Earth loop with TV and car Stereo

[Tony Ross]

Hi Guys,

 

I am getting what sounds like an earth loop buzzing when I try to output the sound from my TV (which is powered by 240v inverter) to the car stereo which is 12v. I do this as the sounds are significantly better than the built in speakers in the TV. The same happens when outputting sound from the laptop when on the mains charger but disappears when running on batteries.

 

Is the correct way to deal with this to knock up a cable using 3.5mm stereo jackplugs and just omit to wire the sleeves, which if I'm not mistaken provide the earth? If not, any other suggestions other than just don't connect the two!

 

As background, both 12v and 240v are negative earthed.

 

Many thanks,

 

Tony

[nbfiresprite]

Sounds like cheap inverter to me, What make and is it a pure sine wave model.

 

Firesprite

 

In the Fens

[Tony Ross]

Sterling 1800w Quasi Sine Wave.

[Nickhlx]

Sterling 1800w Quasi Sine Wave.

 

Assuming its not the inverter, there is a possibility of a ground loop - there exist isolated leads which pass the audio through a small isolation transformer - some car radio shops do them but its a standard item at the bike comms shops - Autocom ( and others) make them - you will probably find them on E-Bay too.

 

The most likely thing though is a noisy mains supply... you could try clamp on ferrites on the mains lead ( and DC lead) but I don't think its likely to help much

 

Nick

Edited September 19, 2011 by Nickhlx

[WotEver]

The same happens when outputting sound from the laptop when on the mains charger but disappears when running on batteries.

Proof positive that it's the inverter.

 

Tony

[Tony Ross]

Proof positive that it's the inverter.

 

Tony

 

Or it could be that with the mains disconnected there is no earth loop! I accept that it could be the inverter but ask any recording studio engineer about earth loops and they will give you a very knowing look so I need to consider the options.

[WotEver]

... ask any recording studio engineer about earth loops and they will give you a very knowing look...

I am a recording studio engineer (or at least was, in a past life). I have also designed and built audio equipment. It sounds nothing like an earth loop to me.

 

Both your TV and Laptop will be double-insulated, meaning there will be no earth connection to them whether the mains is connected or not. Ergo it cannot be an earth loop.

 

Tony

[Tony Ross]

I am a recording studio engineer (or at least was, in a past life). I have also designed and built audio equipment. It sounds nothing like an earth loop to me.

 

Both your TV and Laptop will be double-insulated, meaning there will be no earth connection to them whether the mains is connected or not. Ergo it cannot be an earth loop.

 

Tony

 

Thanks for that Tony,

 

I bow to your better judgement. So, on the basis that it is the inverter, why do the TV or the laptop not buzz on their own? Surely, the buzzing that is transmitted to the Car Stereo would come out of the internal speakers on the TV/Laptop as the audio output socket is just the signal that would normally go to the internal speakers had I not plugged in a 3.5mm jack to send it to the Car Stereo?

 

Is there a way to put a choke on the Inverter or should I just accept the problem and run the TV off the 12v system as well?

 

Tony

[Bob18]

Two problems.

First its a quasi-square wave inverter, its bound to be noisy when using audio equipment on it.

A quick, and "northing's changed" test is to try separating the power and signal cables as much as possible and not running close and parallel to each other, sometimes works, sometimes doesn't, but it costs nothing to try.

 

Second you have an earth loop, but you'd guessed that already. The screen is a sort of earth, but its not a protective earth. You need to have the two signal cores, which should be connected to the tip and first ring of the 3.5mm plug at both ends (I'll assume for now its a 3.5mm plug at each end), disconnecting it from one end only. If it doesn't work one way round reverse the lead and try again. If it still doesn't work there are a few more complicated tricks in the box, but they aren't as easy/cheap to test.

[WotEver]

Second you have an earth loop, but you'd guessed that already.

Why doesn't it do it when on battery power then?

 

Tony

 

So, on the basis that it is the inverter, why do the TV or the laptop not buzz on their own?

My guess is that they do, but with the highly limited bass from such speakers you can't hear it.

 

Tony

[Bob18]

Because the noise is coming from the inverter or charger, and being carried along the screen of the audio cable to the amplifier and and speakers.

[WotEver]

Because the noise is coming from the inverter or charger, and being carried along the screen of the audio cable to the amplifier and and speakers.

That's an induced signal, not an earth loop.

 

Tony

[Guest]

Is there a ring main from your inverter on 240V side? I seem to recall reading about strange effects on sound equipment under these circumstances?

[Tony Ross]

Is there a ring main from your inverter on 240V side? I seem to recall reading about strange effects on sound equipment under these circumstances?

 

 

Yes, there is a run of 5 double sockets coming off the inverter although most of the time only the TV and the Satellite receiver are plugged in.

 

Tony

[WotEver]

To the OP - if the TV is being powered by the inverter and you have nothing connected to the input of your car stereo, but you have the car stereo volume at 'normal' level, do you still hear the buzz?

 

Tony

 

Yes, there is a run of 5 double sockets...

But are they wired as a ring, or just a daisy-chain?

 

Tony

[Tony Ross]

To the OP - if the TV is being powered by the inverter and you have nothing connected to the input of your car stereo, but you have the car stereo volume at 'normal' level, do you still hear the buzz?

 

Tony

 

 

But are they wired as a ring, or just a daisy-chain?

