a router and then onto the two CDJ 900s and by usb to the DJM 5000 mixer. So no red and white connecters involved whereby you'd put that in between your connection from the laptop to the mixer.

Any ideas guys?

Firstly, try to find where the hum is coming from,
I doubt its Ethernet (the signals are isolated as part of the standard)
Unplug the USB to the mixer (if you can)
Unplug the laptop from its PSU (now running on batteries). If the hum is removed, its the laptop.
If the hum remains, get a CrossOver cable and connect the Laptop directly to 1 CDJ900 (leave the other disconnected)

So, its probably coming from USB from the PC.

Before you go too mad, make sure you've earthed your Mixer. My mixer(s) have earth points, that made a noticeable difference when I tied them to Earth. These are actually different to the safety earth you will have on your IEC input.

The hum disappears when I unplug the mains from the laptop and run on battery so it's definitely that. I had this reply on Pioneer's forum which is interesting

"which laptop is it? Most laptop power supplies are 'double insulated' and therefore do not require an earth connection on their uk 3 pin mains plug.
If yours is such a type, simply disconnect the earth from the plug."

My next door neighbour is an electrician so I'll ask his advice on disconnecting the earth.

I picked up a generic laptop power supply moduel for £45 which solved my hum (same problem as you). My original psu had an earth and the new one does not. Problem solved for me. Not a bad idea to have a spare psu - you never know.

I know a guy who did this and it solved the problem, however I don't think I would do it myseld without getting professional advice

Whatever you do, DO NOT disconnect earth!

If your PSU has a 3 pin input, it's there for a reason! Where earth is required/provided, its connected to the 0v or -ve of the output jack of the PSU, and is there because the PSU design needs it.


You can get 2-pin PSUs which can solve this (as MintyDave has suggested above). I've got a Kensington 2-pin one for mine (which will also run off a car battery too). I've also got an FSP Group 120w PSU which comes with various connectors. This is also a 2-pin telefunken input too.


Cheers,

David

just a thought

does your amp have a ground lift switch. This isolates the audio ground from the earth. The kam amps have this facility and helps when you have a loops between amps and psu's. Its no good however if your loop is generated between your laptop and mixer, which is what happend in my case unless i use a different mixer.

just a thought.

Alternatively you can get a pair of ground loop isolators to drop between your mixer and amp. you would need someting like this http://www.dv247.com/studio-equipment/behr...itter-box--4662 which has a ground lift switch. not ideal if the hum is across your mixer and its more gear to carry about, setup and possibly go wrong.

It is 100% safe to remove the earth connection in the mains plug from a laptop power supply.
The power supply is already protected by fuses and double insulation.

The "earth" is only soldered to a foil shield inside the plastic case of the psu via a high valued resistor and has absolutely no safety value or function whatsoever.

The only reason the power supply carries an earth connection is to bleed off some of the enormous amounts of Radio frequency interference the switch mode oscillator inside the psu generates. The foil usually doesn't even cover the live mains, just the bits that tend to emit rfi like the transformer and inductors.

I removed the earth wire from my own laptop ages back as it generated a buzz that no end of ground loop isolators was ever going to fix.

No it isnt. A double-insulated laptop supply wont have an earth by definition. If it has an earth conductor, it isnt double insulated.


No it isnt. The earth also carries the earth current back to the mains from the RFI filters. If you remove the earth, the chassis of the laptop and the chassis of the PSU will float up to 120V or more. This presents a shock hazard.

The earth, where fitted, often constitutes a safety insulation barrier between the high and low voltage sections of the power supply. Remove the earth, and the barrier is in essence useless. So a component failure on the high-voltage side could transmit lethal voltage to the low voltage part. An example would be the chopper transistor (running at 340V) mounted to an earthed aluminium heatsink via insulating mica washer. If the earth is removed, this would mean the washer would be the only thing between 340V DC and your laptop chassis.



Please think and qualify your posts before suggesting potentially life-threatening suggestions in a technical forum!

Thanks guys, alot of good advice. I think my best bet is MintyDave's suggestion of a second power supply.

Yes.

Genuine 2-pin mains adapters are built to different specifications and are of course safe as they are.

This was information I was passing on from another audio forum posted by a very well respected electronics engineer. I can't post links to qualify as it's against the rules and i'll get my account disabled.

Apologies for getting a bit wrong. I remember you posted about the current being carried back to earth.

Quick question: how much is the earth current in question? Can't be many mA surely (as it'd trip the earth RCD breakers in most houses)?

Cheers

I dont know..what power adapter are you thinking of lifting the earth on?