 

Tony

 

Hi Tony,

 

There is no buzz present on the Car Stereo when the TV is on and the Car stereo set to Aux input (nothing connected) at normal volume. Interestingly enough though, when the laptop is plugged into it's charger there is a buzzing on FM channels from the radio. I have blamed this on the switch mode adaptor that the laptop uses. Most other 240v adaptors that I have present no such symptoms, just the laptop charger and one for an ipod dock which is again Switch mode.

 

The plugs are daisy chained

 

Hope that helps,

 

Tony

[by'eck]

Almost certainly induction from the quasi sine wave inverter into the external lead you are using to connect the devices.

[Bob18]

The noise can be both induction or conduction.

Generally if its a low frequency hum then its inducted, but if its a high frequency noise then conduction is more likely

Whichever it is the first port of call is to break the screening that is allowing the noise to get to where it is being amplified.

 

Both Quasi-sine inverters and switchmode power supplies supplies are "good" sources of noise

[WotEver]

Almost certainly induction from the quasi sine wave inverter into the external lead you are using to connect the devices.

Agreed 100%.

 

Tony

[frangar]

I'm afraid you can get earth loops from laptops....they are rarely double insulated....Mac or PC....I'm a Live sound engineer and when we use them on shows we run them thro audio isolating transformers with a signal earth lift (DI box)....some laptops are much worse than others for this...Dells are particularly bad!!

 

My guess is that the AC earth is bonded to the DC Negative so thats why its OK when running on the laptops internal batteries.

 

One fix would be a 1:1 Audio isolator with an earth lift

 

Cheers

 

Gareth

 

PS would post a link but in the middle of a show at the moment!!....will find one when I get a few moments!

[WotEver]

OP said it happened from his TV as well as his lappy. I haven't seen a TV for sale in the UK in the last 20 years that has an earth connection.

 

My first suggestion, apart from experimenting with the actual routing of the audio cable, would be to try a high quality cable with an improved screen.

 

With regard to your comment about DI boxes, non-Apple Laptops are also generally unearthed but of course are also unbalanced. Hence inducted mains hum will be a real problem, as with the OP. Your DI box is eliminating this by balancing the signal, not by removing a non-existant earth loop. I have Dell, Sony, Samsung and IBM laptops at home, and they are all fed with a figure-of-eight mains lead - no earth connection.

 

I don't know about Apple mag-link PSUs 'cos although I have two in front of me I don't have a meter to hand, so I can't comment on those. However OP didn't mention a Mac.

 

It's induced hum from all the nasty harmonics in the MSW inverter output.

 

Tony

[WotEver]

For earth loop avoidance, you can use one of these.

 

To avoid inductance is more expensive if you buy proprietary kit and requires converting the unbalanced TV/Laptop output to balanced as close to the unit as possible, then running with balanced cable to the amp, then unbalancing it just before the amp. You can easily manufacture your own using a couple of transformers if you know what you're doing.

 

Tony

[Pretty Funked Up]

I managed to eliminate most of the noise with this (only my laptop did it though ((no inverter involved at all)) and Ive stopped using that for music now)

Be prepared to lose a huge chunk of your bass though.

[WotEver]

I managed to eliminate most of the noise with this (only my laptop did it though ((no inverter involved at all)) and Ive stopped using that for music now)

Be prepared to lose a huge chunk of your bass though.

I love how they give the spec as "40Hz-32Hz". I assume the 2nd figure is a typo for 32kHz, but it's useless without specifying any margins. It's probably +-6dB, which is huge.

 

Tony

[frangar]

OP said it happened from his TV as well as his lappy. I haven't seen a TV for sale in the UK in the last 20 years that has an earth connection.

 

My first suggestion, apart from experimenting with the actual routing of the audio cable, would be to try a high quality cable with an improved screen.

 

With regard to your comment about DI boxes, non-Apple Laptops are also generally unearthed but of course are also unbalanced. Hence inducted mains hum will be a real problem, as with the OP. Your DI box is eliminating this by balancing the signal, not by removing a non-existant earth loop. I have Dell, Sony, Samsung and IBM laptops at home, and they are all fed with a figure-of-eight mains lead - no earth connection.

 

I don't know about Apple mag-link PSUs 'cos although I have two in front of me I don't have a meter to hand, so I can't comment on those. However OP didn't mention a Mac.

 

It's induced hum from all the nasty harmonics in the MSW inverter output.

 

Tony

 

Mac laptops are one of the few that we dont need to earthlift the signal cable. This very laptop is running straight into the desk via 1/4" mono jacks...no balancing needed!

 

We use the DI Box to isolate the signal path and earth lift as much as balancing the signal.....you can prove this by using 1/4" stereo jack rather than the balanced (XLR or 3 pole jack) connectors

 

I agree that the MSW inverter isnt helping but I think there might be some interesting earth paths going on....possibly even some stray volts on earth which is a problem we have a lot in hotels which dont have a dedicated tech mains.

 

Gareth

 

Edited to add:

 

Both Acers and the Dell that we are using here as well all have "cloverleaf" ie earthed power supplies... the Apple one is a "figure of eight" therefore double insulated....and the Acers and esp the Dell need a decent DI Box.....not ur chinese rubbish to get rid of any earth inducded hum

Edited September 20, 2011 by frangar