You can dump up to 3.5mA down the earth lead of a class-1 appliance before it's a PAT fail; (portable IT equipment is a little less); of course this does not appear as a voltage on any exposed conductors because it's tied to earth via the earth lead.

It's when that current has nowhere to go but down the laptop lead that things get interesting! Random shocks, blown mixer channels, live mics, you name it and it'll happen.

But I'm more concerned by the safety implications of the adapter internal design. Laptop adapters are very compact switch-mode power supplies where the high and low voltage sections are in very close proximity to each other. The switching choke in a class 1 SMPSU is often wound with single section bobbin, with copper interwinding screen tape earthed to the mains earth to act as protection to the user in case of insulation breakdown.
Double insulated SMPSUs will use a split-bobbin with double insulation (the clue's in the name) to fully isolate the primary and secondary windings.

But I'm starting to go on a bit for a disco forum, let's all just agree that mains earths must not be removed to try to cure a ground-hum problem.

If you need a second opinion, here's a link to a Sound-on-Sound article, I think as a community leader I'm allowed to link to it ;-)

http://www.soundonsound.com/sos/aug06/articles/qa0806_4.htm



Back the original OP...the earth loop will be created by the laptop adapter almost certainly; so breaking the earth loop by an audio isolator is still an option. You can't break it obviously at the data leads, so put the isolator between the audio out from the mixer and the audio in of the amprack.

We know the earth loop isn't inducing hum from the laptop to mixer or CDJs because these are data leads which are immune to it!
It's a loop...so you can break it anywhere by an isolator irrespective of which device is actually generating the noise.

Is USB isolated? I don't think so?
As far as I know, its not isolated (but is twisted pair data+/data- )
USB isolators are possible, but are not that common, and are very expensive.

Its a bit late to re-read the thread, but I agree with the consensus that a "floating" power supply (ie no earth) will fix the problem.

One of my (day job) products hates floating PSUs when using RS232 to a PC. We have to earth the product, otherwise we see ~120VAC on the unit on 0V WRT Earth :-=
It does give a little tingle!

Jason

sorry to jump in on this thread but i am suffering a very similar problem the only thing that is different to the op is that i have only just started to get this problem in all my previous gigs i have never had the problem so any ideas why it would just start one suggestion i have had is that it could be a soundcard issue? any ideas gratefully recieved.

Possibly a noisy power adapter, they start developing harmonics as the electrolytics age which can eventually develop into a duff PSU.

OK, sorry if this question has been asked before but I'm useless when it comes to the electrics of my system (software no problem but electrics...nah!).

When everything's set up, there is a hum coming out of the speakers so I'm guessing this is what you call groundloop hum.

Everything (to my knowledge) is wired up and plugged in correctly. I have Wharfedale active speakers. When the music plays you can't hear it so it isn't a massive problem but I'd like to get rid of it.

Is there an idiot-proof widget I can buy that will cure it without me having to check and dismantle every piece of kit? I don't really have the space at home to set everything up to test so if there's a filter or a plug-in I can use then I'd rather go down that route.

Thanks in advance guys..

Are you refering to a regular run of the mill noise that sounds like hmmmmmmmm or shhhhhhhhh

or more of a

HMMMMMMMMMMMMMM

As im sure SSDJ will agree, and im no whizz on this, but there are loads of contributing factors to noise from pa systems. It starts with venues, some are worse than others. Big fridges, beer coolers old flurescent lighting etc can be a root cause. Then there are cheap components in your equipment and leads with poor insulation as well as component degradation. I have different levels of noise depending on where i gig.

A pal of mine runs a PA hire co and he has invested heavily in high end gear (inc quality power conditioners etc etc) and the noise from his 40k rig is practically nothing at concert vol. believe me i have stood in front of the damn thing on countless occasions.

All i can say is use better quaility cables, keeping the length to a minumum and avoid coiling long cables up. Also keep your signal and power cables apart as much as possible, especially with powered speakers.

As i said im no expert but these are things i do remember from styudying electronics 20 years ago.

I'm no techie, but this solved the very same problem for me

http://www.studiosolutions.ie/108-ART-DTI-...r--Isolator.htm

Buzz was gone when I used this between my mixer and powered cabs. It is actually handy as I have short 0.5m xlr cables going from my mixer into the art unit, so when I am plugging in my speakers I just grab the unit, pull it (carefully) out of the flightcase and plug the speakers into it, rather than fiddling with a flashlight trying to plug xlrs into the mixer directly. I also use basic ground loop isolators on both channels on the way into my mixer from my midi player, and my powered cabs are dead quiet when the music is turned off.

That DI box looks pretty good with 3 types of input and output connector... and good to make a balanced signal from an unbalanced and vice-versa (think of those long cable runs!)

David